The Progressive Corporation (PGR) 10-K 27 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($)	12 Months Ended
	Dec. 31, 2017	Jan. 31, 2018	Jun. 30, 2017
Document Information [Line Items]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	PGR
Entity Registrant Name	PROGRESSIVE CORP/OH/
Entity Central Index Key	80,661
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		582,275,989
Entity Public Float			$ 25,242,148,256

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Net premiums earned	$ 25,729.9	$ 22,474	$ 19,899.1
Investment income	563.1	478.9	454.6
Net realized gains (losses) on securities:
Net impairment losses recognized in earnings	(64.5)	(86.8)	(23.8)
Net realized gains (losses) on securities	114.1	137.9	136.5
Total net realized gains (losses) on securities	49.6	51.1	112.7
Fees and other revenues	370.6	332.5	302
Service revenues	126.8	103.3	86.3
Other (gains) losses	(1)	1.6	(0.9)
Total revenues	26,839	23,441.4	20,853.8
Expenses
Losses and loss adjustment expenses	18,808	16,879.6	14,342
Policy acquisition costs	2,124.9	1,863.8	1,651.8
Other underwriting expenses	3,480.7	2,972	2,712.1
Investment expenses	23.9	22.4	22.8
Service expenses	109.5	92	77.5
Interest expense	153.1	140.9	136
Total expenses	24,700.1	21,970.7	18,942.2
Net Income
Income before income taxes	2,138.9	1,470.7	1,911.6
Provision for income taxes	540.8	413.5	611.1
Net income	1,598.1	1,057.2	1,300.5
Net (income) loss attributable to noncontrolling interest (NCI)	(5.9)	(26.2)	(32.9)
Net income attributable to Progressive	1,592.2	1,031	1,267.6
Changes in:
Total net unrealized gains on securities	355.4	130.6	(212.9)
Net unrealized losses on forecasted transactions	(5.4)	(1.2)	(9.7)
Foreign currency translation adjustment	1.1	0.4	(1.2)
Other comprehensive income (loss)	351.1	129.8	(223.8)
Other comprehensive (income) loss attributable to NCI	(2.3)	3.2	1.1
Comprehensive income attributable to Progressive	$ 1,941	$ 1,164	$ 1,044.9
Computation of Per Share Earnings Attributable to Progressive
Average shares outstanding — Basic	580.8	581.7	585.5
Net effect of dilutive stock-based compensation	4.9	3.3	3.7
Total average equivalent shares — Diluted	585.7	585	589.2
Basic: Earnings per share (usd per share)	$ 2.74	$ 1.77	$ 2.16
Diluted: Earnings per share (usd per share)	$ 2.72	$ 1.76	$ 2.15

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Investments – Available-for-sale, at fair value:
Fixed maturities (amortized cost: $20,209.9 and $16,287.1)		$ 20,201.7	$ 16,243.8
Equity securities:
Nonredeemable preferred stocks (cost: $698.6 and $734.2)		803.8	853.5
Common equities (cost: $1,499.0 and $1,437.5)		3,399.8	2,812.4
Short-term investments (amortized cost: $2,869.4 and $3,572.9)		2,869.4	3,572.9
Total investments		27,274.7	23,482.6
Cash		265	211.5
Restricted Cash		10.3	14.9
Total Cash, Cash Equivalents, and Restricted Cash		275.3	226.4
Accrued investment income		119.7	103.9
Premiums receivable, net of allowance for doubtful accounts of $210.9 and $186.8		5,422.5	4,509.2
Reinsurance recoverables, including $103.3 and $83.8 on paid losses and loss adjustment expenses		2,273.4	1,884.8
Prepaid reinsurance premiums		203.3	170.5
Deferred acquisition costs		780.5	651.2
Property and equipment, net of accumulated depreciation of $940.6 and $845.8		1,119.6	1,177.1
Goodwill		452.7	449.4
Intangible assets, net of accumulated amortization of $175.7 and $109.5		366.6	432.8
Other assets		412.9	339.6
Total assets		38,701.2	33,427.5
Liabilities
Unearned premiums		8,903.5	7,468.3
Loss and loss adjustment expense reserves		13,086.9	11,368
Net deferred income taxes		135	111.3
Dividends payable		655.1	395.4
Accounts payable, accrued expenses, and other liabilities	[1]	2,825.9	2,495.5
Debt	[2]	3,306.3	3,148.2
Total liabilities		28,912.7	24,986.7
Redeemable noncontrolling interest (NCI)	[3]	503.7	483.7
Shareholders’ Equity
Common shares, $1.00 par value (authorized 900.0; issued 797.5 including treasury shares of 215.8 and 217.6)		581.7	579.9
Paid-in capital		1,389.2	1,303.4
Retained earnings		6,031.7	5,140.4
Accumulated other comprehensive income:
Net unrealized gains (losses) on securities		1,295	939.6
Net unrealized losses on forecasted transactions		(14.8)	(9.4)
Foreign currency translation adjustment		0	(1.1)
Accumulated other comprehensive (income) loss attributable to NCI		2	4.3
Total accumulated other comprehensive income attributable to Progressive		1,282.2	933.4
Total shareholders’ equity		9,284.8	7,957.1
Total liabilities, redeemable NCI, and shareholders’ equity		$ 38,701.2	$ 33,427.5
[1]	See Note 12 – Litigation and Note 13 – Commitments and Contingencies for further discussion.
[2]	Consists of both short-term and long-term debt. See Note 4 – Debt for further discussion.
[3]	See Note 15 – Redeemable Noncontrolling Interest for further discussion.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 20,209.9	$ 16,287.1
Nonredeemable preferred stocks, cost	698.6	734.2
Common equities, cost	1,499	1,437.5
Short Term Investments Amortized Cost	2,869.4	3,572.9
Premiums receivable, allowance for doubtful accounts	210.9	186.8
Reinsurance recoverables, paid losses and loss adjustment expenses	103.3	83.8
Property and equipment, accumulated depreciation	940.6	845.8
Finite-Lived Intangible Assets, Accumulated Amortization	$ 175.7	$ 109.5
Common Shares, par value (USD per share)	$ 1	$ 1
Common Shares, authorized (shares)	900,000,000	900,000,000
Common Shares, issued (shares)	797,500,000	797,500,000
Common Shares, treasury shares (shares)	215,800,000	217,600,000

Consolidated Statements of Chan

Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Millions	Total	Common Shares, $1.00 Par Value	Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income Attributable to Progressive
Balance, Beginning of year at Dec. 31, 2014		$ 587.8	$ 1,184.3	$ 4,133.4	$ 1,023.1
Tax benefit from vesting of equity-based compensation			16.8
Treasury shares purchased		(7.3)	(15.2)	(186)
Net restricted equity awards issued/vested		3.1	(3.1)
Amortization of equity-based compensation			64.5
Reinvested dividends on restricted stock units			5.7	(5.7)
Adjustment to carrying amount of redeemable noncontrolling interest			(34.2)
Net income attributable to Progressive	$ 1,267.6			1,267.6
Cash dividends declared on common shares ($1.1247, $0.6808, and $0.8882 per share)				(520.5)
Other, net				(2.2)
Attributable to noncontrolling interest	1.1				1.1
Other comprehensive income (loss)					(223.8)
Balance, End of year at Dec. 31, 2015	7,289.4	583.6	1,218.8	4,686.6	800.4
Tax benefit from vesting of equity-based compensation			9.2
Treasury shares purchased		(6.1)	(13.4)	(173)
Net restricted equity awards issued/vested		2.4	(2.4)
Amortization of equity-based compensation			80.9
Reinvested dividends on restricted stock units			6.1	(6.1)
Adjustment to carrying amount of redeemable noncontrolling interest	(4.2)		4.2
Net income attributable to Progressive	1,031			1,031
Cash dividends declared on common shares ($1.1247, $0.6808, and $0.8882 per share)				(394.7)
Other, net				(3.4)
Attributable to noncontrolling interest	3.2				3.2
Other comprehensive income (loss)					129.8
Balance, End of year at Dec. 31, 2016	7,957.1	579.9	1,303.4	5,140.4	933.4
Tax benefit from vesting of equity-based compensation			0
Treasury shares purchased		(1.5)	(3.4)	(57.6)
Net restricted equity awards issued/vested		3.3	(3.3)
Amortization of equity-based compensation			92.9
Reinvested dividends on restricted stock units			8	(8)
Adjustment to carrying amount of redeemable noncontrolling interest	8.4		(8.4)
Net income attributable to Progressive	1,592.2			1,592.2
Cash dividends declared on common shares ($1.1247, $0.6808, and $0.8882 per share)				(654.2)
Other, net				18.9
Attributable to noncontrolling interest	(2.3)				(2.3)
Other comprehensive income (loss)					351.1
Balance, End of year at Dec. 31, 2017	$ 9,284.8	$ 581.7	$ 1,389.2	$ 6,031.7	$ 1,282.2

Consolidated Statements of Cha6

Consolidated Statements of Changes in Shareholders' Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retained Earnings
Cash dividends declared on common shares, per share	$ 1.1247	$ 0.6808	$ 0.6862
Serial Preferred Stock
Preference Shares authorized	20,000,000	20,000,000	20,000,000
Preference Shares issued	0	0	0
Preferred Shares outstanding	0	0	0
Voting Preferred Stock
Preference Shares authorized	5,000,000	5,000,000	5,000,000
Preference Shares issued	0	0	0

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net income		$ 1,598.1	$ 1,057.2	$ 1,300.5
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation		169.9	137.4	103.7
Amortization of intangible assets		66.2	62.1	46.8
Net amortization of fixed-income securities		86.2	77.2	98.4
Amortization of equity-based compensation		95.4	85.2	66.2
Net realized (gains) losses on securities		(49.6)	(51.1)	(112.7)
Net (gains) losses on disposition of property and equipment		7.2	6.6	2
Other (gains) losses		1	(1.6)	0.9
Net loss on exchange transaction		0	4.5	0
Changes in:
Premiums receivable		(913.2)	(518.5)	(421.1)
Reinsurance recoverables		(388.6)	(388.2)	(202.6)
Prepaid reinsurance premiums		(32.8)	48.8	32.5
Deferred acquisition costs		(129.3)	(103.8)	(42.3)
Income taxes		(172.6)	(55.7)	(107.2)
Unearned premiums		1,434.9	830.7	632.4
Loss and loss adjustment expense reserves		1,718.8	1,323.2	917.7
Accounts payable, accrued expenses, and other liabilities		400	308.9	37.9
Other, net		(134.8)	(90.2)	(60.2)
Net cash provided by operating activities		3,756.8	2,732.7	2,292.9
Purchases:
Fixed maturities		(14,587.8)	(11,610.6)	(9,311.1)
Equity securities		(255.6)	(434.2)	(647.1)
Sales:
Fixed maturities		5,382.5	5,694.9	4,913.5
Equity securities		252.9	484.6	402.4
Maturities, paydowns, calls, and other:
Fixed maturities		5,215.8	4,907.4	3,579.5
Equity securities		50	0	12
Net sales (purchases) of short-term investments		727.6	(1,357.2)	20.5
Net unsettled security transactions		(33.6)	50.9	(8.2)
Purchases of property and equipment		(155.7)	(215)	(130.7)
Sales of property and equipment		15.3	6.2	10.6
Net cash disposed in exchange transaction		0	(7.7)	0
Acquisition of additional shares of ARX Holding Corp.		0	0	(12.6)
Net cash used in investing activities		(3,406.7)	(2,480.7)	(1,923.9)
Cash Flows From Financing Activities
Proceeds from exercise of equity options		0.5	0	0.2
Net proceeds from debt issuance		841.1	495.6	382
Payments of debt		(49)	(25.5)	(20.4)
Redemption/reacquisition of subordinated debt		(635.6)	(18.2)	(19.3)
Dividends paid to shareholders		(395.4)	(519)	(403.6)
Acquisition of treasury shares for restricted stock tax liabilities		(57.6)	(25.1)	(30.6)
Acquisition of treasury shares acquired in open market		(4.9)	(167.4)	(177.9)
Tax benefit from vesting of equity-based compensation		0	9.2	16.8
Net cash used in financing activities		(300.9)	(250.4)	(252.8)
Effect of exchange rate changes on cash		(0.3)	0.4	(0.2)
Increase in cash, cash equivalents, and restricted cash		48.9	2	116
Cash, cash equivalents, and restricted cash - Beginning of year		226.4	224.4	108.4
Cash, cash equivalents, and restricted cash - End of year		275.3	226.4	224.4
Insurance Company
Maturities, paydowns, calls, and other:
Acquisition of business, net of cash acquired	[1]	(18.1)	0	0
ARX Holding Corp.
Maturities, paydowns, calls, and other:
Acquisition of business, net of cash acquired		$ 0	$ 0	$ (752.7)
[1]	See Note 1 – Reporting and Accounting Policies for further information.

SCHEDULE I - Summary Of Investm

SCHEDULE I - Summary Of Investments - Other Than Investments in Related Parties	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
SCHEDULE I - Summary Of Investments - Other Than Investments In Related Parties	SCHEDULE I — SUMMARY OF INVESTMENTS — OTHER THAN INVESTMENTS IN RELATED PARTIES THE PROGRESSIVE CORPORATION AND SUBSIDIARIES (millions) December 31, 2017 Type of Investment Cost Fair Value Amount At Which Shown In The Balance Sheet Available-for-sale Fixed maturities: Bonds: United States Government and government agencies and authorities $ 6,688.8 $ 6,645.9 $ 6,645.9 States, municipalities, and political subdivisions 2,285.6 2,297.1 2,297.1 Public utilities 192.5 193.6 193.6 Corporate and other debt securities 4,804.7 4,804.1 4,804.1 Asset-backed securities 6,043.4 6,050.0 6,050.0 Redeemable preferred stocks 194.9 211.0 211.0 Total fixed maturities 20,209.9 20,201.7 20,201.7 Equity securities: Common stocks: Public utilities 106.8 177.5 177.5 Banks, trusts, and insurance companies 285.8 685.0 685.0 Industrial, miscellaneous, and all other 1,106.4 2,537.3 2,537.3 Nonredeemable preferred stocks 698.6 803.8 803.8 Total equity securities 2,197.6 4,203.6 4,203.6 Short-term investments 2,869.4 2,869.4 2,869.4 Total investments $ 25,276.9 $ 27,274.7 $ 27,274.7 Progressive did not have any securities of any one issuer, excluding U.S. government obligations, with an aggregate cost or fair value exceeding 10% of total shareholders’ equity at December 31, 2017 .

SCHEDULE II - Condensed Financi

SCHEDULE II - Condensed Financial Information Of Registrant	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
SCHEDULE II - Condensed Financial Information Of Registrant	SCHEDULE II — CONDENSED FINANCIAL INFORMATION OF REGISTRANT CONDENSED STATEMENTS OF COMPREHENSIVE INCOME THE PROGRESSIVE CORPORATION (PARENT COMPANY) (millions) Years Ended December 31, 2017 2016 2015 Revenues Dividends from subsidiaries $ 867.3 $ 375.5 $ 852.5 Undistributed income (loss) from subsidiaries 866.3 741.9 500.0 Equity in net income of subsidiaries* 1,733.6 1,117.4 1,352.5 Intercompany investment income* 11.3 5.5 3.9 Gains (losses) on extinguishment of debt 0.2 1.6 (0.9 ) Total revenues 1,745.1 1,124.5 1,355.5 Expenses Interest expense 151.1 140.4 136.1 Deferred compensation 1 23.2 5.3 5.3 Other operating costs and expenses 4.6 4.2 5.4 Total expenses 178.9 149.9 146.8 Income before income taxes 1,566.2 974.6 1,208.7 Benefit for income taxes 26.0 56.4 58.9 Net income attributable to Progressive 1,592.2 1,031.0 1,267.6 Other comprehensive income (loss) 348.8 133.0 (222.7 ) Comprehensive income attributable to Progressive $ 1,941.0 $ 1,164.0 $ 1,044.9 * Eliminated in consolidation. 1 See Note 4 – Employee Benefit Plans in these condensed financial statements. See notes to condensed financial statements. CONDENSED BALANCE SHEETS THE PROGRESSIVE CORPORATION (PARENT COMPANY) (millions) December 31, 2017 2016 Assets Investment in affiliate $ 5.0 $ 5.0 Investment in subsidiaries* 11,721.3 10,280.9 Receivable from investment subsidiary* 1,466.1 1,121.9 Intercompany receivable* 578.6 443.3 Net deferred income taxes 67.1 97.1 Other assets 167.3 137.3 Total assets $ 14,005.4 $ 12,085.5 Liabilities and Shareholders’ Equity Accounts payable, accrued expenses, and other liabilities $ 292.6 $ 228.4 Dividend payable 655.1 395.4 Debt 3,269.2 3,020.9 Total liabilities 4,216.9 3,644.7 Redeemable noncontrolling interest (NCI) 503.7 483.7 Shareholders’ Equity Common shares, $1.00 par value (authorized 900.0; issued 797.5 including treasury shares of 215.8 and 217.6) 581.7 579.9 Paid-in capital 1,389.2 1,303.4 Retained earnings 6,031.7 5,140.4 Total accumulated other comprehensive income attributable to Progressive 1,282.2 933.4 Total shareholders’ equity 9,284.8 7,957.1 Total liabilities, redeemable NCI, and shareholders’ equity $ 14,005.4 $ 12,085.5 *Eliminated in consolidation. See notes to condensed financial statements. CONDENSED STATEMENTS OF CASH FLOWS THE PROGRESSIVE CORPORATION (PARENT COMPANY) (millions) Years Ended December 31, 2017 2016 2015 Cash Flows From Operating Activities: Net income attributable to Progressive $ 1,592.2 $ 1,031.0 $ 1,267.6 Adjustments to reconcile net income attributable to Progressive to net cash provided by operating activities: Undistributed (income) loss from subsidiaries (866.3 ) (741.9 ) (500.0 ) Amortization of equity-based compensation 2.1 2.2 2.4 (Gains) losses on extinguishment of debt (0.2 ) (1.6 ) 0.9 Changes in: Intercompany receivable (71.3 ) (37.3 ) 7.0 Accounts payable, accrued expenses, and other liabilities 53.6 24.2 (46.2 ) Income taxes 37.3 (5.0 ) 12.3 Other, net (22.6 ) (13.3 ) (3.1 ) Net cash provided by operating activities 724.8 258.3 740.9 Cash Flows From Investing Activities: Additional investments in equity securities of consolidated subsidiaries (86.7 ) (112.0 ) (40.2 ) Acquisition of an insurance company (18.7 ) 0 0 Acquisition of ARX 0 0 (890.1 ) (Paid to) received from investment subsidiary (344.2 ) 78.6 409.1 Net cash used in investing activities (449.6 ) (33.4 ) (521.2 ) Cash Flows From Financing Activities: Net proceeds from debt issuance 841.1 495.6 394.9 Reacquisitions of debt (594.4 ) (18.2 ) (19.3 ) Dividends paid to shareholders (395.4 ) (519.0 ) (403.6 ) Acquisition of treasury shares for restricted stock tax liabilities (57.6 ) (25.1 ) (30.6 ) Acquisition of treasury shares acquired in open market (4.9 ) (167.4 ) (177.9 ) Loan to ARX Holding Corp. 1 (64.0 ) 0 0 Tax benefit from vesting of equity-based compensation 0 9.2 16.8 Net cash used in financing activities (275.2 ) (224.9 ) (219.7 ) Change in cash, cash equivalents, and restricted cash 0 0 0 Cash, cash equivalents, restricted cash - Beginning of year 0 0 0 Cash, cash equivalents, restricted cash - End of year $ 0 $ 0 $ 0 1 Eliminated in consolidation. See Note 4 – Debt in our Annual Report. See notes to condensed financial statements. NOTES TO CONDENSED FINANCIAL STATEMENTS The accompanying condensed financial statements of The Progressive Corporation (parent company) should be read in conjunction with the consolidated financial statements and notes thereto in the Annual Report to Shareholders of The Progressive Corporation and its subsidiaries, which is included as Exhibit 13 to this Form 10-K. Note 1. Statements of Cash Flows — For the purpose of the Statements of Cash Flows, cash includes only bank demand deposits. The Progressive Corporation does not hold any cash but has unrestricted access to funds maintained in a non-insurance, investment subsidiary to meet its holding company obligations; at year-end 2017 and 2016 , $1.6 billion and $1.3 billion , respectively, of marketable securities were available in this subsidiary. Non-cash activity includes declared but unpaid dividends, the transfer of the previous 5% ownership interest in ARX to The Progressive Corporation from an investment subsidiary in 2015, and the change in redemption value of the redeemable NCI. For the years ended December 31, The Progressive Corporation paid the following: (millions) 2017 2016 2015 Income taxes $ 669.7 $ 450.2 $ 625.0 Interest 142.2 134.2 128.2 Note 2. Income Taxes — The Progressive Corporation files a consolidated federal income tax return with all eligible subsidiaries and acts as an agent for the consolidated tax group when making payments to the Internal Revenue Service. Since The Progressive Corporation owns less than 80% of ARX’s outstanding stock, ARX and its subsidiaries are not eligible to file on a consolidated basis with The Progressive Corporation. The Progressive Corporation consolidated group’s net income taxes currently payable/recoverable are included in other liabilities/assets, respectively, in the accompanying Condensed Balance Sheets based on the balance at the end of the year. The Progressive Corporation and its eligible subsidiaries have adopted, pursuant to a written agreement, a method of allocating consolidated federal income taxes. Amounts allocated to the eligible subsidiaries under the written agreement are included in “Intercompany Receivable” in the accompanying Condensed Balance Sheets. On December 22, 2017, legislation commonly known as the Tax Cuts and Jobs Act of 2017 (the “Tax Act”) was signed into law. One of the provisions of the Tax Act reduced the corporate federal income tax rate from 35% to 21% effective January 1, 2018. Pursuant to current accounting guidance, all deferred tax assets and liabilities were revalued to recognize the tax rate that is expected to apply when the tax effects are ultimately recognized in future periods. The impact of revaluing the deferred tax assets and liabilities from 35% to 21% was a net increase to The Progressive Corporation’s income tax expense of $44.7 million . Note 3. Debt — The information relating to debt is incorporated by reference from Note 4 – Debt in our Annual Report. Note 4. Employee Benefit Plans — The information relating to incentive compensation plans and deferred compensation is incorporated by reference from Note 9 – Employee Benefit Plans in our Annual Report. Note 5. Other Comprehensive Income — On the condensed Statements of Comprehensive Income, other comprehensive income represents activity of the subsidiaries of The Progressive Corporation and includes net unrealized gains (losses) on securities, net unrealized gains on forecasted transactions, and foreign currency translation adjustments. Note 6. Dividends — The information relating to our dividend policy is incorporated by reference from Note 14 – Dividends in our Annual Report.

SCHEDULE III - Supplementary In

SCHEDULE III - Supplementary Insurance Information	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
SCHEDULE III - Supplementary Insurance Information	SCHEDULE III — SUPPLEMENTARY INSURANCE INFORMATION THE PROGRESSIVE CORPORATION AND SUBSIDIARIES (millions) Segment Deferred 1 Future 1 Unearned premiums 1 Other 1 Premium revenue Net 1,2 Benefits, Amortization Other Net written Year ended December 31, 2017: Personal Lines $ 21,947.2 $ 16,141.4 $ 1,656.4 $ 2,954.8 $ 22,928.4 Commercial Lines 2,793.9 1,966.4 309.3 335.3 3,112.7 Property 988.8 700.2 159.2 190.4 1,091.0 Other indemnity 0 0 0 0.2 0 Total $ 780.5 $ 13,086.9 $ 8,903.5 $ 0 $ 25,729.9 $ 539.2 $ 18,808.0 $ 2,124.9 $ 3,480.7 $ 27,132.1 Year ended December 31, 2016: Personal Lines $ 19,188.2 $ 14,591.1 $ 1,446.6 $ 2,549.2 $ 19,819.5 Commercial Lines 2,421.3 1,741.0 266.7 285.4 2,598.3 Property 864.5 546.1 150.5 137.2 935.7 Other indemnity 0 1.4 0 0.2 0 Total $ 651.2 $ 11,368.0 $ 7,468.3 $ 0 $ 22,474.0 $ 456.5 $ 16,879.6 $ 1,863.8 $ 2,972.0 $ 23,353.5 Year ended December 31, 2015: Personal Lines $ 17,294.5 $ 12,748.7 $ 1,331.3 $ 2,379.9 $ 17,703.6 Commercial Lines 1,995.9 1,244.5 219.4 232.6 2,171.2 Property 609.1 349.0 101.1 98.8 689.6 Other indemnity (0.4 ) (0.2 ) 0 0.8 (0.4 ) Total $ 564.1 $ 10,039.0 $ 6,621.8 $ 0 $ 19,899.1 $ 431.8 $ 14,342.0 $ 1,651.8 $ 2,712.1 $ 20,564.0 1 Progressive does not allocate assets, liabilities, or investment income to operating segments. 2 Excludes total net realized gains (losses) on securities.

SCHEDULE IV - Reinsurance

SCHEDULE IV - Reinsurance	12 Months Ended
	Dec. 31, 2017
Text Block [Abstract]
SCHEDULE IV - Reinsurance	SCHEDULE IV — REINSURANCE THE PROGRESSIVE CORPORATION AND SUBSIDIARIES (millions) Year Ended: Gross Amount Ceded to Assumed Net Amount Percentage December 31, 2017 Premiums earned: Property and liability insurance $ 26,425.7 $ 695.8 $ 0 $ 25,729.9 0 % December 31, 2016 Premiums earned: Property and liability insurance $ 23,111.2 $ 637.2 $ 0 $ 22,474.0 0 % December 31, 2015 Premiums earned: Property and liability insurance $ 20,454.1 $ 555.0 $ 0 $ 19,899.1 0 %

Reporting and Accounting Polici

Reporting and Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Reporting and Accounting Policies	REPORTING AND ACCOUNTING POLICIES Nature of Operations The Progressive insurance organization began business in 1937. The Progressive Corporation, an insurance holding company, was formed in 1965. The financial results of The Progressive Corporation include its subsidiaries and affiliates (references to “subsidiaries” in these notes include affiliates as well). Our insurance subsidiaries (collectively the Progressive Group of Insurance Companies) provide personal and commercial auto insurance, residential property insurance, and other specialty property-casualty insurance and related services. Our Personal Lines segment writes insurance for personal autos and recreational vehicles, which we refer to as our special lines products, through both an independent insurance agency channel and a direct channel. Our Commercial Lines segment writes primary liability and physical damage insurance for automobiles and trucks owned and/or operated predominantly by small businesses through both the independent agency and direct channels. Our Property segment writes residential property insurance for homeowners, other property owners, and renters, primarily through the independent insurance agency channel. We operate our businesses throughout the United States. Basis of Consolidation and Reporting The accompanying consolidated financial statements include the accounts of The Progressive Corporation and ARX Holding Corp. (ARX), and their respective wholly owned insurance and non-insurance subsidiaries and affiliates, in which Progressive or ARX has a controlling financial interest. The Progressive Corporation owned 69.0% of the outstanding capital stock of ARX at December 31, 2017 and 69.2% at December 31, 2016 and 2015. The decrease reflects ARX employee stock options that were exercised during 2017. All intercompany accounts and transactions are eliminated in consolidation. Estimates We are required to make estimates and assumptions when preparing our financial statements and accompanying notes in conformity with accounting principles generally accepted in the United States of America (GAAP). As estimates develop into fact (e.g., losses are paid), results may, and will likely, differ from those estimates. Investments Our fixed-maturity securities, equity securities, and short-term investments are accounted for on an available-for-sale basis. See Note 2 – Investments for details regarding the composition of our investment portfolio. Fixed-maturity securities include debt securities and redeemable preferred stocks, which may have fixed or variable principal payment schedules, may be held for indefinite periods of time, and may be used as a part of our asset/liability strategy or sold in response to changes in interest rates, anticipated prepayments, risk/reward characteristics, liquidity needs, or other economic factors. These securities are carried at fair value with the corresponding unrealized gains (losses), net of deferred income taxes, reported in accumulated other comprehensive income. Fair values are obtained from recognized pricing services or are quoted by market makers and dealers, with limited exceptions discussed in Note 3 – Fair Value . Included in the fixed-maturity portfolio are asset-backed securities. The asset-backed securities are generally accounted for under the retrospective method. The retrospective method recalculates yield assumptions (based on changes in interest rates or cash flow expectations) historically to the inception of the investment holding period, and applies the required adjustment, if any, to the cost basis, with the offset recorded to investment income. The prospective method is used primarily for interest-only securities, non-investment-grade asset-backed securities, and certain asset-backed securities with sub-prime loan exposure or where there is a greater risk of non-performance and where it is possible the initial investment may not be substantially recovered. The prospective method requires a calculation of expected future repayments and resets the yield to allow for future period adjustments; no current period impact to investment income or the security’s cost is made based on the cash flow update. Prepayment assumptions are updated quarterly. Equity securities include common stocks, nonredeemable preferred stocks, and other risk investments, and are reported at fair values. Changes in fair value of these securities, net of deferred income taxes, are reflected as unrealized gains (losses) in accumulated other comprehensive income. To the extent we hold any foreign equities or foreign currency hedges, any change in value due to exchange rate fluctuations would be limited by foreign currency hedges, if any, and would be recognized in income in the current period. Short-term investments may include Eurodollar deposits, commercial paper, repurchase transactions, and other securities expected to mature within one year. From time to time, we may also invest in municipal bonds that have maturity dates that are longer than one year, but have either liquidity facilities or mandatory put features within one year. Trading securities are securities bought principally for the purpose of sale in the near term. We do not hold any trading securities. To the extent we have trading securities, changes in fair value would be recognized in income in the current period. Derivative instruments, which may be used for trading purposes or classified as trading derivatives due to the characteristics of the transaction, are discussed below. Derivative instruments may include futures, options, forward positions, foreign currency forwards, interest rate swap agreements, and credit default swaps and may be used in the portfolio for general investment purposes or to hedge the exposure to: • Changes in fair value of an asset or liability (fair value hedge), • Foreign currency of an investment in a foreign operation (foreign currency hedge), or • Variable cash flows of a forecasted transaction (cash flow hedge). We did not have any derivatives outstanding at December 31, 2017 and 2016. To the extent we have derivatives held for general investment purposes, these derivative instruments are recognized as either assets or liabilities and measured at fair value, with changes in fair value recognized in income as a component of net realized gains (losses) on securities during the period of change. Derivatives designated as hedges are required to be evaluated on established criteria to determine the effectiveness of their correlation to, and ability to reduce the designated risk of, specific securities or transactions. Effectiveness is required to be reassessed regularly. Hedges that are deemed to be effective would be accounted for as follows: • Fair value hedge: changes in fair value of the hedge, as well as the hedged item, would be recognized in income in the period of change while the hedge is in effect. • Foreign currency hedge: changes in fair value of the hedge, as well as the hedged item, would be reflected as a change in translation adjustment as part of accumulated other comprehensive income. Gains and losses on the foreign currency hedge would offset the foreign exchange gains and losses on the foreign investment as they are recognized into income. • Cash flow hedge: changes in fair value of the hedge would be reported as a component of accumulated other comprehensive income and subsequently amortized into earnings over the life of the hedged transaction. If a hedge is deemed to become ineffective or discontinued, the following accounting treatment would be applied: • Fair value hedge: the derivative instrument would continue to be adjusted through income, while the adjustment in the change in value of the hedged item would be reflected as a change in unrealized gains (losses) as part of accumulated other comprehensive income. • Foreign currency hedge: changes in the value of the hedged item would continue to be reflected as a change in translation adjustment as part of accumulated other comprehensive income, but the derivative instrument would be adjusted through income for the current period. • Cash flow hedge: changes in fair value of the derivative instrument would be reported in income for the current period. For all derivative positions, net cash requirements are limited to changes in fair values, which may vary as a result of changes in interest rates, currency exchange rates, and other factors. Exposure to credit risk is limited to the carrying value; collateral may be required to limit credit risk. We have elected not to offset fair value amounts that arise from derivative positions with the same counterparty under a master netting arrangement. Investment securities are exposed to various risks such as interest rate, market, credit, and liquidity risk. Fair values of securities fluctuate based on the nature and magnitude of changing market conditions; significant changes in market conditions could materially affect the portfolio’s value in the near term. We regularly monitor our portfolio for price changes, which might indicate potential impairments, and perform detailed reviews of securities with unrealized losses. In such cases, changes in fair value are evaluated to determine the extent to which such changes are attributable to: (i) fundamental factors specific to the issuer, such as financial condition, business prospects, or other factors, (ii) market-related factors, such as interest rates or equity market declines, or (iii) credit-related losses, where the present value of cash flows expected to be collected are lower than the amortized cost basis of the security. We analyze our debt securities that are in a loss position to determine if we intend to sell, or if it is more likely than not that we will be required to sell, the security prior to recovery and, if so, we write down the security to its current fair value, with the entire amount of the write-down recorded to earnings. To the extent that it is more likely than not that we will hold the debt security until recovery (which could be maturity), we determine if any of the decline in value is due to a credit loss (i.e., where the present value of future cash flows expected to be collected is lower than the amortized cost basis of the security) and, if so, we recognize that portion of the impairment as a component of net realized gains (losses) in the comprehensive income statement, with the difference (i.e., non-credit related impairment) recognized as part of our net unrealized gains (losses) in accumulated other comprehensive income. When an equity security (common equity and nonredeemable preferred stock) in our investment portfolio has an unrealized loss in fair value that is deemed to be other-than-temporary, we reduce the book value of such security to its current fair value, recognizing the decline as a realized loss in the comprehensive income statement. Any future changes in fair value, either increases or decreases, are reflected as changes in unrealized gains (losses) as part of accumulated other comprehensive income. Investment income consists of interest, dividends, and accretion net of amortization. In addition to the discussion above for asset-backed securities, interest is recognized on an accrual basis using the effective yield method. Depending on the nature of the equity instruments, dividends are recorded at either the ex-dividend date or on an accrual basis. Realized gains (losses) on securities are computed based on the first-in first-out method and include write-downs on available-for-sale securities considered to have other-than-temporary declines in fair value (excluding non-credit related impairments), as well as holding period valuation changes on derivatives, trading securities, and hybrid instruments (e.g., securities with embedded options, where the option is a feature of the overall change in the value of the instrument). Insurance Premiums and Receivables Insurance premiums written are earned into income on a pro rata basis over the period of risk, based on a daily earnings convention. Accordingly, unearned premiums represent the portion of premiums written that are applicable to the unexpired risk. We provide insurance and related services to individuals and small commercial accounts and offer a variety of payment plans. Generally, premiums are collected prior to providing risk coverage, minimizing our exposure to credit risk. For our vehicle businesses, we perform a policy level evaluation to determine the extent to which the premiums receivable balance exceeds the unearned premiums balance. We then age this exposure to establish an allowance for doubtful accounts based on prior experience. For our Property business, we do not establish an allowance for doubtful accounts since the risk of uncollectibility is relatively low. If premiums are unpaid by the policy due date, we provide advance notice of cancellation in accordance with each state’s requirements and, if the premiums remain unpaid after receipt of notice, cancel the policy and write off any remaining balance. Deferred Acquisition Costs Deferred acquisition costs include commissions, premium taxes, and other variable underwriting and direct sales costs incurred in connection with the successful acquisition or renewal of insurance contracts. These acquisition costs, net of ceding allowances, are deferred and amortized over the policy period in which the related premiums are earned. We consider anticipated investment income in determining the recoverability of these costs. Management believes that these costs will be fully recoverable in the near term. We do not defer any advertising costs. Total advertising costs, which are expensed as incurred, for the years ended December 31, were: (millions) Advertising Costs 2017 $ 1,005.4 2016 756.2 2015 748.3 Loss and Loss Adjustment Expense Reserves Loss reserves represent the estimated liability on claims reported to us, plus reserves for losses incurred but not recorded (IBNR). These estimates are reported net of amounts estimated to be recoverable from salvage and subrogation. Loss adjustment expense reserves represent the estimated expenses required to settle these claims and losses. The methods of making estimates and establishing these reserves are reviewed regularly, and resulting adjustments are reflected in income in the current period. Such loss and loss adjustment expense reserves are susceptible to change in the near term. Reinsurance Our reinsurance transactions include premiums ceded to “Regulated” plans and “Non-Regulated” plans. The Regulated plans in which we participate are governed by insurance regulations and include state-provided reinsurance facilities (Michigan Catastrophic Claims Association, Florida Hurricane Catastrophe Fund, North Carolina Reinsurance Facility), as well as state-mandated involuntary plans for commercial vehicles (Commercial Automobile Insurance Procedures/Plans – “CAIP”) and federally regulated plans for flood (National Flood Insurance Program – “NFIP”); we act as a servicing agent for CAIP and as a participant in the “Write Your Own” program for the NFIP. The Non-Regulated plans are voluntary contractual arrangements and primarily relate to our Property business and transportation network company business written by our Commercial Lines segment. Prepaid reinsurance premiums are earned on a pro rata basis over the period of risk, based on a daily earnings convention, which is consistent with premiums written. See Note 7 – Reinsurance for further discussion. Income Taxes The income tax provision is calculated under the balance sheet approach. Deferred tax assets and liabilities are recorded based on the difference between the financial statement and tax bases of assets and liabilities at the enacted tax rates. The principal items giving rise to such differences are investment securities (e.g., net unrealized gains (losses), write-downs on securities determined to be other-than-temporarily impaired), loss and loss adjustment expense reserves, unearned premiums reserves, deferred acquisition costs, property and equipment, intangible assets, and non-deductible accruals. We review our deferred tax assets regularly for recoverability. The effects of any changes in the tax rate are recorded to our provision for income taxes, including any changes on items initially recognized in accumulated other comprehensive income. See Note 5 – Income Taxes for further discussion. Property and Equipment Property and equipment are recorded at cost, less accumulated depreciation, and include capitalized software developed or acquired for internal use. Depreciation is recognized over the estimated useful lives of the assets using accelerated methods for computer equipment and laptops and the straight-line method for all other fixed assets. We evaluate impairment whenever events or circumstances warrant such a review and write-off the impaired assets if appropriate. Land and buildings comprised 66% and 65% of total property and equipment at December 31, 2017 and 2016 , respectively. The useful lives for property and equipment at December 31, 2017 , were: Useful Lives Computer equipment and laptops 3 years Software licenses (internal use) 1-5 years Capitalized software 3-10 years Buildings, improvements, and integrated components 7-40 years All other property and equipment 3-15 years At December 31, 2017 and 2016, included in other assets in the consolidated balance sheets is $5.3 million and $8.7 million , respectively, of “held for sale” property, which represents the fair value of this property less the estimated costs to sell. Total capitalized interest, which primarily relates to capitalized software projects, for the years ended December 31, was: (millions) Capitalized Interest 2017 $ 2.8 2016 2.9 2015 2.4 Goodwill and Intangible Assets Goodwill is the excess of the purchase price over the estimated fair value of the assets and liabilities acquired and represents the future economic benefits arising from other assets acquired that could not be individually identified and separately recognized. Substantially all of the goodwill recorded as of December 31, 2017 and 2016, relates to the April 1, 2015 acquisition of a controlling interest in ARX. Intangible assets primarily arose through the acquisition of ARX and mainly represent the future premiums that will be recognized from the policies and agency relationships that existed at the acquisition date. The majority of the intangible assets have finite lives, which, at December 31, 2017, had a remaining life range from 2 to 11 years. See Note 16 – Goodwill and Intangible Assets for further discussion. We evaluate our goodwill for impairment at least annually using a qualitative approach. If events or changes in circumstances indicate that the carrying value of goodwill or intangible assets may not be recoverable, we will evaluate such items for impairment using a quantitative approach. Guaranty Fund Assessments We are subject to state guaranty fund assessments, which provide for the payment of covered claims or other insurance obligations of insurance companies deemed insolvent. These assessments are accrued after a formal determination of insolvency has occurred, and we have written the premiums on which the assessments will be based. Assessments that are available for recoupment from policyholders are capitalized when incurred; all other assessments are expensed. Fees and Other Revenues Fees and other revenues primarily represent fees collected from policyholders relating to installment charges in accordance with our bill plans, as well as late payment and insufficient funds fees. Other revenues may include revenue from ceding commissions in excess of acquisition costs, the sale of tax credits, referral fees, rental income, and other revenue transactions. Fees and other revenues are generally earned when collected, except for excess ceding commissions, which are earned over the policy period. Service Revenues and Expenses Our service businesses provide insurance-related services. Service revenues and expenses from our commission-based businesses are recorded in the period in which they are earned or incurred. Service revenues generated from processing business for involuntary CAIP plans are earned on a pro rata basis over the term of the related policies. Service expenses related to these CAIP plans are expensed as incurred. Equity-Based Compensation We issue time-based and performance-based restricted stock unit awards to key members of management (including members of ARX and its subsidiaries in 2017) as our form of equity compensation, and time-based restricted stock awards to non-employee directors. Collectively, we refer to these awards as “restricted equity awards.” Compensation expense for time-based restricted equity awards with installment vesting is recognized over each respective vesting period. For performance-based restricted equity awards, compensation expense is recognized over the respective estimated vesting periods. Dividend equivalent units are credited to outstanding restricted stock unit awards, both time-based and performance-based, at the time a dividend is paid to shareholders. We record an estimate for expected forfeitures of restricted equity awards based on our historical forfeiture rates. In addition, we shorten the vesting periods of certain time-based restricted equity awards based on the “qualified retirement” provisions in our equity compensation plans, under which (among other provisions) if the participant satisfies certain age and years-of-service requirements, the vesting and distribution of 50% of outstanding time-based restricted equity awards accelerates upon reaching eligibility for a qualified retirement and shortly after the grant date for each subsequent award. ARX has nonqualified and incentive stock options outstanding that were issued prior to April 2015 as a form of equity compensation to certain of the officers and employees of ARX and its subsidiaries. These outstanding stock options are subject to the put/call features contained in the current stockholders’ agreement, pursuant to which The Progressive Corporation has the right, and can be required, to purchase a portion or all the shares underlying these awards in 2018 and 2021. The vested stock options, and the shares issuable upon exercise of the stock options, are also subject to repurchase by ARX if the holder’s employment terminates. See Note 15 – Redeemable Noncontrolling Interest for further discussion . These stock options, which are treated for accounting purposes as liability awards, are expensed over the respective vesting periods based on the Black-Scholes value determined at period end. The total compensation expense recognized for equity-based compensation, both our equity and liability awards, for the years ended December 31, was: (millions) 2017 2016 2015 Pretax expense $ 95.4 $ 85.2 $ 66.2 Tax benefit 1 33.4 29.8 23.2 1 Reflected at the 35% corporate federal tax rate; the revaluation to the 21% rate is reflected in the total revaluation adjustment recorded at December 31, 2017 (see N ote 5 – Income Ta xes for further discussion). Earnings Per Share Net income attributable to Progressive is used in our calculation of the per share amounts. Basic earnings per share is computed using the weighted average number of common shares outstanding during the reporting period, excluding unvested time-based and performance-based restricted equity awards that are subject to forfeiture. Diluted earnings per share includes common stock equivalents assumed outstanding during the period. Our common stock equivalents include the incremental shares assumed to be issued for: • earned but unvested time-based restricted equity awards, and • certain unvested performance-based restricted equity awards that satisfied contingency conditions for common stock equivalents during the period. Supplemental Cash Flow Information Cash and cash equivalents include bank demand deposits and daily overnight reverse repurchase commitments of funds held in bank demand deposit accounts on ARX’s subsidiaries, which are primarily collateralized by U.S. Treasury notes. The amount of reverse repurchase commitments held by ARX’s subsidiaries at December 31, 2017, 2016, and 2015, were $247.2 million , $150.0 million , and $174.8 million , respectively. Restricted cash on our consolidated balance sheets at December 31, 2017 and 2016, represents cash that is restricted to pay flood claims under the National Flood Insurance Program’s “Write Your Own” program, for which American Strategic Insurance and other subsidiaries of ARX (ASI) are administrators. Non-cash activity includes declared but unpaid dividends. The cash transferred in the exchange transaction, which occurred in June 2016, was revised to correct the reclassification of a non-cash transaction; there was no overall impact on the increase in cash, cash equivalents, and restricted cash that was reported in our consolidated statement of cash flows for the year ended December 31, 2016. See Note 16 – Goodwill and Intangible Assets for further discussion of the exchange transaction. For the years ended December 31, we paid the following: (millions) 2017 2016 2015 Income taxes $ 715.6 $ 459.4 $ 701.8 Interest 146.3 139.2 132.0 New Accounting Standards Issued In January 2018, the Financial Accounting Standards Board (FASB) proposed an Accounting Standards Update (ASU), which would provide targeted improvements to the new lease accounting guidance issued by the FASB in February 2016 (the “2016 ASU”). The 2016 ASU, which eliminates the off-balance-sheet accounting for leases, will require lessees to report their operating leases as both an asset and liability on the statement of financial position and to disclose key information about leasing arrangements in the financial statement footnotes. Under the 2016 ASU, there will be no change to the recognition of lease expense in our results of operations. The ASU will be effective for fiscal years (including interim periods within those fiscal years) beginning after December 15, 2018 (2019 for calendar-year companies). Under the proposed guidance, companies would have the option to apply the new lease requirements either as of the effective date (i.e., January 1, 2019), with comparative information presented in accordance with the previous standard, or on a modified restrospective basis, which would restate all financial statement information as of the beginning of the earliest period presented and is the transition method under the 2016 ASU. Based on our lease portfolio at December 31, 2017, and in accordance with the accounting elections available in the ASU, we would have recorded an increase to assets and liabilities of approximately $140 million , and there would have been no impact on our results of operations or cash flows. Therefore, we do not expect this standard to have a material impact on our financial condition. In March 2017, the FASB issued an ASU related to premium amortization on purchased callable debt securities. The intent of the standard is to shorten the amortization period for certain purchased callable debt securities held at a premium. Under the ASU, the premium is required to be amortized to the earliest call date. The ASU more closely aligns interest income recorded on bonds held at a premium with the economics of the underlying instrument. The ASU, which is required to be applied on a modified retrospective basis, is effective for fiscal years beginning after December 15, 2018 (2019 for calendar-year companies), and interim periods within those fiscal years. Since we have historically used a yield-to-worst scenario for our securities that were purchased at a premium, and the first call on a premium security most often produces the lowest and most conservative yield, we do not expect this standard to have a significant impact on our financial condition, cash flows, or results of operations. In January 2017, the FASB issued an ASU, which eliminates the requirement to determine the implied value of goodwill in measuring an impairment loss. Upon adoption, the measurement of a goodwill impairment will represent the excess of the reporting unit’s carrying value over fair value, limited to the carrying value of goodwill. This ASU is effective for goodwill impairment tests in fiscal years beginning after December 15, 2019 (2020 for calendar-year companies), with early adoption permitted. We do not expect this standard to have a material impact on our financial position or results of operations. In June 2016, the FASB issued an ASU intended to improve the timing, and enhance the accounting and disclosure, of credit losses on financial assets. Additionally, this update will modify the existing accounting guidance related to the impairment evaluation for available-for-sale debt securities and will result in the creation of an allowance for credit losses as a contra asset account. The ASU will require cumulative-effect changes to retained earnings in the period of adoption, if any occur, and will also require prospective changes on previously recorded impairments. This ASU is effective for fiscal years (including interim periods within those fiscal years) beginning after December 15, 2019 (2020 for calendar-year companies), with early adoption permissible (including interim periods within that fiscal year) beginning after December 15, 2018 (2019 for calendar-year companies). While the ASU creates additional accounting complexities related to the recognition of the impairment losses, and subsequent recoveries, through an allowance for credit losses account, we do not expect that the ASU will have a material impact on our current method of evaluating securities for credit losses or the timing or recognition of the amounts of the impairment losses. In January 2016, the FASB released an ASU intended to improve the recognition and measurement of financial instruments. The new guidance will require the changes in fair value of equity securities to be recognized as a component of net income. The ASU is effective for fiscal years beginning after December 15, 2017 (2018 for calendar-year companies) and requires the prospective method of adoption with a cumulative-effect adjustment recorded to beginning retained earnings upon adoption. In January 2018, we recorded a cumulative-effect adjustment of $1.3 billion , which is net of taxes at the 35% tax rate. The cumulative-effect adjustment represents the amount of after-tax net unrealized gains on equity securities that was recorded as part of accumulated other comprehensive income at December 31, 2017. This ASU will have no impact on comprehensive income. Adopted For the year ended December 31, 2017, we adopted the ASU related to the statement of cash flows and the classification and presentation of changes in restricted cash. This update requires that the statement of cash flows explain the change during the period in the total of cash, cash equivalents, and amounts described as restricted cash. This ASU, which is required to be applied on a retrospective basis, is effective for fiscal years beginning after December 15, 2017, with early adoption permitted. We elected to early adopt this ASU. Since this standard only affected classification and presentation, there was no impact on our results of operations, financial condition, or cash flows. On January 1, 2017, we adopted the ASU to simplify the accounting for employee share-based payment transactions. There were several provisions that could be adopted under this ASU. We did not elect to make any changes to our method of recording forfeitures and are continuing to withhold taxes at the minimum statutory tax rate. We did elect, on a retrospective basis, to disclose the payment of cash to a taxing authority for which we withheld shares for this purpose as a financing activity. Lastly, during the year ended December 31, 2017, we recognized $25.1 million of excess tax benefits as an income tax benefit in our consolidated statements of comprehensive income; this provision was adopted on a prospective basis.

Investments

Investments	12 Months Ended
	Dec. 31, 2017
Investments Schedule [Abstract]
Investments	INVESTMENTS Our securities are reported at fair value, with the changes in fair value of these securities (other than hybrid securities and derivative instruments) reported as a component of accumulated other comprehensive income, net of deferred income taxes. The changes in fair value of the hybrid securities and derivative instruments are recorded as a component of net realized gains (losses) on securities. The following tables present the composition of our investment portfolio by major security type, consistent with our internal classification of how we manage, monitor, and measure the portfolio. The net holding period gains (losses) represent the amounts realized on our hybrid securities only (see discussion below). ($ in millions) Cost Gross Unrealized Gains Gross Unrealized Losses Net Realized Gains (Losses) Fair Value % of Total Fair Value December 31, 2017 Fixed maturities: U.S. government obligations $ 6,688.8 $ 1.1 $ (44.0 ) $ 0 $ 6,645.9 24.4 % State and local government obligations 2,285.6 20.7 (9.3 ) 0.1 2,297.1 8.4 Foreign government obligations 0 0 0 0 0 0 Corporate debt securities 4,997.2 14.8 (14.4 ) 0.1 4,997.7 18.3 Residential mortgage-backed securities 828.8 11.3 (3.4 ) 0 836.7 3.1 Commercial mortgage-backed securities 2,760.1 11.8 (13.3 ) 0 2,758.6 10.1 Other asset-backed securities 2,454.5 4.5 (4.5 ) 0.2 2,454.7 9.0 Redeemable preferred stocks 194.9 17.8 (1.5 ) (0.2 ) 211.0 0.8 Total fixed maturities 20,209.9 82.0 (90.4 ) 0.2 20,201.7 74.1 Equity securities: Nonredeemable preferred stocks 698.6 114.0 (8.8 ) 0 803.8 2.9 Common equities 1,499.0 1,901.0 (0.2 ) 0 3,399.8 12.5 Short-term investments 2,869.4 0 0 0 2,869.4 10.5 Total portfolio 1,2 $ 25,276.9 $ 2,097.0 $ (99.4 ) $ 0.2 $ 27,274.7 100.0 % ($ in millions) Cost Gross Unrealized Gains Gross Unrealized Losses Net Realized Gains (Losses) Fair Value % of Total Fair Value December 31, 2016 Fixed maturities: U.S. government obligations $ 2,899.2 $ 0 $ (29.1 ) $ 0 $ 2,870.1 12.2 % State and local government obligations 2,509.5 13.8 (20.7 ) 0 2,502.6 10.7 Foreign government obligations 24.5 0 0 0 24.5 0.1 Corporate debt securities 4,557.8 17.3 (24.3 ) 0.1 4,550.9 19.4 Residential mortgage-backed securities 1,489.7 23.7 (15.6 ) 1.5 1,499.3 6.4 Commercial mortgage-backed securities 2,266.9 12.0 (25.5 ) 0 2,253.4 9.6 Other asset-backed securities 2,350.7 4.6 (4.4 ) 0.2 2,351.1 10.0 Redeemable preferred stocks 188.8 5.1 (2.0 ) 0 191.9 0.8 Total fixed maturities 16,287.1 76.5 (121.6 ) 1.8 16,243.8 69.2 Equity securities: Nonredeemable preferred stocks 734.2 135.4 (16.1 ) 0 853.5 3.6 Common equities 1,437.5 1,377.0 (2.1 ) 0 2,812.4 12.0 Short-term investments 3,572.9 0 0 0 3,572.9 15.2 Total portfolio 1,2 $ 22,031.7 $ 1,588.9 $ (139.8 ) $ 1.8 $ 23,482.6 100.0 % 1 Our portfolio reflects the effect of unsettled security transactions and collateral on any open derivative positions; at December 31, 2017 , $5.8 million was included in “other assets,” compared to $27.8 million in “other liabilities” at December 31, 2016 . 2 The total fair value of the portfolio at December 31, 2017 and 2016 included $1.6 billion and $1.3 billion , respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company, net of any unsettled security transactions. The increase in fixed-maturity securities, primarily U.S. government obligations, and decrease in short-term investments since December 31, 2016, was due to a decision to slightly lengthen the average maturity of the portfolio late in the fourth quarter 2017 in response to the rising interest rate environment. At December 31, 2017 , bonds and certificates of deposit in the principal amount of $222.6 million were on deposit to meet state insurance regulatory and/or rating agency requirements. We did not hold any securities of any one issuer, excluding U.S. government obligations, with an aggregate cost or fair value exceeding 10% of total shareholders’ equity at December 31, 2017 or 2016 . At December 31, 2017 , we did not hold any debt securities that were non-income producing during the preceding 12 months. Short-Term Investments Our short-term investments may include commercial paper and other investments that are expected to mature or are redeemable within one year. We did not hold any treasury bills issued by the Australian government at December 31, 2017 or 2016 . We did not have any open repurchase or reverse repurchase transactions in our short-term investment portfolio at December 31, 2017 or 2016 . To the extent we had any repurchase and reverse repurchase transactions with the same counterparty and subject to an enforceable master netting arrangement, we could elect to offset these transactions. Consistent with past practice, we have elected not to offset these transactions and, therefore, report these transactions on a gross basis on our balance sheets. Hybrid Securities Included in our fixed maturities are hybrid securities, which are reported at fair value at December 31 : (millions) 2017 2016 State and local government obligations $ 6.1 $ 0 Corporate debt securities 99.8 40.1 Residential mortgage-backed securities 0 170.5 Other asset-backed securities 6.7 8.9 Redeemable preferred stocks 30.3 0 Total hybrid securities $ 142.9 $ 219.5 Certain securities in our portfolio are accounted for as hybrid securities because they contain embedded derivatives that are not deemed to be clearly and closely related to the host investments. Since the embedded derivative does not have an observable intrinsic value (e.g., change-in-control put options, debt-to-equity conversion, or any other feature unrelated to the credit quality or risk of default of the issuer that could impact the amount or timing of our expected future cash flows), we have elected to record the change in fair value of the entire security through income as a realized gain or loss. Fixed Maturities The composition of fixed maturities by maturity at December 31, 2017 , was: (millions) Cost Fair Value Less than one year $ 3,964.1 $ 3,980.0 One to five years 12,706.5 12,671.3 Five to ten years 3,294.4 3,306.9 Ten years or greater 244.9 243.5 Total $ 20,209.9 $ 20,201.7 Asset-backed securities are classified in the maturity distribution table based upon their projected cash flows. All other securities that do not have a single maturity date are reported based upon expected average maturity. Contractual maturities may differ from expected maturities because the issuers of the securities may have the right to call or prepay obligations. Gross Unrealized Losses As of December 31, 2017 , we had $99.2 million of gross unrealized losses in our fixed-income securities (i.e., fixed-maturity securities and nonredeemable preferred stocks) and $0.2 million in our common equities. We currently do not intend to sell the fixed-income securities and determined that it is more likely that we will not be required to sell these securities for the period of time necessary to recover their cost bases. A review of our fixed-income securities indicated that the issuers were current with respect to their interest obligations and that there was no evidence of deterioration of the current cash flow projections that would indicate we would not receive the remaining principal at maturity. For common equities, 96% of our common stock portfolio was indexed to the Russell 1000; as such, this portfolio may contain securities in a loss position for an extended period of time, subject to possible write-downs, as described below. We may retain these securities as long as the portfolio and index correlation remain similar. To the extent there is issuer-specific deterioration, we may write-down the securities of that issuer. The remaining 4% of our common stocks were part of a managed equity strategy selected and administered by an external investment advisor. If our review of loss position securities were to indicate there was a fundamental, or market, impairment on these securities that was determined to be other-than-temporary, we would recognize a write-down in accordance with our stated policy. The following tables show the composition of gross unrealized losses by major security type and by the length of time that individual securities have been in a continuous unrealized loss position: Total No. of Sec. Total Value Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Value Unrealized Losses No. of Sec. Fair Value Unrealized Losses December 31, 2017 Fixed maturities: U.S. government obligations 58 $ 5,817.0 $ (44.0 ) 41 $ 4,869.3 $ (34.6 ) 17 $ 947.7 $ (9.4 ) State and local government obligations 358 1,200.3 (9.3 ) 230 737.6 (4.4 ) 128 462.7 (4.9 ) Corporate debt securities 222 2,979.4 (14.4 ) 171 2,072.9 (9.1 ) 51 906.5 (5.3 ) Residential mortgage-backed securities 201 300.9 (3.4 ) 30 75.1 (0.2 ) 171 225.8 (3.2 ) Commercial mortgage-backed securities 105 1,682.3 (13.3 ) 63 1,221.2 (5.9 ) 42 461.1 (7.4 ) Other asset-backed securities 197 1,837.3 (4.5 ) 134 1,377.8 (3.3 ) 63 459.5 (1.2 ) Redeemable preferred stocks 2 21.8 (1.5 ) 1 10.8 (0.1 ) 1 11.0 (1.4 ) Total fixed maturities 1,143 13,839.0 (90.4 ) 670 10,364.7 (57.6 ) 473 3,474.3 (32.8 ) Equity securities: Nonredeemable preferred stocks 4 127.8 (8.8 ) 1 56.5 (0.5 ) 3 71.3 (8.3 ) Common equities 19 13.4 (0.2 ) 18 13.4 (0.2 ) 1 0 0 Total equity securities 23 141.2 (9.0 ) 19 69.9 (0.7 ) 4 71.3 (8.3 ) Total portfolio 1,166 $ 13,980.2 $ (99.4 ) 689 $ 10,434.6 $ (58.3 ) 477 $ 3,545.6 $ (41.1 ) Total No. of Sec. Total Fair Value Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Value Unrealized Losses No. of Sec. Fair Value Unrealized Losses December 31, 2016 Fixed maturities: U.S. government obligations 30 $ 2,774.0 $ (29.1 ) 30 $ 2,774.0 $ (29.1 ) 0 $ 0 $ 0 State and local government obligations 618 1,497.9 (20.7 ) 584 1,404.3 (19.6 ) 34 93.6 (1.1 ) Corporate debt securities 184 2,615.1 (24.3 ) 175 2,559.9 (24.0 ) 9 55.2 (0.3 ) Residential mortgage-backed securities 233 953.7 (15.6 ) 117 209.7 (1.7 ) 116 744.0 (13.9 ) Commercial mortgage-backed securities 111 1,347.3 (25.5 ) 85 1,061.2 (22.9 ) 26 286.1 (2.6 ) Other asset-backed securities 103 1,605.2 (4.4 ) 89 1,423.3 (3.9 ) 14 181.9 (0.5 ) Redeemable preferred stocks 2 31.0 (2.0 ) 0 0 0 2 31.0 (2.0 ) Total fixed maturities 1,281 10,824.2 (121.6 ) 1,080 9,432.4 (101.2 ) 201 1,391.8 (20.4 ) Equity securities: Nonredeemable preferred stocks 13 329.6 (16.1 ) 8 175.2 (3.8 ) 5 154.4 (12.3 ) Common equities 75 22.1 (2.1 ) 69 19.7 (1.7 ) 6 2.4 (0.4 ) Total equity securities 88 351.7 (18.2 ) 77 194.9 (5.5 ) 11 156.8 (12.7 ) Total portfolio 1,369 $ 11,175.9 $ (139.8 ) 1,157 $ 9,627.3 $ (106.7 ) 212 $ 1,548.6 $ (33.1 ) During 2017 , the number of securities in our fixed-maturity portfolio with unrealized losses decreased, primarily the result of a narrowing of credit spreads during the year. We had no material decreases in valuation as a result of credit rating downgrades during the year. All of the fixed-maturity securities in an unrealized loss position at December 31, 2017 in the table above are current with respect to required principal and interest payments. Since December 31, 2016 , our nonredeemable preferred stocks with unrealized losses decreased to four securities, averaging approximately 6% of their total cost. The decrease in the number of securities was the result of valuation increases in the portfolio. We reviewed these securities and concluded that the unrealized losses are market-related adjustments to the values, which were determined not to be other-than-temporary; we expect to recover our initial investments on these securities. The number of issuers with unrealized losses in our common stock portfolio decreased during 2017. A review of the securities in a loss position did not uncover fundamental issues with the issuers that would indicate other-than-temporary impairments existed. Additionally, consensus analyst expectations for recovery in the next 12 months would put the fair values at or above our current book values. Lastly, we determined, as of the balance sheet date, that it was not likely these securities would be sold prior to that recovery. Other-Than-Temporary Impairment (OTTI) The following table shows the total non-credit portion of the OTTI recorded in accumulated other comprehensive income, reflecting the original non-credit loss at the time the credit impairment was determined (i.e., unadjusted for valuation changes subsequent to the original write-down): December 31, (millions) 2017 2016 Fixed maturities: Residential mortgage-backed securities $ (19.7 ) $ (43.3 ) Commercial mortgage-backed securities (0.3 ) (0.6 ) Total fixed maturities $ (20.0 ) $ (43.9 ) The following tables provide rollforwards of the amounts related to credit losses recognized in earnings for the periods ended December 31, 2017 , 2016 , and 2015 , for which a portion of the OTTI losses were also recognized in accumulated other comprehensive income at the time the credit impairments were determined and recognized: (millions) Residential Mortgage- Backed Commercial Mortgage- Backed Total Total at December 31, 2016 $ 11.1 $ 0.4 $ 11.5 Credit losses for which an OTTI was not previously recognized 0 0.4 0.4 Reductions for securities sold/matured (10.9 ) (0.3 ) (11.2 ) Change in recoveries of future cash flows expected to be collected 1 (0.2 ) 0 (0.2 ) Total at December 31, 2017 $ 0 $ 0.5 $ 0.5 (millions) Residential Mortgage- Backed Commercial Mortgage- Backed Total Total at December 31, 2015 $ 12.4 $ 0.4 $ 12.8 Credit losses for which an OTTI was not previously recognized 0 0 0 Reductions for securities sold/matured 0 0 0 Change in recoveries of future cash flows expected to be collected 1 (1.3 ) 0 (1.3 ) Total at December 31, 2016 $ 11.1 $ 0.4 $ 11.5 (millions) Residential Mortgage- Backed Commercial Mortgage- Backed Total Total at December 31, 2014 $ 12.7 $ 0.4 $ 13.1 Credit losses for which an OTTI was not previously recognized 0 0 0 Reductions for securities sold/matured (1.4 ) 0 (1.4 ) Change in recoveries of future cash flows expected to be collected 1 1.1 0 1.1 Total at December 31, 2015 $ 12.4 $ 0.4 $ 12.8 1 Reflects expected recovery of prior period impairments that will be accreted into income over the remaining life of the security. Although we determined it is more likely that we will not be required to sell the securities prior to the recovery of their respective cost bases (which could be maturity), we are required to measure the amount of potential credit losses on the securities that were in an unrealized loss position. In that process, we considered a number of factors and inputs related to the individual securities. The methodology and significant inputs used to measure the amount of credit losses in our portfolio included: current performance indicators on the business model or underlying assets (e.g., delinquency rates, foreclosure rates, and default rates); credit support (via current levels of subordination); historical credit ratings; and updated cash flow expectations based upon these performance indicators. In order to determine the amount of credit loss, if any, the net present value of the cash flows expected (i.e., expected recovery value) was calculated using the current book yield for each security, and was compared to its current amortized value. In the event that the net present value was below the amortized value, a credit loss would be deemed to exist, and the security would be written down. During the period ended December 31, 2017 , we recorded a credit impairment write-down of $0.4 million . We did not have any credit impairment write-downs for the periods ended December 31, 2016 or 2015 . Realized Gains (Losses) The components of net realized gains (losses) for the years ended December 31, were: (millions) 2017 2016 2015 Gross realized gains on security sales Fixed maturities: U.S. government obligations $ 6.2 $ 24.6 $ 17.5 State and local government obligations 10.5 16.0 7.8 Corporate and other debt securities 20.3 43.3 31.2 Residential mortgage-backed securities 23.8 2.5 4.9 Commercial mortgage-backed securities 4.9 13.3 15.7 Other asset-backed securities 0.3 0 0 Redeemable preferred stocks 8.5 20.9 0.1 Total fixed maturities 74.5 120.6 77.2 Equity securities: Nonredeemable preferred stocks 58.4 11.9 65.3 Common equities 43.0 61.3 50.4 Short-term investments 0 0.1 0 Subtotal gross realized gains on security sales 175.9 193.9 192.9 Gross realized losses on security sales Fixed maturities: U.S. government obligations (28.7 ) (2.4 ) (0.9 ) State and local government obligations (0.1 ) (1.6 ) (0.3 ) Corporate and other debt securities (5.1 ) (2.5 ) (5.0 ) Residential mortgage-backed securities (0.4 ) (0.2 ) (0.8 ) Commercial mortgage-backed securities (5.3 ) (5.6 ) (1.3 ) Other asset-backed securities (0.4 ) 0 0 Redeemable preferred stocks (6.4 ) (6.6 ) 0 Total fixed maturities (46.4 ) (18.9 ) (8.3 ) Equity securities: Nonredeemable preferred stocks (5.9 ) (5.3 ) (3.2 ) Common equities (12.2 ) (15.7 ) (38.4 ) Short-term investments (0.2 ) (0.1 ) 0 Subtotal gross realized losses on security sales (64.7 ) (40.0 ) (49.9 ) Net realized gains (losses) on security sales Fixed maturities: U.S. government obligations (22.5 ) 22.2 16.6 State and local government obligations 10.4 14.4 7.5 Corporate and other debt securities 15.2 40.8 26.2 Residential mortgage-backed securities 23.4 2.3 4.1 Commercial mortgage-backed securities (0.4 ) 7.7 14.4 Other asset-backed securities (0.1 ) 0 0 Redeemable preferred stocks 2.1 14.3 0.1 Total fixed maturities 28.1 101.7 68.9 Equity securities: Nonredeemable preferred stocks 52.5 6.6 62.1 Common equities 30.8 45.6 12.0 Short-term investments (0.2 ) 0 0 Subtotal net realized gains (losses) on security sales 111.2 153.9 143.0 Other-than-temporary impairment losses Fixed maturities: Commercial mortgage-backed securities (0.4 ) 0 0 Redeemable preferred stocks 0 (25.3 ) 0 Total fixed maturities (0.4 ) (25.3 ) 0 Equity securities: Common equities (11.2 ) (0.3 ) (8.7 ) Subtotal investment other-than-temporary impairment losses (11.6 ) (25.6 ) (8.7 ) Other asset impairment (49.6 ) (59.7 ) 0 Subtotal other-than-temporary impairment losses (61.2 ) (85.3 ) (8.7 ) Other gains (losses) Hybrid securities (1.6 ) 2.1 (1.3 ) Derivative instruments 0 (20.0 ) (20.7 ) Litigation settlements 1.2 0.4 0.4 Subtotal other gains (losses) (0.4 ) (17.5 ) (21.6 ) Total net realized gains (losses) on securities $ 49.6 $ 51.1 $ 112.7 Gross realized gains and losses were predominantly the result of sales transactions in our fixed-income portfolio related to movements in credit spreads and interest rates and sales from our equity portfolios. In addition, gains and losses reflect recoveries from litigation settlements related to investments and holding period valuation changes on hybrids and derivatives. Also included are write-downs for securities determined to be other-than-temporarily impaired. The other asset impairment relates to renewable energy investments, which are reflected in “other assets” on the balance sheet, under which the future pretax cash flows are expected to be less than the carrying value of the assets. Net Investment Income The components of net investment income for the years ended December 31, were: (millions) 2017 2016 2015 Fixed maturities: U.S. government obligations $ 72.7 $ 18.2 $ 28.3 State and local government obligations 51.5 52.3 60.7 Foreign government obligations 0.3 0.4 0.4 Corporate debt securities 125.2 110.7 102.4 Residential mortgage-backed securities 34.7 47.3 54.3 Commercial mortgage-backed securities 79.6 81.6 74.6 Other asset-backed securities 47.1 28.0 22.0 Redeemable preferred stocks 11.8 14.9 15.0 Total fixed maturities 422.9 353.4 357.7 Equity securities: Nonredeemable preferred stocks 44.1 48.6 43.7 Common equities 58.3 57.2 51.0 Short-term investments 37.8 19.7 2.2 Investment income 563.1 478.9 454.6 Investment expenses (23.9 ) (22.4 ) (22.8 ) Net investment income $ 539.2 $ 456.5 $ 431.8 The amount of investment income we recognize varies from year to year based on the average assets during the year and the book yields of the securities in our portfolio. In addition to proceeds from debt offerings in each of the last three years, the increase in investment income in both 2017 and 2016, as compared to their prior respective years, reflects an increase in average assets of the portfolio due to the strong underwriting growth and profitability. During 2017, an additional increase was due to a slight increase in overall portfolio yield on new cash and portfolio turnover from our decision to lengthen our portfolio’s duration during the year. The increase in income in 2016 over 2015 also reflects a slight offset due to a lower portfolio yield as a result of the sharp decline in interest rates (which was prevalent during most of 2016), affecting investment yields on new cash and portfolio turnover, as well as our decision to shorten our portfolio duration early in 2016 and invest in a higher amount of short-term paper, which had lower overall yields. Trading Securities At December 31, 2017 and 2016 , we did not hold any trading securities and we did not have any net realized gains (losses) on trading securities for the years ended December 31, 2017 , 2016 , and 2015 . Derivative Instruments For all derivative positions discussed below, realized holding period gains and losses are netted with any upfront cash that may be exchanged under the contract to determine if the net position should be classified either as an asset or liability. To be reported as a net derivative asset and a component of the available-for-sale portfolio, the inception-to-date holding period (realized) gain on the derivative position at period end would have to exceed any upfront cash received. On the other hand, a net derivative liability would include any inception-to-date realized loss plus the amount of upfront cash received (or netted, if upfront cash was paid) and would be reported as a component of other liabilities. These net derivative assets/liabilities are not separately disclosed on the balance sheet due to their immaterial effect on our financial condition, cash flows, and results of operations. The following table shows the status of our derivative instruments at December 31, 2017 and 2016 , and for the years ended December 31, 2017 , 2016 , and 2015 : (millions) Balance Sheet 2 Comprehensive Income Statement Notional Value 1 Assets (Liabilities) Fair Value Pretax Net Realized Gains (Losses) Years ended December 31, December 31, December 31, Derivatives designated as: 2017 2016 2015 Purpose Classification 2017 2016 2017 2016 2015 Hedging instruments Closed: Ineffective cash flow hedge $ 31 $ 370 $ 18 Manage NA $ 0 $ 0 $ 0 $ (1.3 ) $ 0.2 Non-hedging instruments Assets: Interest rate swaps 0 0 750 Manage Investments - fixed 0 0 0 0 (23.4 ) Closed: Interest rate swaps 0 750 0 Manage NA 0 0 0 (19.0 ) 0 U.S. Treasury Note futures 0 135 691 Manage NA 0 0 0 0.3 2.5 Total NA NA NA $ 0 $ 0 $ 0 $ (20.0 ) $ (20.7 ) 1 The amounts represent the value held at year end for open positions and the maximum amount held during the year for closed positions. 2 To the extent we held both derivative assets and liabilities with the same counterparty that were subject to an enforceable master netting arrangement, we reported them on a gross basis on our balance sheets, consistent with our historical presentation. NA = Not Applicable CASH FLOW HEDGES During March 2017 , we entered into a forecasted debt issuance hedge, against a possible rise in interest rates, in conjunction with the $850 million of 4.125% Senior Notes due 2047 issued in April 2017 . Upon issuance, we closed the hedge and recognized, as part of accumulated other comprehensive income, a pretax loss of $8.0 million in April 2017 . During the third quarter 2016 , we entered into a $350 million forecasted transaction to hedge against a possible rise in interest rates in anticipation of a debt offering under which we issued $500 million of 2.45% Senior Notes due 2027. When the contract was closed, the $1.4 million loss on the derivative was immediately recognized as a realized loss. The $31 million in 2017 , the $18 million in 2015, and the remaining $20 million in 2016 , out of the $370 million disclosed in the table above, of our ineffective cash flow hedge resulted from the repurchase of a portion of our 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067, and we reclassified the unrealized gain on forecasted transactions to net realized gains on securities. The portion repurchased in 2017 resulted in an immaterial gain. During 2017 , we reclassified $0.3 million from accumulated other comprehensive income to interest expense on our closed debt issuance cash flow hedges, compared to $1.9 million during 2016 and $1.8 million during 2015 . See Note 4 – Debt for further discussion. INTEREST RATE SWAPS and U.S. TREASURY FUTURES We use interest rate swaps and treasury futures contracts from time to time to manage the fixed-income portfolio duration. We did not hold any interest rate swap positions at December 31, 2017 or 2016 . At December 31, 2015 , we held interest rate swap positions for which we were paying a fixed rate and receiving a variable rate, effectively shortening the duration of our fixed-income portfolio. As of December 31, 2015 , the balance of the cash collateral that we had received from the applicable counterparty on our then open positions was $4.9 million . We did not open any U.S. treasury futures during 2017. We opened and closed treasury futures during 2016 and 2015 ; no positions were outstanding at either year end.

Fair Value

Fair Value	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Fair Value	FAIR VALUE We have categorized our financial instruments, based on the degree of subjectivity inherent in the method by which they are valued, into a fair value hierarchy of three levels, as follows: • Level 1 : Inputs are unadjusted, quoted prices in active markets for identical instruments at the measurement date (e.g., U.S. government obligations, which are continually priced on a daily basis, active exchange-traded equity securities, and certain short-term securities). • Level 2 : Inputs (other than quoted prices included within Level 1) that are observable for the instrument either directly or indirectly (e.g., certain corporate and municipal bonds and certain preferred stocks). This includes: (i) quoted prices for similar instruments in active markets, (ii) quoted prices for identical or similar instruments in markets that are not active, (iii) inputs other than quoted prices that are observable for the instruments, and (iv) inputs that are derived principally from or corroborated by observable market data by correlation or other means. • Level 3 : Inputs that are unobservable. Unobservable inputs reflect our subjective evaluation about the assumptions market participants would use in pricing the financial instrument (e.g., certain structured securities and privately held investments). Determining the fair value of the investment portfolio is the responsibility of management. As part of the responsibility, we evaluate whether a market is distressed or inactive in determining the fair value for our portfolio. We review certain market level inputs to evaluate whether sufficient activity, volume, and new issuances exist to create an active market. Based on this evaluation, we concluded that there was sufficient activity related to the sectors and securities for which we obtained valuations. The composition of the investment portfolio by major security type and our outstanding debt was: Fair Value (millions) Level 1 Level 2 Level 3 Total Cost December 31, 2017 Fixed maturities: U.S. government obligations $ 6,645.9 $ 0 $ 0 $ 6,645.9 $ 6,688.8 State and local government obligations 0 2,297.1 0 2,297.1 2,285.6 Foreign government obligations 0 0 0 0 0 Corporate debt securities 0 4,997.7 0 4,997.7 4,997.2 Subtotal 6,645.9 7,294.8 0 13,940.7 13,971.6 Asset-backed securities: Residential mortgage-backed 0 836.7 0 836.7 828.8 Commercial mortgage-backed 0 2,758.6 0 2,758.6 2,760.1 Other asset-backed 0 2,454.7 0 2,454.7 2,454.5 Subtotal asset-backed securities 0 6,050.0 0 6,050.0 6,043.4 Redeemable preferred stocks: Financials 0 64.1 0 64.1 61.3 Utilities 0 11.4 0 11.4 10.1 Industrials 0 135.5 0 135.5 123.5 Subtotal redeemable preferred stocks 0 211.0 0 211.0 194.9 Total fixed maturities 6,645.9 13,555.8 0 20,201.7 20,209.9 Equity securities: Nonredeemable preferred stocks: Financials 80.6 718.2 0 798.8 693.6 Industrials 0 0 5.0 5.0 5.0 Subtotal nonredeemable preferred stocks 80.6 718.2 5.0 803.8 698.6 Common equities: Common stocks 3,399.5 0 0 3,399.5 1,498.7 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 3,399.5 0 0.3 3,399.8 1,499.0 Total fixed maturities and equity securities 10,126.0 14,274.0 5.3 24,405.3 22,407.5 Short-term investments 1,824.4 1,045.0 0 2,869.4 2,869.4 Total portfolio $ 11,950.4 $ 15,319.0 $ 5.3 $ 27,274.7 $ 25,276.9 Debt $ 0 $ 3,606.5 $ 37.1 $ 3,643.6 $ 3,306.3 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost December 31, 2016 Fixed maturities: U.S. government obligations $ 2,870.1 $ 0 $ 0 $ 2,870.1 $ 2,899.2 State and local government obligations 0 2,502.6 0 2,502.6 2,509.5 Foreign government obligations 24.5 0 0 24.5 24.5 Corporate debt securities 0 4,550.9 0 4,550.9 4,557.8 Subtotal 2,894.6 7,053.5 0 9,948.1 9,991.0 Asset-backed securities: Residential mortgage-backed 0 1,499.3 0 1,499.3 1,489.7 Commercial mortgage-backed 0 2,253.1 0.3 2,253.4 2,266.9 Other asset-backed 0 2,351.1 0 2,351.1 2,350.7 Subtotal asset-backed securities 0 6,103.5 0.3 6,103.8 6,107.3 Redeemable preferred stocks: Financials 0 59.5 0 59.5 59.8 Utilities 0 30.9 0 30.9 30.5 Industrials 0 101.5 0 101.5 98.5 Subtotal redeemable preferred stocks 0 191.9 0 191.9 188.8 Total fixed maturities 2,894.6 13,348.9 0.3 16,243.8 16,287.1 Equity securities: Nonredeemable preferred stocks: Financials 138.1 715.4 0 853.5 734.2 Industrials 0 0 0 0 0 Subtotal nonredeemable preferred stocks 138.1 715.4 0 853.5 734.2 Common equities: Common stocks 2,812.0 0 0 2,812.0 1,437.1 Other risk investments 0 0 0.4 0.4 0.4 Subtotal common equities 2,812.0 0 0.4 2,812.4 1,437.5 Total fixed maturities and equity securities 5,844.7 14,064.3 0.7 19,909.7 18,458.8 Short-term investments 3,009.3 563.6 0 3,572.9 3,572.9 Total portfolio $ 8,854.0 $ 14,627.9 $ 0.7 $ 23,482.6 $ 22,031.7 Debt $ 0 $ 3,188.5 $ 127.3 $ 3,315.8 $ 3,148.2 Our portfolio valuations, excluding short-term investments, classified as either Level 1 or Level 2 in the above tables are priced exclusively by external sources, including: pricing vendors, dealers/market makers, and exchange-quoted prices. During 2017 and 2016 , we did not have any transfers between Level 1 and Level 2. Our short-term security holdings classified as Level 1 are highly liquid, actively marketed, and have a very short duration, primarily 90 days or less to redemption. These securities are held at their original cost, adjusted for any accretion of discount, since that value very closely approximates what an active market participant would be willing to pay for such securities. The remainder of our short-term securities are classified as Level 2 and are not priced externally since these securities continually trade at par value. These securities are classified as Level 2 since they are primarily longer-dated auction securities issued by municipalities that contain a redemption put feature back to the auction pool with a redemption period typically less than seven days. The auction pool is created by a liquidity provider and if the auction is not available at the end of the seven days, we have the right to put the security back to the issuer at par. At December 31, 2017 , vendor-quoted prices represented 66% of our Level 1 classifications (excluding short-term investments), compared to 52% at December 31, 2016 . The securities quoted by vendors in Level 1 primarily represent our holdings in U.S. Treasury Notes, which are frequently traded and the quotes are considered similar to exchange-traded quotes. The balance of our Level 1 pricing comes from quotes obtained directly from trades made on active exchanges. At December 31, 2017 , vendor-quoted prices comprised 98% of our Level 2 classifications (excluding short-term investments), while dealer-quoted prices represented 2% , compared to 99% and 1% at December 31, 2016 , respectively. In our process for selecting a source (e.g., dealer, pricing service) to provide pricing for securities in our portfolio, we reviewed documentation from the sources that detailed the pricing techniques and methodologies used by these sources and determined if their policies adequately considered market activity, either based on specific transactions for the particular security type or based on modeling of securities with similar credit quality, duration, yield, and structure that were recently transacted. Once a source is chosen, we continue to monitor any changes or modifications to their processes by reviewing their documentation on internal controls for pricing and market reviews. We review quality control measures of our sources as they become available to determine if any significant changes have occurred from period to period that might indicate issues or concerns regarding their evaluation or market coverage. As part of our pricing procedures, we obtain quotes from more than one source to help us fully evaluate the market price of securities. However, our internal pricing policy is to use a consistent source for individual securities in order to maintain the integrity of our valuation process. Quotes obtained from the sources are not considered binding offers to transact. Under our policy, when a review of the valuation received from our selected source appears to be outside of what is considered market level activity (which is defined as trading at spreads or yields significantly different than those of comparable securities or outside the general sector level movement without a reasonable explanation), we may use an alternate source’s price. To the extent we determine that it may be prudent to substitute one source’s price for another, we will contact the initial source to obtain an understanding of the factors that may be contributing to the significant price variance, which often leads the source to adjust their pricing input data for future pricing. To allow us to determine if our initial source is providing a price that is outside of a reasonable range, we review our portfolio pricing on a weekly basis. We frequently challenge prices from our sources when a price provided does not match our expectations based on our evaluation of market trends and activity. Initially, we perform a review of our portfolio by sector to identify securities whose prices appear outside of a reasonable range. We then perform a more detailed review of fair values for securities disclosed as Level 2. We review dealer bids and quotes for these and/or similar securities to determine the market level context for our valuations. We then evaluate inputs relevant for each class of securities disclosed in the preceding hierarchy tables. For our structured debt securities, including commercial, residential, and asset-backed securities, we evaluate available market-related data for these and similar securities related to collateral, delinquencies, and defaults for historical trends and reasonably estimable projections, as well as historical prepayment rates and current prepayment assumptions and cash flow estimates. We further stratify each class of our structured debt securities into more finite sectors (e.g., planned amortization class, first pay, second pay, senior, subordinated, etc.) and use duration, credit quality, and coupon to determine if the fair value is appropriate. For our corporate debt and preferred stock (redeemable and nonredeemable) portfolios, as well as the notes and debentures issued by The Progressive Corporation (see Note 4 – Debt ), we review securities by duration, coupon, and credit quality, as well as changes in interest rate and credit spread movements within that stratification. The review also includes recent trades, including: volume traded at various levels that establish a market, issuer specific fundamentals, and industry specific economic news as it comes to light. For our municipal securities (e.g., general obligations, revenue, and housing), we stratify the portfolio to evaluate securities by type, coupon, credit quality, and duration to review price changes relative to credit spread and interest rate changes. Additionally, we look to economic data as it relates to geographic location as an indication of price-to-call or maturity predictors. For municipal housing securities, we look to changes in cash flow projections, both historical and reasonably estimable projections, to understand yield changes and their effect on valuation. Lastly, for our short-term securities, we look at acquisition price relative to the coupon or yield. Since our short-term securities are typically 90 days or less to maturity, with the majority listed in Level 2 being seven days or less to redemption, we believe that acquisition price is the best estimate of fair value. We also review data assumptions as supplied by our sources to determine if that data is relevant to current market conditions. In addition, we independently review each sector for transaction volumes, new issuances, and changes in spreads, as well as the overall movement of interest rates along the yield curve to determine if sufficient activity and liquidity exists to provide a credible source for our market valuations. During each valuation period, we create internal estimations of portfolio valuation (performance returns), based on current market-related activity (i.e., interest rate and credit spread movements and other credit-related factors) within each major sector of our portfolio. We compare our internally generated portfolio results with those generated based on quotes we received externally and research material valuation differences. We compare our results to index returns for each major sector adjusting for duration and credit quality differences to better understand our portfolio’s results. Additionally, we review on a monthly basis our external sales transactions and compare the actual final market sales price to a previous market valuation price. This review provides us further validation that our pricing sources are providing market level prices, since we are able to explain significant price changes (i.e., greater than 2%) as known events occur in the marketplace and affect a particular security’s price at sale. This analysis provides us with additional comfort regarding each source’s process, the quality of its review, and its willingness to improve its analysis based on feedback from clients. We believe this effort helps ensure that we are reporting the most representative fair values for our securities. Except as described below, our Level 3 securities are also priced externally; however, due to several factors (e.g., nature of the securities, level of inactivity, and lack of similar securities trading to obtain observable market level inputs), these valuations are more subjective in nature. Certain private equity investments and fixed-income investments included in the Level 3 category are valued using external pricing supplemented by internal review and analysis. After all the valuations are received and our review is complete, if the inputs used by vendors are determined to not contain sufficient observable market information, we will reclassify the affected security valuations to Level 3. At December 31, 2017 , we did not have any securities in our fixed-maturity portfolio listed as Level 3. At December 31, 2016 , the security in our fixed-maturity portfolio listed as Level 3 was thinly held with little liquidity and based on these factors, it was difficult to independently verify the observable market inputs that were used to generate the external valuation we received. Despite the lack of sufficient observable market information for our Level 3 security, we believe the valuation received, in conjunction with our procedures for evaluating third-party prices, supports the fair value reported in the financial statement. At December 31, 2017 , we held one private nonredeemable preferred security with a value of $5.0 million that was priced internally. The security was purchased during the third quarter 2017 and the value at December 31, 2017 equals the cost at acquisition. A review of their latest available financial statements did not reveal any significant changes that would impact the security’s fair value. We did not hold any internally-priced securities at December 31, 2016 . We review the prices from our external sources for reasonableness using internally developed assumptions to derive prices for the securities, which are then compared to the prices we received. During 2017 or 2016 , there were no material assets or liabilities measured at fair value on a nonrecurring basis. Based on our review, all the prices received from external sources remain unadjusted. The following tables provide a summary of changes in fair value associated with Level 3 assets for the years ended December 31, 2017 and 2016 : Level 3 Fair Value (millions) Fair Value at Dec. 31, 2016 Calls/ Maturities/ Paydowns Purchases Sales Net Realized (Gain)/Loss on Sales Change in Valuation Net Transfers In (Out) Fair Value at Dec. 31, 2017 Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 0.3 $ (0.3 ) $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 Total fixed maturities 0.3 (0.3 ) 0 0 0 0 0 0 Equity securities: Nonredeemable preferred stocks: Industrials 0 0 5.0 0 0 0 0 5.0 Common equities: Other risk investments 0.4 (0.1 ) 0 0 0 0 0 0.3 Total Level 3 securities $ 0.7 $ (0.4 ) $ 5.0 $ 0 $ 0 $ 0 $ 0 $ 5.3 Level 3 Fair Value (millions) Fair Value at Dec. 31, 2015 Calls/ Maturities/ Paydowns Purchases Sales Net Realized (Gain)/Loss on Sales Change in Valuation Net Transfers In (Out) Fair Value at Dec. 31, 2016 Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 9.9 $ (9.6 ) $ 0 $ 0 $ 0 $ 0 $ 0 $ 0.3 Total fixed maturities 9.9 (9.6 ) 0 0 0 0 0 0.3 Equity securities: Nonredeemable preferred stocks: Industrials 0 0 0 0 0 0 0 0 Common equities: Other risk investments 0.3 0 0 0 0 0.1 0 0.4 Total Level 3 securities $ 10.2 $ (9.6 ) $ 0 $ 0 $ 0 $ 0.1 $ 0 $ 0.7 The following tables provide a summary of the quantitative information about Level 3 fair value measurements for our applicable securities at December 31 : Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at Dec. 31, 2017 Valuation Technique Unobservable Input Unobservable Input Assumption Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 0 NA NA NA Total fixed maturities 0 Equity securities: Nonredeemable preferred stocks: Industrials 1 5.0 Internal price Unadjusted purchase price 3.9 Subtotal Level 3 securities 5.0 Third-party pricing exemption securities 2 0.3 Total Level 3 securities $ 5.3 1 The security was internally-priced since it is privately held and it was valued at December 31, 2017 using the purchase price. 2 The fair values for these securities were obtained from non-binding external sources where unobservable inputs are not reasonably available to us. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at Dec. 31, 2016 Valuation Technique Unobservable Input Unobservable Input Assumption Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 0.3 External vendor Prepayment rate 1 0 Total fixed maturities 0.3 Equity securities: Nonredeemable preferred stocks: Industrials 0 NA NA NA Subtotal Level 3 securities 0.3 Third-party pricing exemption securities 2 0.4 Total Level 3 securities $ 0.7 1 Assumes that one security has 0% of the principal amount of the underlying loans that will be paid off prematurely in each year. 2 The fair values for these securities were obtained from non-binding external sources where unobservable inputs are not reasonably available to us. Due to the relative size of the Level 3 securities’ fair values compared to the total portfolio’s fair value, any changes in pricing methodology would not have a significant change in valuation that would materially impact net or comprehensive income.

Debt

Debt	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Debt	DEBT Debt at December 31, consisted of: 2017 2016 (millions) Carrying Value Fair Value Carrying Value Fair Value 3.75% Senior Notes due 2021 (issued: $500.0, August 2011) $ 498.8 $ 520.7 $ 498.4 $ 528.8 2.45% Senior Notes due 2027 (issued: $500.0, August 2016) 496.1 477.9 495.8 464.6 6 5/8% Senior Notes due 2029 (issued: $300.0, March 1999) 296.1 382.3 295.9 380.1 6.25% Senior Notes due 2032 (issued: $400.0, November 2002) 395.3 516.9 395.2 499.0 4.35% Senior Notes due 2044 (issued: $350.0, April 2014) 346.5 388.7 346.4 362.3 3.70% Senior Notes due 2045 (issued: $400.0, January 2015) 395.2 402.9 395.1 372.5 4.125% Senior Notes due 2047 (issued: $850.0, April 2017) 841.2 917.1 0 0 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 (issued: $1,000.0, June 2007; outstanding: $0 and $594.6) 0 0 594.1 581.2 Other debt instruments 37.1 37.1 127.3 127.3 Total $ 3,306.3 $ 3,643.6 $ 3,148.2 $ 3,315.8 The other debt instruments reported in the table above represent ARX indebtedness and consist of: December 31, 2017 December 31, 2016 Type of debt instrument Number of Instruments Carrying Value Number of Instruments Carrying Stated Maturity Date(s) Term loans 2 $ 37.1 2 $ 62.1 December 2018 and 2019 Junior subordinated notes 0 0 2 41.2 NA Senior notes 0 0 4 24.0 NA Total $ 37.1 $ 127.3 NA - Not applicable as the notes were redeemed during 2017. Aggregate required principal payments on debt outstanding at December 31, 2017, are as follows: (millions) Year Payments 2018 $ 25.0 2019 12.1 2020 0 2021 500.0 2022 0 Thereafter 2,800.0 Total $ 3,337.1 The Progressive Corporation Debt Excluding the other debt instruments, all of the outstanding debt was issued by The Progressive Corporation, the ultimate holding company. The holding company debt includes amounts that were borrowed and contributed to the capital of its insurance subsidiaries or used, or made available for use, for other business purposes. Fair values for these debt instruments are obtained from external sources. There are no restrictive financial covenants or credit rating triggers on The Progressive Corporation debt. Interest on all debt issued by The Progressive Corporation is payable semiannually at the stated rates. All principal on the Senior Notes is due at the maturity stated in the tables above. The Senior Notes are redeemable, in whole or in part, at any time; however, the redemption price will equal the greater of the principal amount of the Senior Notes or a “make whole” amount calculated by reference to the present values of remaining scheduled principal and interest payments under the Senior Notes. The Progressive Corporation issued $850 million of 4.125% Senior Notes due 2047 (the “ 4.125% Senior Notes”) in April 2017, and $500 million of our 2.45% Senior Notes due 2027 (the “ 2.45% Senior Notes”) in August 2016, in underwritten public offerings. We received proceeds, after deducting underwriter’s discounts, commissions and other issuance costs, of approximately $841.1 million and $495.6 million , respectively, and paid approximately $1.5 million and $0.9 million of costs related to the issuance of the 4.125% Senior Notes and 2.45% Senior Notes, respectively. During 2017, we redeemed our 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 (the “ 6.70% Debentures”), at par, in the aggregate principal amount of $563.7 million . During 2017 and 2016, prior to the redemption, we repurchased, in the open market, $30.9 million and $19.8 million , respectively, in aggregate principal amount of our 6.70% Debentures. Since the carrying value of the debt we repurchased differed from the amount paid to extinguish the debt, we recognized a gain of $0.2 million during 2017 and $1.6 million during 2016. Prior to issuance of our debt securities, we entered into forecasted transactions to hedge against possible rises in interest rates. When the contracts were closed upon issuance of the applicable debt securities, we recognized unrealized gains (losses) as part of accumulated other comprehensive income for all of the Senior Notes, except for the 2.45% Senior Notes. Upon issuance of the 2.45% Senior Notes, we recognized a realized loss of $1.4 million (See Note 2 – Investments for further discussion). The following table shows the original gain (loss) recognized at debt issuance and the unamortized balance at December 31, 2017 , on a pretax basis: (millions) Unrealized Gain (Loss) at Debt Issuance Unamortized Balance at December 31, 2017 3.75% Senior Notes $ (5.1 ) $ (2.1 ) 6 5/8% Senior Notes (4.2 ) (2.7 ) 6.25% Senior Notes 5.1 3.6 4.35% Senior Notes (1.6 ) (1.5 ) 3.70% Senior Notes (12.9 ) (12.2 ) 4.125% Senior Notes (8.0 ) (7.9 ) These unrealized gains (losses) are being amortized as adjustments to interest expense over the life of the related Senior Notes. ARX Debt (i.e., Other debt instruments) The other debt instruments were issued by ARX, prior to The Progressive Corporation acquiring a controlling interest in 2015. ARX, not The Progressive Corporation or any of its other subsidiaries, is responsible for the other debt, which includes amounts that were borrowed and contributed to the capital of ARX’s insurance subsidiaries or used, or made available for use, for other business purposes. In estimating the fair values of the other debt instruments, it was determined that the fair values of these notes are equal to the carrying value, based on the current rates offered for debt of similar maturities and interest rates. During 2017, ARX redeemed its junior subordinated notes and senior notes, in their entirety, in the aggregate principal amount of $65.2 million , with proceeds from a 5 -year, fixed-rate loan made by The Progressive Corporation to fund the redemptions; this intercompany transaction is eliminated in consolidation. Monthly interest and principal payments are made on the term loans, with interest calculated based on the 30-day LIBOR plus 2.25% . Principal payments of $25.0 million are required to be paid during the next twelve months on these term loans. The term loans are secured by 100% of the outstanding common stock of three subsidiaries of ARX. The term loans require ARX and its subsidiaries to maintain specified debt leverage and fixed charge coverage ratios, as well as maintain a minimum risk-based capital ratio and minimum financial strength and credit ratings, as provided by A.M. Best Company, Inc. As of December 31, 2017 , ARX did not maintain the specified fixed charge coverage ratio or the minimum risk-based capital ratio on one of the subsidiaries that is pledged as collateral on the term loans. Subsequent to year-end, ARX received a waiver from the bank in regards to both of these items. The Progressive Corporation Line of Credit During 2017, The Progressive Corporation entered into a new line of credit with PNC Bank, National Association (PNC) in the maximum principal amount of $250 million . This line of credit replaced a previous line of credit with a maximum principal amount of $100 million that expired in April 2017. Subject to the terms and conditions of the line of credit documents, advances under the line of credit (if any) will bear interest at a variable rate equal to the higher of PNC’s Prime Rate or the sum of the Federal Funds Open Rate plus 50 basis points. Each advance must be repaid on the 30 th day after the advance or, if earlier, on April 30, 2018, the expiration date of the Line of Credit. Prepayments are permitted without penalty. The Line of Credit is uncommitted and, as such, all advances are subject to PNC’s discretion. We had no borrowings under either line of credit in 2017 or 2016 .

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	INCOME TAXES The components of our income tax provision were as follows: (millions) 2017 2016 2015 Current tax provision Federal $ 680.9 $ 469.6 $ 655.3 State 12.8 12.7 14.7 Deferred tax expense (benefit) Federal (149.4 ) (66.3 ) (47.7 ) State (3.5 ) (2.5 ) (11.2 ) Total income tax provision $ 540.8 $ 413.5 $ 611.1 The provision for income taxes in the accompanying consolidated statements of comprehensive income differed from the statutory rate as follows: ($ in millions) 2017 2016 2015 Income before income taxes $ 2,138.9 $ 1,470.7 $ 1,911.6 Tax at statutory federal rate $ 748.6 35 % $ 514.8 35 % $ 669.1 35 % Tax effect of: Net deferred tax liability revaluation 1 (99.5 ) (5 ) 0 0 0 0 Tax credits (52.4 ) (2 ) (62.2 ) (4 ) (1.9 ) 0 Stock-based compensation 2 (25.1 ) (1 ) 0 0 0 0 Dividends received deduction (20.7 ) (1 ) (22.6 ) (2 ) (19.8 ) (1 ) Exempt interest income (16.9 ) (1 ) (15.7 ) (1 ) (17.8 ) (1 ) Tax-deductible dividends (9.7 ) 0 (6.1 ) 0 (7.9 ) 0 Nondeductible compensation expense 3 10.1 0 0.5 0 0.3 0 State income taxes, net of federal taxes 6.0 0 6.6 0 2.3 0 Other items, net 0.4 0 (1.8 ) 0 0.6 0 Non-taxable gain 4 0 0 0 0 (13.8 ) (1 ) Total income tax provision $ 540.8 25 % $ 413.5 28 % $ 611.1 32 % 1 Pursuant to the recently enacted legislation commonly known as the Tax Cuts and Jobs Act of 2017 (the “Tax Act”); see further discussion below. 2 Represents excess tax benefits associated with share-based payments awards. Prior to 2017, these excess tax benefits were recorded directly to additional paid-in capital under the then existing accounting guidance. 3 Increase in 2017 primarily reflects changes in compensation that qualifies for deduction under the Tax Act. 4 Represents the tax effect of holding period gains on the 5% interest in ARX we owned prior to acquisition of a controlling interest on April 1, 2015. On December 22, 2017, legislation commonly known as the Tax Cuts and Jobs Act of 2017 (the “Tax Act”) was signed into law. One of the provisions of the Tax Act reduced the corporate federal income tax rate from 35% to 21% effective January 1, 2018. Pursuant to current accounting guidance, all deferred tax assets and liabilities were revalued to recognize the tax rate that is expected to apply when the tax effects are ultimately recognized in future periods. The impact of revaluing the deferred tax assets and liabilities from 35% to 21% was a net reduction to income tax expense of $99.5 million , as disclosed in the table above. This revaluation adjustment included a $275.7 million reduction related to the deferred tax liability associated with the net unrealized gains on our investment portfolio, which was originally recorded as a component of other comprehensive income and not through the tax provision. The remaining $176.2 million , which increased the tax provision, was associated with our other deferred tax assets and liabilities identified in the table below. As of December 31, 2017, the company had not fully completed its accounting for the tax effects of the enactment of the Tax Act with regard to the following: • The company recorded a provisional tax amount of $4.5 million related to deductibility of compensation expense for certain covered executives due to uncertainty surrounding the appropriate tax treatment of outstanding performance-based awards. This amount may change when the Internal Revenue Service (IRS) issues additional guidance. • The company did not record any amounts related to the changes in loss reserve discounting required by the Tax Act. These changes require the IRS to publish new discount factors based on loss payment patterns and interest rates determined under the Tax Act, and the IRS has not yet published this information. We are unable to make a reasonable estimate at this time; however, the ultimate adjustment is not expected to have any net impact on the current year balance sheet or income statement. Deferred income taxes reflect the effect for financial statement reporting purposes of temporary differences between the financial statement carrying amounts and the tax bases of assets and liabilities. The following table shows the components of the net deferred tax liability at December 31, 2017 and 2016 . As noted above, the federal deferred tax assets and liabilities at December 31, 2017, have been revalued to reflect the new 21% federal corporate income tax rate under the Tax Act. (millions) 2017 2016 Federal deferred tax assets: Unearned premiums reserve $ 368.9 $ 515.6 Non-deductible accruals 166.0 259.9 Loss and loss adjustment expense reserves 48.4 76.5 Hedges on forecasted transactions 4.8 5.1 Investment basis differences 0 31.3 Other 7.3 8.9 Federal deferred tax liabilities: Net unrealized gains on securities (419.5 ) (507.2 ) Deferred acquisition costs (163.9 ) (227.9 ) Property and equipment (75.5 ) (120.2 ) Intangible assets (66.6 ) (145.3 ) Prepaid expenses (7.1 ) (9.2 ) Investment basis differences (5.5 ) 0 Deferred gain on extinguishment of debt (0.4 ) (1.5 ) Other (6.1 ) (7.7 ) Net federal deferred tax liability (149.2 ) (121.7 ) Net state deferred tax asset 14.2 10.4 Net deferred tax liability $ (135.0 ) $ (111.3 ) Although realization of the deferred tax assets is not assured, management believes that it is more likely than not that the deferred tax assets will be realized based on our expectation that we will be able to fully utilize the deductions that are ultimately recognized for tax purposes and, therefore, no valuation allowance was needed at December 31, 2017 or 2016 . At December 31, 2017 and 2016, we had $23.8 million and $41.2 million , respectively, of net taxes payable (included in other liabilities on the balance sheet). The Progressive Corporation and its wholly-owned subsidiaries file a consolidated income tax return. This group has been a participant in the Compliance Assurance Program (CAP) since 2007. Under CAP, the IRS begins its examination process for the tax year before the tax return is filed, by examining significant transactions and events as they occur. The goal of the CAP program is to expedite the exam process and to reduce the level of uncertainty regarding a taxpayer’s tax filing positions. All federal income tax years prior to 2014 are closed to examination for both Progressive and ARX. The IRS exams for 2014-2016 for Progressive have been completed and the returns were accepted as filed. We consider these years to be effectively settled. The 2017 tax year remains open to examination. ARX and its wholly owned subsidiaries file their own consolidated income tax return since we owned less than 80% of their outstanding stock at December 31, 2017. This group was last examined by the IRS for the 2011 and 2012 tax years, which we consider to be effectively settled. The statute of limitations has expired for the 2013 tax year, while the 2014-2017 tax years remain open to examination. The statute of limitations for state income tax purposes generally remains open for three to four years from the return filing date, depending upon the jurisdiction. There has been no significant state income tax audit activity. We recognize interest and penalties, if any, as a component of income tax expense. For the years ended December 31, 2017, 2016, and 2015, $0.2 million , less than $0.1 million , and $0.1 million , respectively, of interest and penalties expense has been recorded in the tax provision. We have not recorded any unrecognized tax benefits, or any related interest and penalties, as of December 31, 2017 and 2016 .

Loss And Loss Adjustment Expens

Loss And Loss Adjustment Expense Reserves	12 Months Ended
	Dec. 31, 2017
Insurance [Abstract]
Loss And Loss Adjustment Expense Reserves	LOSS AND LOSS ADJUSTMENT EXPENSE RESERVES We write personal and commercial auto insurance, residential property insurance, and other specialty property-casualty insurance and related services throughout the United States. As a property-casualty insurance company, we are exposed to hurricanes or other catastrophes. To help mitigate this risk, we maintain excess of loss and aggregate stop-loss reinsurance coverage on our Property business. Although the occurrence of a major catastrophe could have a significant effect on our monthly or quarterly results, we believe that, based on historical experience, such an event would not be so material as to disrupt the overall normal operations of Progressive. We are unable to predict the frequency or severity of any such events that may occur in the near term or thereafter. As we are primarily an insurer of motor vehicles and residential property, we have limited exposure to environmental, asbestos, and general liability claims. We have established reserves for such exposures, in amounts that we believe to be adequate based on information currently known. These claims are not expected to have a material effect on our liquidity, financial condition, cash flows, or results of operations. Loss and Loss Adjustment Expense Reserves Loss and loss adjustment expense (LAE) reserves represent our best estimate of our ultimate liability for losses and LAE relating to events that occurred prior to the end of any given accounting period but have not yet been paid. For our vehicle businesses, which represent about 98% of our total carried reserves, Progressive’s actuarial staff reviews over 400 subsets of business data, which are at a combined state, product, and line coverage level (the “products”), to calculate the needed loss and LAE reserves. We begin our review of a set of data by producing multiple estimates of needed reserves, using both paid and incurred data, to determine if a reserve change is required. In the event of a wide variation among results generated by the different projections, our actuarial group will further analyze the data using additional quantitative analysis. Each review develops a point estimate for a relatively small subset of the business, which allows us to establish meaningful reserve levels for that subset. We believe our comprehensive process of reviewing at a subsegment level provides us more meaningful estimates of our aggregate loss reserves. The actuarial staff completes separate projections of needed case and incurred but not recorded (IBNR) reserves. Since a large majority of the parties involved in an accident report their claims within a short time period after the occurrence, we do not carry a significant amount of IBNR reserves for older accident years. Based on the methodology we use to estimate case reserves for our vehicle businesses, we do not have expected development on reported claims included in our IBNR reserves. We do, however, include anticipated salvage and subrogation recoveries in our IBNR reserves, which could result in negative carried IBNR reserves, primarily in our physical damage reserves. Changes in historical data may reduce the predictiveness of our projected future loss costs. Internal considerations that are process-related, which generally result from changes in our claims organization’s activities, include claim closure rates, the number of claims that are closed without payment, and the level of the claims representatives’ estimates of the needed case reserve for each claim. These changes and their effect on the historical data are studied at the state level versus on a larger, less indicative, countrywide basis. External items considered include the litigation atmosphere, state-by-state changes in medical costs, and the availability of services to resolve claims. These also are better understood at the state level versus at a more macro, countrywide level. The actuarial staff takes these changes into consideration when making their assumptions to determine needed reserve levels. Similar to our vehicle businesses, our actuarial staff analyzes loss and LAE property data on an accident period basis. Many of the methodologies and key parameters reviewed are similar. Unlike our vehicle businesses, primarily due to the low levels of reserves, data is reviewed at a macro level and a range of reserves are generated to determine a reasonable range. In addition, for our Property business, since claims adjusters primarily establish the case reserves, the actuarial staff includes expected development on case reserves as a component of the overall IBNR reserves. Activity in the loss and loss adjustment expense reserves is summarized as follows: (millions) 2017 2016 2015 Balance at January 1 $ 11,368.0 $ 10,039.0 $ 8,857.4 Less reinsurance recoverables on unpaid losses 1,801.0 1,442.7 1,185.9 Net balance at January 1 9,567.0 8,596.3 7,671.5 Net loss and loss adjustment reserves (disposed) acquired 1 0 (2.5 ) 222.4 Total beginning reserves 9,567.0 8,593.8 7,893.9 Incurred related to: Current year 18,782.1 16,967.1 14,657.1 Prior years 25.9 (87.5 ) (315.1 ) Total incurred 18,808.0 16,879.6 14,342.0 Paid related to: Current year 12,201.5 11,149.0 9,577.3 Prior years 5,256.7 4,757.4 4,062.3 Total paid 17,458.2 15,906.4 13,639.6 Net balance at December 31 10,916.8 9,567.0 8,596.3 Plus reinsurance recoverables on unpaid losses 2,170.1 1,801.0 1,442.7 Balance at December 31 $ 13,086.9 $ 11,368.0 $ 10,039.0 1 During 2016, $2.5 million net reserves were disposed by ARX in an exchange transaction (see Note 16 – Goodwill and Intangible Assets for further discussion). During 2015, $222.4 million net reserves were acquired in the ARX acquisition. We experienced unfavorable reserve development of $25.9 million in 2017, and favorable development of $87.5 million in 2016 and $315.1 million in 2015, which is reflected as “Incurred related to prior years” in the table above. 2017 • Approximately $64 million of unfavorable prior year reserve development was attributable to accident years 2016 and 2015. This unfavorable development was offset by $38 million of favorable development attributable to accident year 2014 and prior accident years. • Our personal auto products incurred $70 million of unfavorable loss and LAE reserve development, primarily in the Agency business, in part reflecting an increase in costs related to property damage and higher LAE costs, primarily due to an increase in incentive compensation and defense counsel spend. • Our Property business experienced $37 million in favorable development primarily due to lower severity and frequency than anticipated for accident year 2016 and development of losses eligible to be ceded under our catastrophe bond reinsurance program. • Our commercial auto products had less than $1 million of unfavorable reserve development. 2016 • Approximately $56 million of the favorable prior year reserve development was attributable to accident year 2015, and approximately $51 million of favorable development was attributable to accident years 2013 and prior. This favorable development was partially offset by $19 million of unfavorable development attributable to accident year 2014. • Our Personal Lines and Property businesses incurred $54 million and $52 million , respectively, of favorable loss and LAE reserve development, partially offset by the unfavorable loss and LAE development in Commercial Lines. In our Property business, both the severity and frequency of late reported claims was less than anticipated. • Our personal auto product developed favorably $40 million , almost evenly split between Direct and Agency. • Our personal auto business incurred favorable case development, primarily in bodily injury due to a lower than anticipated severity. • Our personal auto and Commercial Lines businesses incurred unfavorable IBNR loss reserve development, primarily due to a higher severity and frequency of late reported claims than anticipated for accident year 2015, due in part to storms in late December 2015, resulting in a greater number of claims being reported in January 2016 than anticipated. • In addition, our Commercial Lines business experienced unfavorable case reserve development for accident year 2014, primarily due to a higher severity than anticipated on our largest limits, while case reserve development for accident years 2015 and 2013 and prior was favorable. 2015 • Approximately $239 million of the favorable prior year reserve development was primarily attributable to accident year 2014. • Favorable reserve development occurred in all segments; our combined Agency auto business and Direct auto business experienced approximately $217 million of total development, with the remainder split between our Commercial Lines and Property businesses. • In our personal auto and Commercial Lines businesses, we incurred favorable case loss reserve development, primarily in bodily injury and uninsured motorist bodily injury coverages, due to lower than anticipated severity. • Our Property business development was favorable due to lower than anticipated severity and frequency across all products, primarily in accident years 2014 and 2013. Incurred and Paid Claims Development by Accident Year The tables below present our incurred and paid claims development by accident year for the last five years, which generally represents the maximum development period for claims in any of our segments. The tables below include inception-to-date information for companies acquired and wholly exclude companies disposed of, rather than including information from the date of acquisition, or until the date of disposition. We believe that the most meaningful presentation of claims development is through the retrospective approach by presenting all relevant historical information for all periods presented. We have elected to present our incurred and paid claims development consistent with our GAAP reportable segments (see Note 10 – Segment Information for a discussion of our segment reporting), with a further disaggregation of our Personal Lines and Commercial Lines claims development between liability and physical damage, since the loss patterns are significantly different between them. The other business primarily includes reserves for our run-off products, which are not considered material, and, therefore, we are not including separate claims development tables. The following tables show incurred and paid claims development, net of reinsurance, by accident year. Only 2017 is audited; all prior years are considered required supplementary information and, therefore, are unaudited. Expected development on our case reserves is excluded from the IBNR reserves on our vehicle businesses, as discussed above. For the Property business, the IBNR reserves include expected case development based on the methodology used in establishing the case reserves for that segment. The cumulative number of incurred claims are based on accident coverages (e.g., bodily injury, collision, comprehensive, personal injury protection, property damage) related to opened claims. Coverage counts related to claims closed without payment are excluded from the cumulative number of incurred claims. Personal Lines - Agency - Liability ($ in millions) As of Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance December 31, 2017 For the years ended December 31, Total of IBNR Liabilities Plus Expected Development on Reported Claims Cumulative Number of Incurred Claim Counts Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 3,506.0 $ 3,520.9 $ 3,518.2 $ 3,528.0 $ 3,484.2 $ 0 696,624 2014 3,702.1 3,627.7 3,633.2 3,654.4 50.9 701,787 2015 3,774.9 3,773.8 3,798.8 59.2 704,990 2016 4,082.9 4,130.0 158.7 738,563 2017 4,474.8 679.6 765,553 Total $ 19,542.2 Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance For the years ended December 31, Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 1,684.0 $ 2,794.1 $ 3,173.1 $ 3,362.9 $ 3,437.6 2014 1,809.0 2,868.1 3,284.5 3,482.3 2015 1,793.1 2,976.0 3,416.5 2016 1,941.6 3,231.5 2017 2,074.0 Total $ 15,641.9 All outstanding liabilities before 2013, net of reinsurance 1 55.2 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 3,955.5 1 Required supplementary information (unaudited) Personal Lines - Agency - Physical Damage ($ in millions) As of Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance December 31, 2017 For the years ended December 31, Total of IBNR Liabilities Plus Expected Development on Reported Claims Cumulative Number of Incurred Claim Counts Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 2,014.0 $ 2,001.4 $ 2,000.1 $ 1,999.4 $ 2,001.1 $ 0 1,347,920 2014 2,107.5 2,090.3 2,089.9 2,088.0 (3.3 ) 1,374,666 2015 2,136.8 2,137.2 2,134.4 (2.6 ) 1,336,268 2016 2,423.4 2,398.9 (12.1 ) 1,398,371 2017 2,635.5 (101.8 ) 1,496,298 Total $ 11,257.9 Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance For the years ended December 31, Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 1,952.7 $ 2,003.9 $ 2,002.0 $ 2,001.3 $ 2,000.8 2014 2,078.8 2,091.6 2,090.6 2,090.7 2015 2,106.2 2,138.1 2,134.4 2016 2,391.0 2,406.9 2017 2,599.8 Total $ 11,232.6 All outstanding liabilities before 2013, net of reinsurance 1 0.9 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 26.2 1 Required supplementary information (unaudited) Personal Lines - Direct - Liability ($ in millions) As of Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance December 31, 2017 For the years ended December 31, Total of IBNR Liabilities Plus Expected Development on Reported Claims Cumulative Number of Incurred Claim Counts Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 2,619.4 $ 2,621.8 $ 2,615.8 $ 2,649.8 $ 2,612.9 $ 0 550,166 2014 2,946.8 2,887.4 2,898.1 2,913.6 38.2 592,185 2015 3,330.5 3,328.3 3,354.2 48.6 659,102 2016 3,819.0 3,843.9 143.0 733,195 2017 4,209.5 598.6 761,280 Total $ 16,934.1 Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance For the years ended December 31, Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 1,252.0 $ 2,085.1 $ 2,375.5 $ 2,521.3 $ 2,577.8 2014 1,413.0 2,278.0 2,624.2 2,780.0 2015 1,545.2 2,615.0 3,021.0 2016 1,780.6 2,991.1 2017 1,912.6 Total $ 13,282.5 All outstanding liabilities before 2013, net of reinsurance 1 33.5 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 3,685.1 1 Required supplementary information (unaudited) Personal Lines - Direct - Physical Damage ($ in millions) As of Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance December 31, 2017 For the years ended December 31, Total of IBNR Liabilities Plus Expected Development on Reported Claims Cumulative Number of Incurred Claim Counts Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 1,653.7 $ 1,635.6 $ 1,634.6 $ 1,633.4 $ 1,635.0 $ 0 1,367,341 2014 1,889.3 1,862.2 1,861.7 1,859.2 (3.2 ) 1,471,016 2015 2,110.7 2,097.7 2,093.5 (3.0 ) 1,539,790 2016 2,521.0 2,475.4 (14.3 ) 1,675,405 2017 2,750.6 (145.3 ) 1,779,665 Total $ 10,813.7 Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance For the years ended December 31, Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 1,612.9 $ 1,638.2 $ 1,636.2 $ 1,635.3 $ 1,635.0 2014 1,874.6 1,864.1 1,862.7 1,861.8 2015 2,094.7 2,100.1 2,094.7 2016 2,505.0 2,485.8 2017 2,742.1 Total $ 10,819.4 All outstanding liabilities before 2013, net of reinsurance 1 0.1 Liabilities for claims and claim adjustment expenses, net of reinsurance $ (5.6 ) 1 Required supplementary information (unaudited) Commercial Lines - Liability ($ in millions) As of Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance December 31, 2017 For the years ended December 31, Total of IBNR Liabilities Plus Expected Development on Reported Claims Cumulative Number of Incurred Claim Counts Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 864.2 $ 864.6 $ 865.4 $ 865.0 $ 840.8 $ 0 78,075 2014 822.5 795.4 820.3 823.0 15.9 74,724 2015 897.6 911.1 914.8 23.7 77,807 2016 1,185.8 1,204.8 63.1 92,196 2017 1,374.1 219.2 94,992 Total $ 5,157.5 Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance For the years ended December 31, Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 233.3 $ 509.4 $ 659.1 $ 752.5 $ 802.6 2014 234.0 438.7 610.0 715.3 2015 238.4 501.5 675.0 2016 298.6 639.9 2017 325.8 Total $ 3,158.6 All outstanding liabilities before 2013, net of reinsurance 1 28.5 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 2,027.4 1 Required supplementary information (unaudited) Commercial Lines - Physical Damage ($ in millions) As of Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance December 31, 2017 For the years ended December 31, Total of IBNR Liabilities Plus Expected Development on Reported Claims Cumulative Number of Incurred Claim Counts Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 256.2 $ 254.2 $ 254.5 $ 253.7 $ 254.2 $ 0 62,673 2014 240.3 239.7 238.6 238.0 (0.5 ) 59,622 2015 274.4 274.1 273.5 (0.1 ) 62,593 2016 379.6 379.8 (0.9 ) 74,217 2017 415.4 (5.7 ) 77,767 Total $ 1,560.9 Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance For the years ended December 31, Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 239.4 $ 253.1 $ 253.9 $ 253.7 $ 253.8 2014 224.6 238.3 237.7 237.9 2015 248.5 271.9 272.0 2016 336.7 376.9 2017 369.0 Total $ 1,509.6 All outstanding liabilities before 2013, net of reinsurance 1 0.4 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 51.7 1 Required supplementary information (unaudited) Property Business ($ in millions) As of Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance December 31, 2017 For the years ended December 31, Total of IBNR Liabilities Plus Expected Development on Reported Claims Cumulative Number of Incurred Claim Counts Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 307.3 $ 283.3 $ 254.3 $ 254.1 $ 253.9 $ 2.5 30,600 2014 415.5 389.1 379.7 376.3 3.6 40,984 2015 460.0 416.5 403.6 11.5 41,957 2016 568.6 541.2 24.8 53,323 2017 672.8 99.7 64,487 Total $ 2,247.8 Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance For the years ended December 31, Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 185.1 $ 234.2 $ 244.9 $ 249.5 $ 250.5 2014 269.2 351.5 365.9 370.3 2015 280.3 372.8 383.5 2016 415.2 498.2 2017 506.7 Total $ 2,009.2 All outstanding liabilities before 2013, net of reinsurance 1 7.2 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 245.8 1 Required supplementary information (unaudited) The following table reconciles the net incurred and paid claims development tables to the liability for claims and claim adjustment expenses: (millions) 2017 2016 Net outstanding liabilities Personal Lines Agency, Liability $ 3,955.5 $ 3,555.9 Agency, Physical Damage 26.2 30.1 Direct, Liability 3,685.1 3,219.1 Direct, Physical Damage (5.6 ) 11.2 Commercial Lines Liability 2,027.4 1,675.3 Physical Damage 51.7 46.6 Property 245.8 224.7 Other business 43.7 37.1 Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance 10,029.8 8,800.0 Reinsurance recoverable on unpaid claims Personal Lines Agency, Liability 769.8 724.5 Agency, Physical Damage 0 0 Direct, Liability 838.1 724.3 Direct, Physical Damage 0 0 Commercial Lines Liability 105.1 59.5 Physical Damage 0.1 0 Property 243.8 132.7 Other business 200.9 154.9 Total reinsurance recoverable on unpaid claims $ 2,157.8 $ 1,795.9 Unallocated claims adjustment expense related to: Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance 887.0 767.0 Reinsurance recoverable on unpaid claims 12.3 5.1 Total gross liability for unpaid claims and claim adjustment expense $ 13,086.9 $ 11,368.0 The following table shows the average historical claims duration as of December 31, 2017 : (Required Supplementary Information - Unaudited) Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 Personal Lines Agency, Liability 47.6% 30.8% 11.3% 5.4% 2.1% Agency, Physical Damage 98.9% 1.3% (0.1)% 0% 0% Direct, Liability 46.7% 31.3% 11.7% 5.5% 2.2% Direct, Physical Damage 100.1% 0% (0.2)% (0.1)% 0% Commercial Lines Liability 25.8% 28.7% 19.2% 11.9% 6.0% Physical Damage 90.9% 7.9% 0% 0% 0% Property 73.7% 19.5% 3.5% 1.4% 0.4%

Reinsurance

Reinsurance	12 Months Ended
	Dec. 31, 2017
Reinsurance Disclosures [Abstract]
Reinsurance	REINSURANCE The effect of reinsurance on premiums written and earned for the years ended December 31, was as follows: 2017 2016 2015 (millions) Written Earned Written Earned Written Earned Direct premiums $ 27,860.7 $ 26,425.7 $ 23,941.9 $ 23,111.2 $ 21,086.5 $ 20,454.1 Ceded premiums: Regulated plans (505.9 ) (479.6 ) (439.4 ) (425.1 ) (358.0 ) (362.6 ) Non-Regulated plans (222.7 ) (216.2 ) (149.0 ) (212.1 ) (164.5 ) (192.4 ) Total ceded premiums (728.6 ) (695.8 ) (588.4 ) (637.2 ) (522.5 ) (555.0 ) Net premiums $ 27,132.1 $ 25,729.9 $ 23,353.5 $ 22,474.0 $ 20,564.0 $ 19,899.1 The Regulated plans primarily include the following: • Federal reinsurance plan • National Flood Insurance Program (NFIP) • State-provided reinsurance facilities • Michigan Catastrophic Claims Association (MCCA) • North Carolina Reinsurance Facility (NCRF) • Florida Hurricane Catastrophe Fund (FHCF) • State-mandated involuntary plans • Commercial Automobile Insurance Procedures/Plans (CAIP) The Non-Regulated plans primarily include amounts ceded on Property business under catastrophic and aggregate stop-loss reinsurance agreements, as well as amounts ceded on transportation network company (TNC) business under a quota-share reinsurance agreement. The increase in ceded written premiums for 2017 was primarily driven by growth in the Regulated plans, a full year of ceded premiums on the reinsurance coverage on the TNC business, and the aggregate stop-loss agreement that was effective January 1, 2017. For 2016, the increase reflected the beginning of reinsurance coverage on the TNC pilot program and a full year of ceded premiums on catastrophe reinsurance held by ASI, partially offset by ASI’s termination of a quota-share contract, which reduced ceded earned premium in 2016. Losses and loss adjustment expenses were net of losses ceded of $1,094.2 million in 2017 , $764.4 million in 2016 , and $457.3 million in 2015 . The increase in losses and loss adjustment expenses ceded in 2017 and 2016, compared to the prior years, was primarily driven by the impact of catastrophic flooding events on the business serviced by ASI and other subsidiaries of ARX as part of its participation in the NFIP. For 2017, ASI also ceded additional losses and loss adjustment expenses due to the hurricanes and other severe storms that were reinsured by ASI. Our prepaid reinsurance premiums and reinsurance recoverables were comprised of the following at December 31: Prepaid Reinsurance Premiums Reinsurance Recoverables ($ in millions) 2017 2016 2017 2016 Regulated plans: MCCA $ 44.3 22 % $ 36.5 21 % $ 1,611.5 71 % $ 1,452.7 77 % CAIP 50.0 25 40.2 24 218.0 10 170.6 9 NCRF 31.5 15 27.5 16 74.2 3 67.0 4 NFIP 53.8 26 49.1 29 148.8 7 78.5 4 Other 0.3 0 0.1 0 8.2 0 3.5 0 Total Regulated plans 179.9 88 153.4 90 2,060.7 91 1,772.3 94 Non-Regulated plans: Property 9.1 5 3.2 2 138.6 6 62.9 3 Other 14.3 7 13.9 8 74.1 3 49.6 3 Total Non-Regulated plans 23.4 12 17.1 10 212.7 9 112.5 6 Total $ 203.3 100 % $ 170.5 100 % $ 2,273.4 100 % $ 1,884.8 100 % The increase in reinsurance recoverables during 2017 reflects both new claims and development under the MCCA program, catastrophic flooding on business under the NFIP, and recoverables under the other catastrophe reinsurance programs covering hurricanes and other severe storms. In addition, part of the increase reflects business growth and reserve development in our TNC business, which is covered by a quota-share reinsurance agreement. Reinsurance contracts do not relieve us from our obligations to policyholders. Failure of reinsurers to honor their obligations could result in losses to us. Our exposure to losses from the failure of Regulated plans is minimal, since these plans are funded by the federal government or by mechanisms supported by insurance companies in applicable states. We evaluate the financial condition of our other reinsurers and monitor concentrations of credit risk to minimize our exposure to significant losses from reinsurer insolvencies.

Statutory Financial Information

Statutory Financial Information	12 Months Ended
	Dec. 31, 2017
Statutory Financial Information Disclosures [Abstract]
Statutory Financial Information	STATUTORY FINANCIAL INFORMATION Consolidated statutory surplus was $9,664.4 million and $8,560.0 million at December 31, 2017 and 2016 , respectively. Statutory net income was $1,416.2 million , $1,022.3 million , and $1,333.1 million for the years ended December 31, 2017 , 2016 , and 2015 , respectively. At December 31, 2017 , $830.1 million of consolidated statutory surplus represented net admitted assets of our insurance subsidiaries and affiliates that are required to meet minimum statutory surplus requirements in such entities’ states of domicile. The companies may be licensed in states other than their states of domicile, which may have higher minimum statutory surplus requirements. Generally, the net admitted assets of insurance companies that, subject to other applicable insurance laws and regulations, are available for transfer to the parent company cannot include the net admitted assets required to meet the minimum statutory surplus requirements of the states where the companies are licensed. During 2017 , the insurance subsidiaries paid aggregate cash dividends of $872.8 million to their parent company. Based on the dividend laws in effect at December 31, 2017, the insurance subsidiaries could pay aggregate dividends of $1,390.5 million in 2018 without prior approval from regulatory authorities, provided the dividend payments are not made within 12 months of previous dividends paid by the applicable subsidiary.

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Dec. 31, 2017
Employee Benefit Plans Disclosures [Abstract]
Employee Benefit Plans	EMPLOYEE BENEFIT PLANS Beginning July 1, 2017, employees of ARX and its subsidiaries were included in the benefit plans described below. References in this Note 9 to Progressive refer to The Progressive Corporation and its subsidiaries, including ARX and its subsidiaries. Prior to July 1, 2017, ARX maintained employee benefit plans that were separate from the plans that covered employees of The Progressive Corporation’s other subsidiaries. Retirement Plans Progressive has a defined contribution pension plan (401(k) Plan) that covers employees who have been employed with the company for at least 30 days . Under Progressive’s 401(k) Plan, we match up to a maximum of 6% of an employee’s eligible compensation contributed to the plan. Employee and company matching contributions are invested, at the direction of the employee, in a number of investment options available under the plan, including various mutual funds, a self-directed brokerage option, and a Progressive common stock fund. Progressive’s common stock fund is an employee stock ownership program (ESOP) within the 401(k) Plan. At December 31, 2017, the ESOP held 24.6 million of our common shares, all of which are included in shares outstanding. Dividends on these shares are reinvested in common shares or paid out in cash, at the election of the participant, and the related tax benefit is recorded as part of our tax provision. Previously, ARX employees were covered by separate qualified defined contribution plans. Matching contributions to these plans for the year ended December 31, 2017, 2016, and 2015 (including the contributions made subsequent to The Progressive Corporation acquiring a controlling interest in ARX) were $97.3 million , $86.8 million , and $79.1 million for the years ended December 31, 2017, 2016, and 2015, respectively. Postemployment Benefits Progressive provides various postemployment benefits to former or inactive employees who meet eligibility requirements, and to their beneficiaries and covered dependents. Postemployment benefits include salary continuation and disability-related benefits, including workers’ compensation and, if elected, continuation of health-care benefits for specified limited periods. The liability for these benefits was $19.5 million and $21.7 million at December 31, 2017 and 2016, respectively. Incentive Compensation Plans – Employees Progressive’s incentive compensation programs include both non-equity incentive plans (cash) and equity incentive plans. Progressive’s cash incentive compensation includes an annual cash incentive program for a limited number of senior executives and Progressive’s Gainsharing program for other employees, and for 2017, a separate gainsharing program for ARX employees; the structures of these programs are similar in nature. Progressive’s equity incentive compensation plans provide for the granting of restricted stock awards and restricted stock unit awards (collectively, “restricted equity awards”) to key members of management. Since 2010, Progressive has only issued restricted stock units as the form of equity compensation. In addition, ARX provides periodic cash bonuses to its employees and, prior to 2017, annual cash bonuses to its employees. ARX also has an equity compensation plan under which it has granted stock option awards, exercisable for shares of ARX common stock, to certain of its key employees. These stock option awards include both nonqualified and incentive stock options; all such stock options are subject to the put and call provisions of the ARX stockholders’ agreement (See Note 15 – Redeemable Noncontrolling Interest for further discussion ). As a result of these provisions, and the determination that the ultimate settlement of these awards would be in cash, the ARX stock options are treated as liability awards for accounting purposes. The amounts charged to income for Progressive and ARX incentive compensation plans for the years ended December 31, were: 2017 2016 2015 (millions) Pretax After Tax Pretax After Tax Pretax After Tax Non-equity incentive plans – cash $ 461.3 $ 299.8 $ 386.8 $ 251.4 $ 337.7 $ 219.5 Equity incentive plans: Equity awards 92.9 60.4 80.9 52.6 64.5 41.9 Liability awards 2.5 1.6 4.3 2.8 1.7 1.1 The increase in expense for the equity awards during 2017 primarily reflects an increase in management’s expectation of the percentage of certain performance-based awards that will ultimately vest (discussed below). The after-tax amounts are determined using the 35% corporate federal tax rate; the revaluation to the 21% rate is reflected in the total revaluation adjustment recorded at December 31, 2017 (see Note 5 – Income Taxes for further discussion). Progressive’s 2003 Incentive Plan has expired, and no new awards may be made under this plan; all awards granted prior to the plan’s expiration have vested, been forfeited, or expired, prior to December 31, 2015. Progressive’s 2010 Equity Incentive Plan and 2015 Equity Incentive Plan, which provide for the granting of equity-based compensation to officers and other key employees, originally authorized awards for up to 18.0 million shares and up to 13.0 million shares, respectively. The restricted equity awards are issued as either time-based or performance-based awards. All restricted stock units are settled at or after vesting in Progressive common shares from existing treasury shares on a one-to-one basis. The time-based awards vest in equal installments upon the lapse of specified periods of time, typically three , four , and five years. The performance-based awards were granted to approximately 45 Progressive executives and senior managers (including certain ARX managers) in 2017 in addition to their time-based awards, if applicable, to provide additional incentive to achieve pre-established profitability and growth targets, relative investment performance, or specific growth measures. Vesting of performance-based awards is contingent upon the achievement of predetermined performance goals within specified time periods. The targets for the performance-based awards, as well as the number of units that ultimately may vest, vary by grant. All performance-based awards include a specified number of units that will vest if performance meets a specified target and minimum performance goals. If at least the minimum performance goals are achieved, the range at which an award can vest is determined by the type of measurement goals included in the award, as follows: Performance Measurement Year(s) of Grant Vesting range (as a percentage of target) Growth of our personal and commercial auto businesses, compared to market 2013-2017 0-250% 2012 0-200% Investment results relative to peer group 2012-2017 0-200% Growth in percentage of auto policies bundled with other specified types of policies 2015 0% or 100-200% Unit growth in a particular customer segment 2016-2017 0% or 85-150% Generally, time-based and performance-based equity awards are expensed pro rata over their respective vesting periods based on the market value of the awards at the time of grant. Performance-based equity awards that contain variable vesting criteria are expensed based on management’s expectation of the percentage of the award, if any, that will ultimately vest. These estimates can change periodically throughout the measurement period. A summary of all employee restricted equity award activity during the years ended December 31, follows: 2017 2016 2015 Restricted Equity Awards Number of Shares 1 Weighted Average Grant Date Fair Value Number of Shares 1 Weighted Average Grant Date Fair Value Number of Shares 1 Weighted Average Grant Date Fair Value Beginning of year 6,951,373 $ 26.18 7,725,227 $ 23.37 9,051,564 $ 21.27 Add (deduct): Granted 2 2,383,475 32.01 1,870,660 31.54 2,489,976 25.20 Vested (3,220,671 ) 22.53 (2,422,700 ) 21.50 (3,682,644 ) 19.53 Forfeited (255,329 ) 28.03 (221,814 ) 24.64 (133,669 ) 21.63 End of year 3,4 5,858,848 $ 30.47 6,951,373 $ 26.18 7,725,227 $ 23.37 1 Includes restricted stock units; 2015 also includes restricted stock. Upon vesting, all units will be converted on a one-for-one basis into Progressive common shares funded from existing treasury shares. All performance-based awards are included at their target amounts. 2 We reinvest dividend equivalents on restricted stock units. For 2017 , 2016 , and 2015 , the number of units “granted” shown in the table above includes 157,396 , 165,045 , and 196,947 of dividend equivalent units, respectively, at a weighted average grant date fair value of $0 , since the dividends were factored into the grant date fair value of the original grant. 3 At December 31, 2017 , the number of shares included 1,513,779 performance-based units at their target amounts. We expect 3,244,111 units to vest based upon our current estimates of the likelihood of achieving the pre-determined performance goals applicable to each award. 4 At December 31, 2017 , the total unrecognized compensation cost related to unvested equity awards was $87.1 million , which includes performance-based awards at their currently estimated vesting value. This compensation expense will be recognized into the income statement over the weighted average vesting period of 2.2 years. The aggregate fair value of the restricted equity awards that vested during the years ended December 31, 2017 , 2016 , and 2015 , was $130.5 million , $77.0 million , and $105.4 million , respectively, based on the actual stock price on the applicable vesting date. As a result of the put and call rights described in Note 15 – Redeemable Noncontrolling Interest , all outstanding stock options awarded to ARX employees prior to April 1, 2015, are treated as liability awards for accounting purposes; however, the awards maintain the specific features per the original award agreements. The value of each option is based upon our good faith estimate of the fair market value as of the end of the reporting period and the pro rata expense is recognized. A summary of all ARX employee stock option activity since acquisition, follows: 2017 2016 2015 Options Outstanding Number of Shares Weighted Average Number of Shares Weighted Average Number of Shares Weighted Average Beginning of year 24,995 $ 526.46 24,995 $ 526.46 NA NA At acquisition date 4/1/2015 NA NA NA NA 26,000 $ 513.72 Add (deduct): Exercised 1 (2,445 ) 174.65 0 0 (1,005 ) 197.01 End of year 22,550 $ 564.60 24,995 $ 526.46 24,995 $ 526.46 Exercisable, end of year 17,950 $ 517.75 16,995 $ 438.77 12,995 $ 386.69 NA = Not Applicable 1 At the time of exercise in 2017 and 2015, the value earned by the option holders was $2.9 million and $1.1 million , respectively. 2017 2016 2015 Non-Vested Options Outstanding Number of Shares Weighted Average Number of Shares Weighted Average Number of Shares Weighted Average Beginning of year 8,000 $ 712.74 12,000 $ 677.81 NA NA At acquisition date 4/1/2015 NA NA NA NA 14,800 $ 675.55 Add (deduct): Vested (3,400 ) 665.79 (4,000 ) 607.95 (2,800 ) 665.85 End of year 1 4,600 $ 747.45 8,000 $ 712.74 12,000 $ 677.81 NA = Not Applicable 1 At December 31, 2017, 2016, and 2015, the remaining unrecognized compensation cost related to unvested options was $0.7 million , $1.6 million , and $2.9 million , respectively, and the remaining weighted average vesting period on the unvested awards was 1.01 years, 1.36 years, and 1.72 years, respectively. Incentive Compensation Plans – Directors Progressive’s 2003 Directors Equity Incentive Plan, has expired and no new awards may be made under this plan; in 2017, shareholders approved the Progressive 2017 Directors Equity Incentive Plan and it provides for the granting of equity-based awards, including restricted stock awards, to non-employee directors, and originally authorized awards for up to 0.5 million shares. Beginning in 2016, The Progressive Corporation permits each non-employee director to indicate a preference to receive either 100% of their compensation in the form of a restricted stock award or 60% in the form of a restricted stock award and 40% in the form of cash. If the director does not state a preference, it is presumed that he or she preferred to receive 100% of their compensation in the form of restricted stock. After considering such preferences, the Board’s Compensation Committee determines the awards (restricted stock, or restricted stock and cash) for each non-employee director. Prior to 2016, The Progressive Corporation granted restricted stock awards as the sole form of compensation to non-employee directors. The restricted stock awards are issued as time-based awards. The vesting period (i.e., requisite service period) is typically 11 months from the date of each grant. To the extent a director is newly appointed during the year, or a director’s committee assignments change, the vesting period may be shorter. Both the restricted stock awards and cash, if elected, are expensed pro rata over their respective vesting periods based on the market value of the awards at the time of grant. A summary of all directors’ restricted stock activity during the years ended December 31, follows: 2017 2016 2015 Restricted Stock Number of Shares Weighted Average Grant Date Fair Value Number of Shares Weighted Average Grant Date Fair Value Number of Shares Weighted Average Grant Date Fair Value Beginning of year 55,839 $ 33.24 89,427 $ 27.23 81,579 $ 25.45 Add (deduct): Granted 53,284 40.54 55,839 33.24 89,427 27.23 Vested (55,839 ) 33.24 (89,427 ) 27.23 (81,579 ) 25.45 End of year 53,284 $ 40.54 55,839 $ 33.24 89,427 $ 27.23 The aggregate fair value of the restricted stock vested, during the years ended December 31, 2017 , 2016 , and 2015 , was $2.2 million , $3.0 million , and $2.2 million , respectively, based on the actual stock price at time of vesting. Deferred Compensation The Progressive Corporation Executive Deferred Compensation Plan (Deferral Plan) permits eligible Progressive executives to defer receipt of some or all of their annual incentive payments and all of their annual equity awards. Deferred cash compensation is deemed invested in one or more investment funds, including Progressive common shares, offered under the Deferral Plan and elected by the participant. All Deferral Plan distributions attributable to deferred cash compensation will be paid in cash. For all equity awards granted in or after March 2005, and deferred pursuant to the Deferral Plan, the deferred amounts are deemed invested in our common shares and are ineligible for transfer to other investment funds in the Deferral Plan; distributions of these deferred awards will be made in Progressive common shares. For all restricted stock awards granted prior to that date, the deferred amounts are eligible to be transferred to any of the investment funds in the Deferral Plan; distributions of these deferred awards will be made in cash. We reserved 11.1 million of our common shares for issuance under the Deferral Plan. An irrevocable grantor trust has been established to provide a source of funds to assist us in meeting our liabilities under the Deferral Plan. The Deferral Plan Irrevocable Grantor Trust account held the following assets at December 31: (millions) 2017 2016 Progressive common shares 1 $ 128.2 $ 122.2 Other investment funds 2 167.0 136.9 Total $ 295.2 $ 259.1 1 Included 4.4 million and 4.7 million common shares as of December 31, 2017 and 2016 , respectively, to be distributed in common shares. 2 Amount is included in other assets on the balance sheet.

Segment Information

Segment Information	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Segment Information	SEGMENT INFORMATION We write personal and commercial auto insurance, residential property insurance, and other specialty property-casualty insurance and related services. Our Personal Lines segment writes insurance for personal autos and recreational vehicles (our special lines products). The Personal Lines segment is comprised of both the Agency and Direct businesses. The Agency business includes business written by our network of more than 35,000 independent insurance agencies, including brokerages in New York and California, and strategic alliance business relationships (other insurance companies, financial institutions, and national agencies). The Direct business includes business written directly by us online, by phone, or on mobile devices. We operate our personal auto businesses throughout the United States. In 2017, we ceased writing and servicing personal auto physical damage and auto property damage liability insurance in Australia. Our Commercial Lines segment writes primary liability and physical damage insurance for automobiles and trucks owned and/or operated predominantly by small businesses in the business auto, for-hire transportation, contractor, for-hire specialty, tow, and for-hire livery markets. This segment operates throughout the United States and is distributed through both the independent agency and direct channels. Our Property segment writes residential property insurance for homeowners, other property owners, and renters primarily through the independent agency channel in 41 states and the District of Columbia as of December 31, 2017 . Our Property business primarily consists of the operations of ASI and other insurance subsidiaries. ASI also acts as a participant in the “Write Your Own” program for the National Flood Insurance Program and, as such, writes flood insurance in 43 states and the District of Columbia. Our other indemnity businesses manage our run-off businesses. Our service businesses provide insurance-related services, including processing CAIP business, and serving as an agent for homeowners, general liability, and workers’ compensation insurance, among other products, through our programs with ASI and unaffiliated insurance companies. All segment revenues are generated from external customers and we do not have a reliance on any major customer. All intercompany transactions, including those between Progressive and ASI, are eliminated in consolidation. We evaluate profitability based on pretax underwriting profit (loss) for the Personal Lines, Commercial Lines, and Property segments and for the other indemnity businesses. Pretax underwriting profit (loss) is calculated as net premiums earned plus fees and other revenues, less: (i) losses and loss adjustment expenses; (ii) policy acquisition costs; and (iii) other underwriting expenses. Service business pretax profit (loss) is the difference between service business revenues and service business expenses. Expense allocations are based on certain assumptions and estimates primarily related to revenue and volume; stated segment operating results would change if different methods were applied. We do not allocate assets or income taxes to operating segments. In addition, we do not separately identify depreciation expense by segment, and such allocation would be impractical. Companywide depreciation expense was $169.9 million in 2017 , $137.4 million in 2016 , and $103.7 million in 2015 . The accounting policies of the operating segments are the same as those described in Note 1 – Reporting and Accounting Policies . Following are the operating results for the years ended December 31: 2017 2016 2015 (millions) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Personal Lines Agency $ 11,177.6 $ 839.6 $ 9,791.7 $ 492.8 $ 9,108.6 $ 713.2 Direct 10,769.6 683.7 9,396.5 412.2 8,185.9 403.4 Total Personal Lines 1 21,947.2 1,523.3 19,188.2 905.0 17,294.5 1,116.6 Commercial Lines 2,793.9 214.1 2,421.3 155.2 1,995.9 318.3 Property 2 988.8 (50.3 ) 864.5 32.5 609.1 61.3 Other indemnity 3 0 (0.2 ) 0 (1.6 ) (0.4 ) (1.0 ) Total underwriting operations 25,729.9 1,686.9 22,474.0 1,091.1 19,899.1 1,495.2 Fees and other revenues 4 370.6 NA 332.5 NA 302.0 NA Service businesses 126.8 17.3 103.3 11.3 86.3 8.8 Investments 5 612.7 588.8 530.0 507.6 567.3 544.5 Other gains (losses) (1.0 ) (1.0 ) 1.6 1.6 (0.9 ) (0.9 ) Interest expense NA (153.1 ) NA (140.9 ) NA (136.0 ) Consolidated total $ 26,839.0 $ 2,138.9 $ 23,441.4 $ 1,470.7 $ 20,853.8 $ 1,911.6 NA = Not Applicable 1 Personal auto insurance accounted for 93% of the total Personal Lines segment net premiums earned in 2017 , compared to 92% in 2016 and 2015 ; insurance for our special lines products (e.g., motorcycles, ATVs, RVs, watercraft, and snowmobiles) accounted for the balance of the Personal Lines net premiums earned. 2 We began reporting our Property business as a segment on April 1, 2015, upon acquisition of a controlling interest in ARX; therefore, the year ended 2015 only includes results for nine months and is not comparable to results reported for 2017 or 2016. During 2017 , 2016 , and 2015, pretax profit (loss) also includes $66.2 million , $62.1 million , and $46.8 million , respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX. Although this expense is included in our Property segment, it is not reported in the consolidated results of ARX and, therefore, will not affect the value of the net income attributable to the noncontrolling interest. 3 Our professional liability group recognized $0.4 million of reinstatement premiums paid to our reinsurers pursuant to their reinsurance contracts during 2015. This premium reduction was reflected in our companywide total results. 4 Pretax profit (loss) for fees and other revenues are allocated to operating segments. 5 Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit is net of investment expenses. Our management uses underwriting margin and combined ratio as primary measures of underwriting profitability. Underwriting profitability is calculated by subtracting losses and loss adjustment expenses, policy acquisition costs, and other underwriting expenses from the total of net premiums earned and fees and other revenues. The underwriting margin is the pretax underwriting profit (loss) expressed as a percentage of net premiums earned (i.e., revenues from underwriting operations). Combined ratio is the complement of the underwriting margin. Following are the underwriting margins/combined ratios for our underwriting operations for the years ended December 31: 2017 2016 2015 Underwriting Margin Combined Ratio Underwriting Margin Combined Ratio Underwriting Margin Combined Ratio Personal Lines Agency 7.5 % 92.5 5.0 % 95.0 7.8 % 92.2 Direct 6.3 93.7 4.4 95.6 4.9 95.1 Total Personal Lines 6.9 93.1 4.7 95.3 6.5 93.5 Commercial Lines 7.7 92.3 6.4 93.6 15.9 84.1 Property 1 (5.1 ) 105.1 3.8 96.2 10.1 89.9 Total underwriting operations 6.6 93.4 4.9 95.1 7.5 92.5 1 We began reporting our Property business as a segment on April 1, 2015, when we acquired a controlling interest in ARX; therefore, the year ended 2015 only includes results for nine months and is not comparable to results reported for 2017 or 2016. Included in 2017 , 2016 , and 2015 is 6.7 points, 7.2 points, and 7.7 points, respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX.

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Other Comprehensive Income (Loss)	OTHER COMPREHENSIVE INCOME (LOSS) The components of other comprehensive income (loss), including reclassification adjustments by income statement line item, for the years ended December 31, were as follows: Components of Changes in Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income Total tax (provision) benefit After tax total accumulated other comprehensive income Total net unrealized gains (losses) on securities Net unrealized gains on forecasted transactions Foreign currency translation adjustment Loss attributable to NCI Total at December 31, 2016 $ 1,439.5 $ (506.1 ) $ 933.4 $ 939.6 $ (9.4 ) $ (1.1 ) $ 4.3 Other comprehensive income (loss) before reclassifications: Investment securities 636.9 (224.0 ) 412.9 412.9 0 0 0 Net non-credit related OTTI losses, adjusted for valuation changes 0 0 0 0 0 0 0 Forecasted transactions (8.0 ) 2.8 (5.2 ) 0 (5.2 ) 0 0 Foreign currency translation adjustment 0.4 (0.1 ) 0.3 0 0 0.3 0 Loss attributable to noncontrolling interest (NCI) (3.5 ) 1.2 (2.3 ) 0 0 0 (2.3 ) Total other comprehensive income (loss) before reclassifications 625.8 (220.1 ) 405.7 412.9 (5.2 ) 0.3 (2.3 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (14.9 ) 5.3 (9.6 ) (9.6 ) 0 0 0 Net realized gains (losses) on securities 103.3 (36.2 ) 67.1 67.1 0 0 0 Other gains (losses) 1 (1.2 ) 0.4 (0.8 ) 0 0 (0.8 ) 0 Interest expense 0.3 (0.1 ) 0.2 0 0.2 0 0 Total reclassification adjustment for amounts realized in net income 87.5 (30.6 ) 56.9 57.5 0.2 (0.8 ) 0 Total other comprehensive income (loss) 538.3 (189.5 ) 348.8 355.4 (5.4 ) 1.1 (2.3 ) Total at December 31, 2017 $ 1,977.8 $ (695.6 ) $ 1,282.2 $ 1,295.0 $ (14.8 ) $ 0 $ 2.0 1 During 2017, we ceased writing insurance in Australia resulting in a loss of $1.2 million relating to the foreign currency translation adjustment. The loss is netted against a gain on extinguishment of debt (see Note 4 – Debt) in other gains (losses) on our consolidated statements of comprehensive income for the year end December 31, 2017. Components of Changes in Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income Total tax (provision) benefit After tax total accumulated other comprehensive income Total net unrealized gains (losses) on securities Net unrealized gains on forecasted transactions Foreign currency translation adjustment Loss attributable to NCI Total at December 31, 2015 $ 1,234.5 $ (434.1 ) $ 800.4 $ 809.0 $ (8.2 ) $ (1.5 ) $ 1.1 Other comprehensive income (loss) before reclassifications: Investment securities 320.5 (112.6 ) 207.9 207.9 0 0 0 Net non-credit related OTTI losses, adjusted for valuation changes (0.1 ) 0.1 0 0 0 0 0 Forecasted transactions 0 0 0 0 0 0 0 Foreign currency translation adjustment 0.6 (0.2 ) 0.4 0 0 0.4 0 Loss attributable to noncontrolling interest (NCI) 5.1 (1.9 ) 3.2 0 0 0 3.2 Total other comprehensive income (loss) before reclassifications 326.1 (114.6 ) 211.5 207.9 0 0.4 3.2 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (27.1 ) 9.5 (17.6 ) (17.6 ) 0 0 0 Net realized gains (losses) on securities 146.3 (51.4 ) 94.9 94.9 0 0 0 Other gains (losses) 0 0 0 0 0 0 0 Interest expense 1.9 (0.7 ) 1.2 0 1.2 0 0 Total reclassification adjustment for amounts realized in net income 121.1 (42.6 ) 78.5 77.3 1.2 0 0 Total other comprehensive income (loss) 205.0 (72.0 ) 133.0 130.6 (1.2 ) 0.4 3.2 Total at December 31, 2016 $ 1,439.5 $ (506.1 ) $ 933.4 $ 939.6 $ (9.4 ) $ (1.1 ) $ 4.3 Components of Changes in Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income Total tax (provision) benefit After tax total accumulated other comprehensive income Total net unrealized gains (losses) on securities Net unrealized gains on forecasted transactions Foreign currency translation adjustment Loss attributable to NCI Total at December 31, 2014 $ 1,574.0 $ (550.9 ) $ 1,023.1 $ 1,021.9 $ 1.5 $ (0.3 ) $ 0 Other comprehensive income (loss) before reclassifications: Investment securities (198.7 ) 67.5 (131.2 ) (131.2 ) 0 0 0 Net non-credit related OTTI losses, adjusted for valuation changes 0 0 0 0 0 0 0 Forecasted transactions (12.9 ) 4.5 (8.4 ) 0 (8.4 ) 0 0 Foreign currency translation adjustment (1.8 ) 0.6 (1.2 ) 0 0 (1.2 ) 0 Loss attributable to noncontrolling interest (NCI) 1.6 (0.5 ) 1.1 0 0 0 1.1 Total other comprehensive income (loss) before reclassifications (211.8 ) 72.1 (139.7 ) (131.2 ) (8.4 ) (1.2 ) 1.1 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (23.8 ) 8.4 (15.4 ) (15.4 ) 0 0 0 Net realized gains (losses) on securities 149.7 (52.5 ) 97.2 97.1 0.1 0 0 Other gains (losses) 0 0 0 0 0 0 0 Interest expense 1.8 (0.6 ) 1.2 0 1.2 0 0 Total reclassification adjustment for amounts realized in net income 127.7 (44.7 ) 83.0 81.7 1.3 0 0 Total other comprehensive income (loss) (339.5 ) 116.8 (222.7 ) (212.9 ) (9.7 ) (1.2 ) 1.1 Total at December 31, 2015 $ 1,234.5 $ (434.1 ) $ 800.4 $ 809.0 $ (8.2 ) $ (1.5 ) $ 1.1 In an effort to manage interest rate risk, we entered into forecasted transactions on each of The Progressive Corporation’s outstanding debt issuances. We expect to reclassify $1.0 million (pretax) into interest expense during the next 12 months, related to net unrealized losses on forecasted transactions (see Note 4 – Debt for further discussion).

Litigation

Litigation	12 Months Ended
	Dec. 31, 2017
Litigation Disclosures [Abstract]
Litigation	LITIGATION The Progressive Corporation and/or its insurance subsidiaries are named as defendants in various lawsuits arising out of claims made under insurance policies written by our insurance subsidiaries in the ordinary course of business. We consider all legal actions relating to such claims in establishing our loss and loss adjustment expense reserves. In addition, The Progressive Corporation and/or its insurance and other subsidiaries have been named as defendants in a number of class action or individual lawsuits that challenge certain of the operations of the subsidiaries. Other insurance companies face many of these same issues. We describe litigation contingencies for which a loss is probable. In addition, we establish accruals for these lawsuits when we can reasonably estimate potential loss exposure, which may include a range of loss, and we will disclose such amount or range of loss if material. As to lawsuits for which the loss is considered probable but not estimable, we do not establish an accrual. Nevertheless, we continue to evaluate this pending litigation to determine if any losses not deemed probable and estimable become so, at which point we would establish an accrual at our best estimate of the loss or range of loss. We also describe litigation contingencies for which a loss is reasonably possible (but not probable). When disclosing reasonably possible litigation contingencies, we will disclose the amount or range of possible loss, if we are able to make that determination and if material. We review all reasonably possible losses on an ongoing basis to determine whether the likelihood of incurring a loss has become probable, or whether the circumstances have changed such that we may now reasonably estimate a range of loss. We may also be exposed to litigation contingencies that are remote. Remote litigation contingencies are those for which the likelihood of a loss is slight at period end. We do not disclose, or establish accruals for, remote litigation contingencies, but we evaluate these contingencies on an ongoing basis to determine whether the likelihood of a loss has increased. Each year, certain of our pending litigation matters are brought to conclusion and/or settlement. Many of these concluded matters involve the same or similar fact patterns as the matters described below. For cases that have settled, but for which settlement is not complete, an accrual is established at our best estimate of the loss exposure. We regularly review these and other accruals to ensure they are adequate, and that there is not the possibility of material losses in excess of our accruals. Settlements that are complete are fully reflected in our financial statements. The amounts accrued and/or paid for settlements during the periods presented were not material to our consolidated financial condition, cash flows, or results of operations. The pending lawsuits summarized below are in various stages of development, and the outcomes are uncertain at this time. At period end, except to the extent an immaterial accrual has been established, we do not consider the losses from these pending cases to be both probable and estimable, and we are unable to estimate a range of loss at this time. It is not possible to determine loss exposure for a number of reasons, including, without limitation, one or more of the following: liability appears to be remote; putative class action lawsuits generally pose immaterial exposure until a class is actually certified, which, historically, has not been granted by the courts in the vast majority of our cases in which class certification has been sought; class definitions are often indefinite and preclude detailed exposure analysis; and complaints rarely state an amount sought as relief, and when such amount is stated, it is often a function of pleading requirements and may be unrelated to the potential exposure. We plan to contest these suits vigorously, but may pursue settlement negotiations in some cases, if appropriate. In the event that any one or more of these cases results in a substantial judgment against, or settlement by, us or if our accruals (if any) prove to be inadequate, the resulting liability could have a material adverse effect on our consolidated financial condition, cash flows, and/or results of operations. Based on information currently known, we do not believe that the outcome of any pending cases described below will have a material impact on our consolidated financial condition, cash flows, and/or results of operations. At December 31, 2017, pending lawsuits as described above that challenge certain of the operations of our subsidiaries included: Lawsuits seeking class/collective action status : • alleging we sell or charge insureds for illusory coverage or coverage lower than amounts allowed by law for personal injury protection (PIP) coverage and pay related claims at levels lower than allowed by law. • challenging how physical damage claims are handled, adjusted and ultimately paid, including how we value total loss claims, the payment of fees and taxes associated with total losses, and the payment of diminution damages. • challenging our practice in Florida of adjusting PIP payments. • challenging our assessment of fees. • challenging our adjustment of medical bills submitted by insureds in medical claims. • challenging the manner in which we grant a discount for anti-theft devices. • challenging our claim settlement practices where the tortfeasor and the injured party are both Progressive insureds. • challenging general claim practices, such as those relating to subrogation, rental reimbursement, and setting off certain claim payments based on other coverages and payments. • challenging our payment and reimbursement practices to Medicare Advantage Plans on first party medical, PIP, and bodily injury claims. • alleging we improperly sell secondary PIP coverage to Medicare and Medicaid beneficiaries in New Jersey. • challenging our rating practices. Lawsuits certified or conditionally certified as class/collective actions : • alleging that we undervalued total loss claims through the use of certain valuation tools. • alleging that we fail to provide proper uninsured motorist coverage. • challenging our practice in Florida of adjusting PIP payments. • alleging that we fail to pay overtime to certain employees who we classify as exempt from overtime pay requirements. Individual lawsuits : • challenging the estimation of physical damage and payment practices for physical damage repairs, and allegations of tortious interference with contract and insurance industry antitrust practice asserted by body shops outside our network program. • challenging, on a representative basis, certain of our pay practices.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	COMMITMENTS AND CONTINGENCIES We have certain noncancelable operating lease commitments with lease terms greater than one year for property and computer equipment. The minimum commitments under these agreements at December 31, 2017 , were as follows: (millions) Commitments 2018 $ 53.6 2019 44.2 2020 28.5 2021 16.9 2022 8.0 Thereafter 1.1 Total $ 152.3 Some of the leases have options to renew at the end of the lease periods. The expense we incurred for the leases disclosed above, as well as other operating leases that may be cancelable or have terms less than one year, was: (millions) Expense 2017 $ 77.2 2016 72.9 2015 66.6 We also have certain noncancelable purchase obligations. The minimum commitment under these agreements at December 31, 2017 , was $546.3 million . The insurance operations of ARX have several multiple-layer property catastrophe excess of loss reinsurance contracts with various reinsurers with terms ranging from one to three years. The minimum commitment under these contracts was $90.7 million at December 31, 2017.

Dividends

Dividends	12 Months Ended
	Dec. 31, 2017
Dividends [Abstract]
Dividends	DIVIDENDS We maintain a policy of paying an annual variable dividend that, if declared, would be payable shortly after the close of the year. This annual variable dividend is based on a target percentage of after-tax underwriting income multiplied by a performance factor (Gainshare factor), which, beginning in 2017, is determined by reference to the Agency auto, Direct auto, special lines, Commercial Lines, and Property business units, with minor exclusions and adjustments, and subject to the limitations discussed below. The target percentage is determined by our Board of Directors on an annual basis and announced to shareholders and the public. In December 2016 , the Board determined the target percentage for 2017 to be 33-1/3% of annual after-tax underwriting income, which is unchanged from the target percentage in both 2016 and 2015 . Since the inception of our variable dividend, we have applied a tax rate of 35% to calculate the after-tax underwriting income. Beginning in 2018, we will apply a rate of 21% to calculate the after-tax underwriting income. The Gainshare factor can range from zero to two and is determined by comparing our operating performance for the specified business units for the year to certain predetermined profitability and growth objectives approved by the Compensation Committee of the Board. This Gainshare factor is also used in the annual cash incentive program currently in place for our employees (our “Gainsharing program”). For 2017 , the Gainshare factor was 1.79 , compared to 1.67 in 2016 and 1.60 in 2015 . Our annual dividend program will result in a variable payment to shareholders each year, subject to certain limitations. If the Gainshare factor is zero or if our comprehensive income is less than after-tax underwriting income, no dividend would be payable under our annual variable dividend policy. In addition, the ultimate decision on whether or not a dividend will be paid is in the discretion of the Board of Directors. The Board could decide to alter our policy, or not to pay the annual variable dividend, at any time prior to the declaration of the dividend for the year. Such an action by the Board could result from, among other reasons, changes in the insurance marketplace, changes in our performance or capital needs, changes in the U.S. federal income tax laws, disruptions of national or international capital markets, or other events affecting our business, liquidity, or financial position. Following is a summary of our shareholder dividends that were declared in the last three years: (millions, except per share amounts) Amount Dividend Type Declared Paid Per Total 1 Annual – Variable December 2017 February 2018 $ 1.1247 $ 655.1 Annual – Variable December 2016 February 2017 0.6808 395.4 Annual – Variable December 2015 February 2016 0.8882 519.2 1 Based on an estimate of shares outstanding as of the record date. For the dividends declared in December 2016 and 2015, we paid $395.4 million and $519.0 million , respectively.

Redeemable Noncontrolling Inter

Redeemable Noncontrolling Interest (Notes)	12 Months Ended
	Dec. 31, 2017
Noncontrolling Interest [Abstract]
Noncontrolling Interest Disclosure [Text Block]	REDEEMABLE NONCONTROLLING INTEREST In connection with the April 2015 acquisition of a controlling interest in ARX, The Progressive Corporation entered into a stockholders’ agreement with the other ARX stockholders. As part of the stockholders’ agreement, the minority ARX shareholders have the right to “put” their ARX shares to Progressive in two installments, one in early 2018 and one in early 2021, and Progressive has the ability to “call” a portion of the outstanding shares shortly thereafter. If these rights are exercised in full when available, our ownership stake in ARX capital stock will exceed 80% in 2018 and will reach 100% in 2021. The purchase prices for shares to be purchased by Progressive pursuant to these put or call rights will be determined by adding (A) the price per share paid at the closing on April 1, 2015, to (B) the product of the change in the fully diluted net tangible book value per share of ARX between December 31 , 2014 a nd December 31, 2017 (for the 2018 put or call purchases) or December 31, 2020 (for the 2021 put or call purchases) times a multiple of between 1.0 and 2.0 . The multiple will be determined based on the growth and profitability of ARX’s business over the applicable time period, pursuant to criteria included in the stockholders’ agreement. Among other provisions, the stockholders’ agreement also prohibits ARX from taking a number of actions, including the payment of dividends, without the consent of The Progressive Corporation and two other stockholders. Since these securities are redeemable upon the occurrence of an event that is not solely within the control of Progressive, we have recorded the redeemable noncontrolling interest (NCI) as mezzanine equity on our consolidated balance sheets, which represents the minority shares at the current estimated purchase price pursuant to the put and call provisions of the stockholders’ agreement. The estimated purchase price is based, in part, on the change in tangible net book value of ARX from December 31, 2014 to the balance sheet dates. The redeemable noncontrolling interest was initially recorded at a fair value of $411.5 million , representing the minority shares at the net acquisition price adjusted for the fair value of the put and call rights. The value of the put and call rights on the acquisition date was based on an internally developed modified binomial model. Subsequent changes to the redeemable noncontrolling interest are based on the maximum redemption value at the end of the reporting period, as determined in accordance with the stockholders’ agreement. In addition to these minority shares, at December 31, 2017, ARX employees hold options to purchase 22,550 ARX shares. These options and any shares issued upon exercise are subject to the stockholders’ agreement, including the right to “put ” these shares to Progressive, as described above. Until the options are exercised, the underlying obligation of approximately $32.5 million is not recorded as part of redeemable NCI. The changes in the components of redeemable NCI during the year ended December 31, 2017, 2016, and 2015, were: (millions) December 31, 2017 December 31, 2016 December 31, 2015 Balance, Beginning of period $ 483.7 $ 464.9 $ 0 Fair value at date of acquisition 0 0 411.5 Net income attributable to NCI 5.9 26.2 32.9 Other comprehensive income (loss) attributable to NCI 2.3 (3.2 ) (1.1 ) Exercise of employee stock options 3.4 0 0 Purchase of shares from NCI 0 0 (12.6 ) Change in redemption value of NCI 8.4 (4.2 ) 34.2 Balance, End of period $ 503.7 $ 483.7 $ 464.9

Goodwill and Intangible Assets

Goodwill and Intangible Assets	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	GOODWILL AND INTANGIBLE ASSETS Goodwill Goodwill recorded at December 31, 2017 and 2016 was $452.7 million and $449.4 million , respectively. No accumulated goodwill impairment losses exist. During 2017 and 2016, the carrying amount of goodwill increased $3.3 million and $1.8 million , respectively. In 2017, we acquired a small excess and surplus lines insurance company to provide us flexibility in our Commercial Lines business. In 2016, ARX entered into an exchange transaction with a third party, whereby ARX acquired 100% of the equity interest in a residential property insurance company and disposed of 100% of the equity interest in a commercial property insurance company. Intangible Assets The following table is a summary of the net carrying amount of other intangible assets as of December 31, 2017 and 2016 : (millions) December 31, 2017 December 31, 2016 Intangible assets subject to amortization $ 354.2 $ 420.4 Indefinite-lived intangible assets 1 12.4 12.4 Total $ 366.6 $ 432.8 1 Indefinite-lived intangible assets are comprised of state insurance and agent licenses. State insurance licenses were previously subject to amortization under superseded accounting guidance and have $0.6 million of accumulated amortization for both periods presented. Intangible assets subject to amortization consisted of the following: (millions) December 31, 2017 December 31, 2016 Category Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Policies in force $ 256.2 $ 100.7 $ 155.5 $ 256.2 $ 64.1 $ 192.1 Agency relationships 159.2 31.3 127.9 159.2 19.9 139.3 Software rights 79.1 29.4 49.7 79.1 18.8 60.3 Trade name 34.8 13.7 21.1 34.8 6.1 28.7 Total $ 529.3 $ 175.1 $ 354.2 $ 529.3 $ 108.9 $ 420.4 Amortization expense was $66.2 million , $62.1 million , and $46.8 million for the years ended December 31, 2017 , 2016 , and 2015, respectively. During 2017, we revised our estimate of the economic useful life of our trade name intangible asset from an original life of 10 years to a remaining life of 2 years . The decrease in the useful life represents the estimated length of time that it is expected to take to transition the branding of our Property business from the ASI trade name to “Progressive Home.” As of December 31, 2017, the remaining average life of all of our intangible assets was 4.9 years . The estimated aggregate amortization on these intangible assets for each of the next five years as of December 31, 2017, is as follows: (millions) Year Amortization Expense 2018 $ 71.9 2019 66.4 2020 56.9 2021 56.6 2022 29.2

Reporting and Accounting Poli28

Reporting and Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Nature Of Operations Policy	Nature of Operations The Progressive insurance organization began business in 1937. The Progressive Corporation, an insurance holding company, was formed in 1965. The financial results of The Progressive Corporation include its subsidiaries and affiliates (references to “subsidiaries” in these notes include affiliates as well). Our insurance subsidiaries (collectively the Progressive Group of Insurance Companies) provide personal and commercial auto insurance, residential property insurance, and other specialty property-casualty insurance and related services. Our Personal Lines segment writes insurance for personal autos and recreational vehicles, which we refer to as our special lines products, through both an independent insurance agency channel and a direct channel. Our Commercial Lines segment writes primary liability and physical damage insurance for automobiles and trucks owned and/or operated predominantly by small businesses through both the independent agency and direct channels. Our Property segment writes residential property insurance for homeowners, other property owners, and renters, primarily through the independent insurance agency channel.
Basis of Consolidation and Reporting	Basis of Consolidation and Reporting The accompanying consolidated financial statements include the accounts of The Progressive Corporation and ARX Holding Corp. (ARX), and their respective wholly owned insurance and non-insurance subsidiaries and affiliates, in which Progressive or ARX has a controlling financial interest. The Progressive Corporation owned 69.0% of the outstanding capital stock of ARX at December 31, 2017 and 69.2% at December 31, 2016 and 2015. The decrease reflects ARX employee stock options that were exercised during 2017. All intercompany accounts and transactions are eliminated in consolidation.
Estimates	Estimates We are required to make estimates and assumptions when preparing our financial statements and accompanying notes in conformity with accounting principles generally accepted in the United States of America (GAAP). As estimates develop into fact (e.g., losses are paid), results may, and will likely, differ from those estimates.
Investments	Investments Our fixed-maturity securities, equity securities, and short-term investments are accounted for on an available-for-sale basis. See Note 2 – Investments for details regarding the composition of our investment portfolio. Fixed-maturity securities include debt securities and redeemable preferred stocks, which may have fixed or variable principal payment schedules, may be held for indefinite periods of time, and may be used as a part of our asset/liability strategy or sold in response to changes in interest rates, anticipated prepayments, risk/reward characteristics, liquidity needs, or other economic factors. These securities are carried at fair value with the corresponding unrealized gains (losses), net of deferred income taxes, reported in accumulated other comprehensive income. Fair values are obtained from recognized pricing services or are quoted by market makers and dealers, with limited exceptions discussed in Note 3 – Fair Value . Included in the fixed-maturity portfolio are asset-backed securities. The asset-backed securities are generally accounted for under the retrospective method. The retrospective method recalculates yield assumptions (based on changes in interest rates or cash flow expectations) historically to the inception of the investment holding period, and applies the required adjustment, if any, to the cost basis, with the offset recorded to investment income. The prospective method is used primarily for interest-only securities, non-investment-grade asset-backed securities, and certain asset-backed securities with sub-prime loan exposure or where there is a greater risk of non-performance and where it is possible the initial investment may not be substantially recovered. The prospective method requires a calculation of expected future repayments and resets the yield to allow for future period adjustments; no current period impact to investment income or the security’s cost is made based on the cash flow update. Prepayment assumptions are updated quarterly. Equity securities include common stocks, nonredeemable preferred stocks, and other risk investments, and are reported at fair values. Changes in fair value of these securities, net of deferred income taxes, are reflected as unrealized gains (losses) in accumulated other comprehensive income. To the extent we hold any foreign equities or foreign currency hedges, any change in value due to exchange rate fluctuations would be limited by foreign currency hedges, if any, and would be recognized in income in the current period. Short-term investments may include Eurodollar deposits, commercial paper, repurchase transactions, and other securities expected to mature within one year. From time to time, we may also invest in municipal bonds that have maturity dates that are longer than one year, but have either liquidity facilities or mandatory put features within one year. Trading securities are securities bought principally for the purpose of sale in the near term. We do not hold any trading securities. To the extent we have trading securities, changes in fair value would be recognized in income in the current period. Derivative instruments, which may be used for trading purposes or classified as trading derivatives due to the characteristics of the transaction, are discussed below. Derivative instruments may include futures, options, forward positions, foreign currency forwards, interest rate swap agreements, and credit default swaps and may be used in the portfolio for general investment purposes or to hedge the exposure to: • Changes in fair value of an asset or liability (fair value hedge), • Foreign currency of an investment in a foreign operation (foreign currency hedge), or • Variable cash flows of a forecasted transaction (cash flow hedge). We did not have any derivatives outstanding at December 31, 2017 and 2016. To the extent we have derivatives held for general investment purposes, these derivative instruments are recognized as either assets or liabilities and measured at fair value, with changes in fair value recognized in income as a component of net realized gains (losses) on securities during the period of change. Derivatives designated as hedges are required to be evaluated on established criteria to determine the effectiveness of their correlation to, and ability to reduce the designated risk of, specific securities or transactions. Effectiveness is required to be reassessed regularly. Hedges that are deemed to be effective would be accounted for as follows: • Fair value hedge: changes in fair value of the hedge, as well as the hedged item, would be recognized in income in the period of change while the hedge is in effect. • Foreign currency hedge: changes in fair value of the hedge, as well as the hedged item, would be reflected as a change in translation adjustment as part of accumulated other comprehensive income. Gains and losses on the foreign currency hedge would offset the foreign exchange gains and losses on the foreign investment as they are recognized into income. • Cash flow hedge: changes in fair value of the hedge would be reported as a component of accumulated other comprehensive income and subsequently amortized into earnings over the life of the hedged transaction. If a hedge is deemed to become ineffective or discontinued, the following accounting treatment would be applied: • Fair value hedge: the derivative instrument would continue to be adjusted through income, while the adjustment in the change in value of the hedged item would be reflected as a change in unrealized gains (losses) as part of accumulated other comprehensive income. • Foreign currency hedge: changes in the value of the hedged item would continue to be reflected as a change in translation adjustment as part of accumulated other comprehensive income, but the derivative instrument would be adjusted through income for the current period. • Cash flow hedge: changes in fair value of the derivative instrument would be reported in income for the current period. For all derivative positions, net cash requirements are limited to changes in fair values, which may vary as a result of changes in interest rates, currency exchange rates, and other factors. Exposure to credit risk is limited to the carrying value; collateral may be required to limit credit risk. We have elected not to offset fair value amounts that arise from derivative positions with the same counterparty under a master netting arrangement. Investment securities are exposed to various risks such as interest rate, market, credit, and liquidity risk. Fair values of securities fluctuate based on the nature and magnitude of changing market conditions; significant changes in market conditions could materially affect the portfolio’s value in the near term. We regularly monitor our portfolio for price changes, which might indicate potential impairments, and perform detailed reviews of securities with unrealized losses. In such cases, changes in fair value are evaluated to determine the extent to which such changes are attributable to: (i) fundamental factors specific to the issuer, such as financial condition, business prospects, or other factors, (ii) market-related factors, such as interest rates or equity market declines, or (iii) credit-related losses, where the present value of cash flows expected to be collected are lower than the amortized cost basis of the security. We analyze our debt securities that are in a loss position to determine if we intend to sell, or if it is more likely than not that we will be required to sell, the security prior to recovery and, if so, we write down the security to its current fair value, with the entire amount of the write-down recorded to earnings. To the extent that it is more likely than not that we will hold the debt security until recovery (which could be maturity), we determine if any of the decline in value is due to a credit loss (i.e., where the present value of future cash flows expected to be collected is lower than the amortized cost basis of the security) and, if so, we recognize that portion of the impairment as a component of net realized gains (losses) in the comprehensive income statement, with the difference (i.e., non-credit related impairment) recognized as part of our net unrealized gains (losses) in accumulated other comprehensive income. When an equity security (common equity and nonredeemable preferred stock) in our investment portfolio has an unrealized loss in fair value that is deemed to be other-than-temporary, we reduce the book value of such security to its current fair value, recognizing the decline as a realized loss in the comprehensive income statement. Any future changes in fair value, either increases or decreases, are reflected as changes in unrealized gains (losses) as part of accumulated other comprehensive income. Investment income consists of interest, dividends, and accretion net of amortization. In addition to the discussion above for asset-backed securities, interest is recognized on an accrual basis using the effective yield method. Depending on the nature of the equity instruments, dividends are recorded at either the ex-dividend date or on an accrual basis. Realized gains (losses) on securities are computed based on the first-in first-out method and include write-downs on available-for-sale securities considered to have other-than-temporary declines in fair value (excluding non-credit related impairments), as well as holding period valuation changes on derivatives, trading securities, and hybrid instruments (e.g., securities with embedded options, where the option is a feature of the overall change in the value of the instrument).
Insurance Premiums and Receivables	Insurance Premiums and Receivables Insurance premiums written are earned into income on a pro rata basis over the period of risk, based on a daily earnings convention. Accordingly, unearned premiums represent the portion of premiums written that are applicable to the unexpired risk. We provide insurance and related services to individuals and small commercial accounts and offer a variety of payment plans. Generally, premiums are collected prior to providing risk coverage, minimizing our exposure to credit risk. For our vehicle businesses, we perform a policy level evaluation to determine the extent to which the premiums receivable balance exceeds the unearned premiums balance. We then age this exposure to establish an allowance for doubtful accounts based on prior experience. For our Property business, we do not establish an allowance for doubtful accounts since the risk of uncollectibility is relatively low. If premiums are unpaid by the policy due date, we provide advance notice of cancellation in accordance with each state’s requirements and, if the premiums remain unpaid after receipt of notice, cancel the policy and write off any remaining balance.
Deferred Acquisition Costs	Deferred Acquisition Costs Deferred acquisition costs include commissions, premium taxes, and other variable underwriting and direct sales costs incurred in connection with the successful acquisition or renewal of insurance contracts. These acquisition costs, net of ceding allowances, are deferred and amortized over the policy period in which the related premiums are earned. We consider anticipated investment income in determining the recoverability of these costs. Management believes that these costs will be fully recoverable in the near term.
Advertising Costs	We do not defer any advertising costs. Total advertising costs, which are expensed as incurred
Loss and Loss Adjustment Expense Reserves	Loss and Loss Adjustment Expense Reserves Loss reserves represent the estimated liability on claims reported to us, plus reserves for losses incurred but not recorded (IBNR). These estimates are reported net of amounts estimated to be recoverable from salvage and subrogation. Loss adjustment expense reserves represent the estimated expenses required to settle these claims and losses. The methods of making estimates and establishing these reserves are reviewed regularly, and resulting adjustments are reflected in income in the current period. Such loss and loss adjustment expense reserves are susceptible to change in the near term.
Reinsurance	Reinsurance Our reinsurance transactions include premiums ceded to “Regulated” plans and “Non-Regulated” plans. The Regulated plans in which we participate are governed by insurance regulations and include state-provided reinsurance facilities (Michigan Catastrophic Claims Association, Florida Hurricane Catastrophe Fund, North Carolina Reinsurance Facility), as well as state-mandated involuntary plans for commercial vehicles (Commercial Automobile Insurance Procedures/Plans – “CAIP”) and federally regulated plans for flood (National Flood Insurance Program – “NFIP”); we act as a servicing agent for CAIP and as a participant in the “Write Your Own” program for the NFIP. The Non-Regulated plans are voluntary contractual arrangements and primarily relate to our Property business and transportation network company business written by our Commercial Lines segment. Prepaid reinsurance premiums are earned on a pro rata basis over the period of risk, based on a daily earnings convention, which is consistent with premiums written. See Note 7 – Reinsurance for further discussion.
Income Taxes	Income Taxes The income tax provision is calculated under the balance sheet approach. Deferred tax assets and liabilities are recorded based on the difference between the financial statement and tax bases of assets and liabilities at the enacted tax rates. The principal items giving rise to such differences are investment securities (e.g., net unrealized gains (losses), write-downs on securities determined to be other-than-temporarily impaired), loss and loss adjustment expense reserves, unearned premiums reserves, deferred acquisition costs, property and equipment, intangible assets, and non-deductible accruals. We review our deferred tax assets regularly for recoverability. The effects of any changes in the tax rate are recorded to our provision for income taxes, including any changes on items initially recognized in accumulated other comprehensive income. See Note 5 – Income Taxes for further discussion.
Property and Equipment	Property and Equipment Property and equipment are recorded at cost, less accumulated depreciation, and include capitalized software developed or acquired for internal use. Depreciation is recognized over the estimated useful lives of the assets using accelerated methods for computer equipment and laptops and the straight-line method for all other fixed assets. We evaluate impairment whenever events or circumstances warrant such a review and write-off the impaired assets if appropriate. Land and buildings comprised 66% and 65% of total property and equipment at December 31, 2017 and 2016 , respectively. The useful lives for property and equipment at December 31, 2017 , were: Useful Lives Computer equipment and laptops 3 years Software licenses (internal use) 1-5 years Capitalized software 3-10 years Buildings, improvements, and integrated components 7-40 years All other property and equipment 3-15 years At December 31, 2017 and 2016, included in other assets in the consolidated balance sheets is $5.3 million and $8.7 million , respectively, of “held for sale” property, which represents the fair value of this property less the estimated costs to sell.
Goodwill and Intangible Assets	Goodwill and Intangible Assets Goodwill is the excess of the purchase price over the estimated fair value of the assets and liabilities acquired and represents the future economic benefits arising from other assets acquired that could not be individually identified and separately recognized. Substantially all of the goodwill recorded as of December 31, 2017 and 2016, relates to the April 1, 2015 acquisition of a controlling interest in ARX. Intangible assets primarily arose through the acquisition of ARX and mainly represent the future premiums that will be recognized from the policies and agency relationships that existed at the acquisition date. The majority of the intangible assets have finite lives, which, at December 31, 2017, had a remaining life range from 2 to 11 years. See Note 16 – Goodwill and Intangible Assets for further discussion. We evaluate our goodwill for impairment at least annually using a qualitative approach. If events or changes in circumstances indicate that the carrying value of goodwill or intangible assets may not be recoverable, we will evaluate such items for impairment using a quantitative approach.
Guaranty Fund Assessments	Guaranty Fund Assessments We are subject to state guaranty fund assessments, which provide for the payment of covered claims or other insurance obligations of insurance companies deemed insolvent. These assessments are accrued after a formal determination of insolvency has occurred, and we have written the premiums on which the assessments will be based. Assessments that are available for recoupment from policyholders are capitalized when incurred; all other assessments are expensed.
Fees and Other Revenues	Fees and Other Revenues Fees and other revenues primarily represent fees collected from policyholders relating to installment charges in accordance with our bill plans, as well as late payment and insufficient funds fees. Other revenues may include revenue from ceding commissions in excess of acquisition costs, the sale of tax credits, referral fees, rental income, and other revenue transactions. Fees and other revenues are generally earned when collected, except for excess ceding commissions, which are earned over the policy period.
Service Revenues and Expenses	Service Revenues and Expenses Our service businesses provide insurance-related services. Service revenues and expenses from our commission-based businesses are recorded in the period in which they are earned or incurred. Service revenues generated from processing business for involuntary CAIP plans are earned on a pro rata basis over the term of the related policies. Service expenses related to these CAIP plans are expensed as incurred.
Equity-Based Compensation	Equity-Based Compensation We issue time-based and performance-based restricted stock unit awards to key members of management (including members of ARX and its subsidiaries in 2017) as our form of equity compensation, and time-based restricted stock awards to non-employee directors. Collectively, we refer to these awards as “restricted equity awards.” Compensation expense for time-based restricted equity awards with installment vesting is recognized over each respective vesting period. For performance-based restricted equity awards, compensation expense is recognized over the respective estimated vesting periods. Dividend equivalent units are credited to outstanding restricted stock unit awards, both time-based and performance-based, at the time a dividend is paid to shareholders. We record an estimate for expected forfeitures of restricted equity awards based on our historical forfeiture rates. In addition, we shorten the vesting periods of certain time-based restricted equity awards based on the “qualified retirement” provisions in our equity compensation plans, under which (among other provisions) if the participant satisfies certain age and years-of-service requirements, the vesting and distribution of 50% of outstanding time-based restricted equity awards accelerates upon reaching eligibility for a qualified retirement and shortly after the grant date for each subsequent award. ARX has nonqualified and incentive stock options outstanding that were issued prior to April 2015 as a form of equity compensation to certain of the officers and employees of ARX and its subsidiaries. These outstanding stock options are subject to the put/call features contained in the current stockholders’ agreement, pursuant to which The Progressive Corporation has the right, and can be required, to purchase a portion or all the shares underlying these awards in 2018 and 2021. The vested stock options, and the shares issuable upon exercise of the stock options, are also subject to repurchase by ARX if the holder’s employment terminates. See Note 15 – Redeemable Noncontrolling Interest for further discussion . These stock options, which are treated for accounting purposes as liability awards, are expensed over the respective vesting periods based on the Black-Scholes value determined at period end.
Earnings Per Share	Earnings Per Share Net income attributable to Progressive is used in our calculation of the per share amounts. Basic earnings per share is computed using the weighted average number of common shares outstanding during the reporting period, excluding unvested time-based and performance-based restricted equity awards that are subject to forfeiture. Diluted earnings per share includes common stock equivalents assumed outstanding during the period. Our common stock equivalents include the incremental shares assumed to be issued for: • earned but unvested time-based restricted equity awards, and • certain unvested performance-based restricted equity awards that satisfied contingency conditions for common stock equivalents during the period.
Cash Flow, Supplemental Disclosures	Supplemental Cash Flow Information Cash and cash equivalents include bank demand deposits and daily overnight reverse repurchase commitments of funds held in bank demand deposit accounts on ARX’s subsidiaries, which are primarily collateralized by U.S. Treasury notes. The amount of reverse repurchase commitments held by ARX’s subsidiaries at December 31, 2017, 2016, and 2015, were $247.2 million , $150.0 million , and $174.8 million , respectively. Restricted cash on our consolidated balance sheets at December 31, 2017 and 2016, represents cash that is restricted to pay flood claims under the National Flood Insurance Program’s “Write Your Own” program, for which American Strategic Insurance and other subsidiaries of ARX (ASI) are administrators. Non-cash activity includes declared but unpaid dividends. The cash transferred in the exchange transaction, which occurred in June 2016, was revised to correct the reclassification of a non-cash transaction; there was no overall impact on the increase in cash, cash equivalents, and restricted cash that was reported in our consolidated statement of cash flows for the year ended December 31, 2016. See Note 16 – Goodwill and Intangible Assets for further discussion of the exchange transaction. For the years ended December 31, we paid the following: (millions) 2017 2016 2015 Income taxes $ 715.6 $ 459.4 $ 701.8 Interest 146.3 139.2 132.0
New Accounting Standards	New Accounting Standards Issued In January 2018, the Financial Accounting Standards Board (FASB) proposed an Accounting Standards Update (ASU), which would provide targeted improvements to the new lease accounting guidance issued by the FASB in February 2016 (the “2016 ASU”). The 2016 ASU, which eliminates the off-balance-sheet accounting for leases, will require lessees to report their operating leases as both an asset and liability on the statement of financial position and to disclose key information about leasing arrangements in the financial statement footnotes. Under the 2016 ASU, there will be no change to the recognition of lease expense in our results of operations. The ASU will be effective for fiscal years (including interim periods within those fiscal years) beginning after December 15, 2018 (2019 for calendar-year companies). Under the proposed guidance, companies would have the option to apply the new lease requirements either as of the effective date (i.e., January 1, 2019), with comparative information presented in accordance with the previous standard, or on a modified restrospective basis, which would restate all financial statement information as of the beginning of the earliest period presented and is the transition method under the 2016 ASU. Based on our lease portfolio at December 31, 2017, and in accordance with the accounting elections available in the ASU, we would have recorded an increase to assets and liabilities of approximately $140 million , and there would have been no impact on our results of operations or cash flows. Therefore, we do not expect this standard to have a material impact on our financial condition. In March 2017, the FASB issued an ASU related to premium amortization on purchased callable debt securities. The intent of the standard is to shorten the amortization period for certain purchased callable debt securities held at a premium. Under the ASU, the premium is required to be amortized to the earliest call date. The ASU more closely aligns interest income recorded on bonds held at a premium with the economics of the underlying instrument. The ASU, which is required to be applied on a modified retrospective basis, is effective for fiscal years beginning after December 15, 2018 (2019 for calendar-year companies), and interim periods within those fiscal years. Since we have historically used a yield-to-worst scenario for our securities that were purchased at a premium, and the first call on a premium security most often produces the lowest and most conservative yield, we do not expect this standard to have a significant impact on our financial condition, cash flows, or results of operations. In January 2017, the FASB issued an ASU, which eliminates the requirement to determine the implied value of goodwill in measuring an impairment loss. Upon adoption, the measurement of a goodwill impairment will represent the excess of the reporting unit’s carrying value over fair value, limited to the carrying value of goodwill. This ASU is effective for goodwill impairment tests in fiscal years beginning after December 15, 2019 (2020 for calendar-year companies), with early adoption permitted. We do not expect this standard to have a material impact on our financial position or results of operations. In June 2016, the FASB issued an ASU intended to improve the timing, and enhance the accounting and disclosure, of credit losses on financial assets. Additionally, this update will modify the existing accounting guidance related to the impairment evaluation for available-for-sale debt securities and will result in the creation of an allowance for credit losses as a contra asset account. The ASU will require cumulative-effect changes to retained earnings in the period of adoption, if any occur, and will also require prospective changes on previously recorded impairments. This ASU is effective for fiscal years (including interim periods within those fiscal years) beginning after December 15, 2019 (2020 for calendar-year companies), with early adoption permissible (including interim periods within that fiscal year) beginning after December 15, 2018 (2019 for calendar-year companies). While the ASU creates additional accounting complexities related to the recognition of the impairment losses, and subsequent recoveries, through an allowance for credit losses account, we do not expect that the ASU will have a material impact on our current method of evaluating securities for credit losses or the timing or recognition of the amounts of the impairment losses. In January 2016, the FASB released an ASU intended to improve the recognition and measurement of financial instruments. The new guidance will require the changes in fair value of equity securities to be recognized as a component of net income. The ASU is effective for fiscal years beginning after December 15, 2017 (2018 for calendar-year companies) and requires the prospective method of adoption with a cumulative-effect adjustment recorded to beginning retained earnings upon adoption. In January 2018, we recorded a cumulative-effect adjustment of $1.3 billion , which is net of taxes at the 35% tax rate. The cumulative-effect adjustment represents the amount of after-tax net unrealized gains on equity securities that was recorded as part of accumulated other comprehensive income at December 31, 2017. This ASU will have no impact on comprehensive income. Adopted For the year ended December 31, 2017, we adopted the ASU related to the statement of cash flows and the classification and presentation of changes in restricted cash. This update requires that the statement of cash flows explain the change during the period in the total of cash, cash equivalents, and amounts described as restricted cash. This ASU, which is required to be applied on a retrospective basis, is effective for fiscal years beginning after December 15, 2017, with early adoption permitted. We elected to early adopt this ASU. Since this standard only affected classification and presentation, there was no impact on our results of operations, financial condition, or cash flows. On January 1, 2017, we adopted the ASU to simplify the accounting for employee share-based payment transactions. There were several provisions that could be adopted under this ASU. We did not elect to make any changes to our method of recording forfeitures and are continuing to withhold taxes at the minimum statutory tax rate. We did elect, on a retrospective basis, to disclose the payment of cash to a taxing authority for which we withheld shares for this purpose as a financing activity. Lastly, during the year ended December 31, 2017, we recognized $25.1 million of excess tax benefits as an income tax benefit in our consolidated statements of comprehensive income; this provision was adopted on a prospective basis.

Reporting and Accounting Poli29

Reporting and Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Advertising Costs	Total advertising costs, which are expensed as incurred, for the years ended December 31, were: (millions) Advertising Costs 2017 $ 1,005.4 2016 756.2 2015 748.3
Property and Equipment	The useful lives for property and equipment at December 31, 2017 , were: Useful Lives Computer equipment and laptops 3 years Software licenses (internal use) 1-5 years Capitalized software 3-10 years Buildings, improvements, and integrated components 7-40 years All other property and equipment 3-15 years
Interest Capitalized	Total capitalized interest, which primarily relates to capitalized software projects, for the years ended December 31, was: (millions) Capitalized Interest 2017 $ 2.8 2016 2.9 2015 2.4
Equity Based Compensation and Related Tax Benefits	The total compensation expense recognized for equity-based compensation, both our equity and liability awards, for the years ended December 31, was: (millions) 2017 2016 2015 Pretax expense $ 95.4 $ 85.2 $ 66.2 Tax benefit 1 33.4 29.8 23.2 1 Reflected at the 35% corporate federal tax rate; the revaluation to the 21% rate is reflected in the total revaluation adjustment recorded at December 31, 2017 (see N ote 5 – Income Ta xes for further discussion).
Supplemental Cash Flow Information	For the years ended December 31, we paid the following: (millions) 2017 2016 2015 Income taxes $ 715.6 $ 459.4 $ 701.8 Interest 146.3 139.2 132.0

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2017
Investments Schedule [Abstract]
Investment Portfolio by Major Security Type	The following tables present the composition of our investment portfolio by major security type, consistent with our internal classification of how we manage, monitor, and measure the portfolio. The net holding period gains (losses) represent the amounts realized on our hybrid securities only (see discussion below). ($ in millions) Cost Gross Unrealized Gains Gross Unrealized Losses Net Realized Gains (Losses) Fair Value % of Total Fair Value December 31, 2017 Fixed maturities: U.S. government obligations $ 6,688.8 $ 1.1 $ (44.0 ) $ 0 $ 6,645.9 24.4 % State and local government obligations 2,285.6 20.7 (9.3 ) 0.1 2,297.1 8.4 Foreign government obligations 0 0 0 0 0 0 Corporate debt securities 4,997.2 14.8 (14.4 ) 0.1 4,997.7 18.3 Residential mortgage-backed securities 828.8 11.3 (3.4 ) 0 836.7 3.1 Commercial mortgage-backed securities 2,760.1 11.8 (13.3 ) 0 2,758.6 10.1 Other asset-backed securities 2,454.5 4.5 (4.5 ) 0.2 2,454.7 9.0 Redeemable preferred stocks 194.9 17.8 (1.5 ) (0.2 ) 211.0 0.8 Total fixed maturities 20,209.9 82.0 (90.4 ) 0.2 20,201.7 74.1 Equity securities: Nonredeemable preferred stocks 698.6 114.0 (8.8 ) 0 803.8 2.9 Common equities 1,499.0 1,901.0 (0.2 ) 0 3,399.8 12.5 Short-term investments 2,869.4 0 0 0 2,869.4 10.5 Total portfolio 1,2 $ 25,276.9 $ 2,097.0 $ (99.4 ) $ 0.2 $ 27,274.7 100.0 % ($ in millions) Cost Gross Unrealized Gains Gross Unrealized Losses Net Realized Gains (Losses) Fair Value % of Total Fair Value December 31, 2016 Fixed maturities: U.S. government obligations $ 2,899.2 $ 0 $ (29.1 ) $ 0 $ 2,870.1 12.2 % State and local government obligations 2,509.5 13.8 (20.7 ) 0 2,502.6 10.7 Foreign government obligations 24.5 0 0 0 24.5 0.1 Corporate debt securities 4,557.8 17.3 (24.3 ) 0.1 4,550.9 19.4 Residential mortgage-backed securities 1,489.7 23.7 (15.6 ) 1.5 1,499.3 6.4 Commercial mortgage-backed securities 2,266.9 12.0 (25.5 ) 0 2,253.4 9.6 Other asset-backed securities 2,350.7 4.6 (4.4 ) 0.2 2,351.1 10.0 Redeemable preferred stocks 188.8 5.1 (2.0 ) 0 191.9 0.8 Total fixed maturities 16,287.1 76.5 (121.6 ) 1.8 16,243.8 69.2 Equity securities: Nonredeemable preferred stocks 734.2 135.4 (16.1 ) 0 853.5 3.6 Common equities 1,437.5 1,377.0 (2.1 ) 0 2,812.4 12.0 Short-term investments 3,572.9 0 0 0 3,572.9 15.2 Total portfolio 1,2 $ 22,031.7 $ 1,588.9 $ (139.8 ) $ 1.8 $ 23,482.6 100.0 % 1 Our portfolio reflects the effect of unsettled security transactions and collateral on any open derivative positions; at December 31, 2017 , $5.8 million was included in “other assets,” compared to $27.8 million in “other liabilities” at December 31, 2016 . 2 The total fair value of the portfolio at December 31, 2017 and 2016 included $1.6 billion and $1.3 billion , respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company, net of any unsettled security transactions.
Hybrid Securities	Included in our fixed maturities are hybrid securities, which are reported at fair value at December 31 : (millions) 2017 2016 State and local government obligations $ 6.1 $ 0 Corporate debt securities 99.8 40.1 Residential mortgage-backed securities 0 170.5 Other asset-backed securities 6.7 8.9 Redeemable preferred stocks 30.3 0 Total hybrid securities $ 142.9 $ 219.5
Composition of Fixed Maturities by Maturity	The composition of fixed maturities by maturity at December 31, 2017 , was: (millions) Cost Fair Value Less than one year $ 3,964.1 $ 3,980.0 One to five years 12,706.5 12,671.3 Five to ten years 3,294.4 3,306.9 Ten years or greater 244.9 243.5 Total $ 20,209.9 $ 20,201.7
Gross Unrealized Losses by Major Security	The following tables show the composition of gross unrealized losses by major security type and by the length of time that individual securities have been in a continuous unrealized loss position: Total No. of Sec. Total Value Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Value Unrealized Losses No. of Sec. Fair Value Unrealized Losses December 31, 2017 Fixed maturities: U.S. government obligations 58 $ 5,817.0 $ (44.0 ) 41 $ 4,869.3 $ (34.6 ) 17 $ 947.7 $ (9.4 ) State and local government obligations 358 1,200.3 (9.3 ) 230 737.6 (4.4 ) 128 462.7 (4.9 ) Corporate debt securities 222 2,979.4 (14.4 ) 171 2,072.9 (9.1 ) 51 906.5 (5.3 ) Residential mortgage-backed securities 201 300.9 (3.4 ) 30 75.1 (0.2 ) 171 225.8 (3.2 ) Commercial mortgage-backed securities 105 1,682.3 (13.3 ) 63 1,221.2 (5.9 ) 42 461.1 (7.4 ) Other asset-backed securities 197 1,837.3 (4.5 ) 134 1,377.8 (3.3 ) 63 459.5 (1.2 ) Redeemable preferred stocks 2 21.8 (1.5 ) 1 10.8 (0.1 ) 1 11.0 (1.4 ) Total fixed maturities 1,143 13,839.0 (90.4 ) 670 10,364.7 (57.6 ) 473 3,474.3 (32.8 ) Equity securities: Nonredeemable preferred stocks 4 127.8 (8.8 ) 1 56.5 (0.5 ) 3 71.3 (8.3 ) Common equities 19 13.4 (0.2 ) 18 13.4 (0.2 ) 1 0 0 Total equity securities 23 141.2 (9.0 ) 19 69.9 (0.7 ) 4 71.3 (8.3 ) Total portfolio 1,166 $ 13,980.2 $ (99.4 ) 689 $ 10,434.6 $ (58.3 ) 477 $ 3,545.6 $ (41.1 ) Total No. of Sec. Total Fair Value Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Value Unrealized Losses No. of Sec. Fair Value Unrealized Losses December 31, 2016 Fixed maturities: U.S. government obligations 30 $ 2,774.0 $ (29.1 ) 30 $ 2,774.0 $ (29.1 ) 0 $ 0 $ 0 State and local government obligations 618 1,497.9 (20.7 ) 584 1,404.3 (19.6 ) 34 93.6 (1.1 ) Corporate debt securities 184 2,615.1 (24.3 ) 175 2,559.9 (24.0 ) 9 55.2 (0.3 ) Residential mortgage-backed securities 233 953.7 (15.6 ) 117 209.7 (1.7 ) 116 744.0 (13.9 ) Commercial mortgage-backed securities 111 1,347.3 (25.5 ) 85 1,061.2 (22.9 ) 26 286.1 (2.6 ) Other asset-backed securities 103 1,605.2 (4.4 ) 89 1,423.3 (3.9 ) 14 181.9 (0.5 ) Redeemable preferred stocks 2 31.0 (2.0 ) 0 0 0 2 31.0 (2.0 ) Total fixed maturities 1,281 10,824.2 (121.6 ) 1,080 9,432.4 (101.2 ) 201 1,391.8 (20.4 ) Equity securities: Nonredeemable preferred stocks 13 329.6 (16.1 ) 8 175.2 (3.8 ) 5 154.4 (12.3 ) Common equities 75 22.1 (2.1 ) 69 19.7 (1.7 ) 6 2.4 (0.4 ) Total equity securities 88 351.7 (18.2 ) 77 194.9 (5.5 ) 11 156.8 (12.7 ) Total portfolio 1,369 $ 11,175.9 $ (139.8 ) 1,157 $ 9,627.3 $ (106.7 ) 212 $ 1,548.6 $ (33.1 )
Total Non-Credit Portion of Other-Than-Temporary Impairment Recorded in Accumulated Other Comprehensive Income, Reflecting Original Non-Credit Loss at the Time Credit Impairment	The following table shows the total non-credit portion of the OTTI recorded in accumulated other comprehensive income, reflecting the original non-credit loss at the time the credit impairment was determined (i.e., unadjusted for valuation changes subsequent to the original write-down): December 31, (millions) 2017 2016 Fixed maturities: Residential mortgage-backed securities $ (19.7 ) $ (43.3 ) Commercial mortgage-backed securities (0.3 ) (0.6 ) Total fixed maturities $ (20.0 ) $ (43.9 )
OTTI Credit Losses Recognized in Earnings	The following tables provide rollforwards of the amounts related to credit losses recognized in earnings for the periods ended December 31, 2017 , 2016 , and 2015 , for which a portion of the OTTI losses were also recognized in accumulated other comprehensive income at the time the credit impairments were determined and recognized: (millions) Residential Mortgage- Backed Commercial Mortgage- Backed Total Total at December 31, 2016 $ 11.1 $ 0.4 $ 11.5 Credit losses for which an OTTI was not previously recognized 0 0.4 0.4 Reductions for securities sold/matured (10.9 ) (0.3 ) (11.2 ) Change in recoveries of future cash flows expected to be collected 1 (0.2 ) 0 (0.2 ) Total at December 31, 2017 $ 0 $ 0.5 $ 0.5 (millions) Residential Mortgage- Backed Commercial Mortgage- Backed Total Total at December 31, 2015 $ 12.4 $ 0.4 $ 12.8 Credit losses for which an OTTI was not previously recognized 0 0 0 Reductions for securities sold/matured 0 0 0 Change in recoveries of future cash flows expected to be collected 1 (1.3 ) 0 (1.3 ) Total at December 31, 2016 $ 11.1 $ 0.4 $ 11.5 (millions) Residential Mortgage- Backed Commercial Mortgage- Backed Total Total at December 31, 2014 $ 12.7 $ 0.4 $ 13.1 Credit losses for which an OTTI was not previously recognized 0 0 0 Reductions for securities sold/matured (1.4 ) 0 (1.4 ) Change in recoveries of future cash flows expected to be collected 1 1.1 0 1.1 Total at December 31, 2015 $ 12.4 $ 0.4 $ 12.8 1 Reflects expected recovery of prior period impairments that will be accreted into income over the remaining life of the security.
Components of Net Realized Gains (Losses)	The components of net realized gains (losses) for the years ended December 31, were: (millions) 2017 2016 2015 Gross realized gains on security sales Fixed maturities: U.S. government obligations $ 6.2 $ 24.6 $ 17.5 State and local government obligations 10.5 16.0 7.8 Corporate and other debt securities 20.3 43.3 31.2 Residential mortgage-backed securities 23.8 2.5 4.9 Commercial mortgage-backed securities 4.9 13.3 15.7 Other asset-backed securities 0.3 0 0 Redeemable preferred stocks 8.5 20.9 0.1 Total fixed maturities 74.5 120.6 77.2 Equity securities: Nonredeemable preferred stocks 58.4 11.9 65.3 Common equities 43.0 61.3 50.4 Short-term investments 0 0.1 0 Subtotal gross realized gains on security sales 175.9 193.9 192.9 Gross realized losses on security sales Fixed maturities: U.S. government obligations (28.7 ) (2.4 ) (0.9 ) State and local government obligations (0.1 ) (1.6 ) (0.3 ) Corporate and other debt securities (5.1 ) (2.5 ) (5.0 ) Residential mortgage-backed securities (0.4 ) (0.2 ) (0.8 ) Commercial mortgage-backed securities (5.3 ) (5.6 ) (1.3 ) Other asset-backed securities (0.4 ) 0 0 Redeemable preferred stocks (6.4 ) (6.6 ) 0 Total fixed maturities (46.4 ) (18.9 ) (8.3 ) Equity securities: Nonredeemable preferred stocks (5.9 ) (5.3 ) (3.2 ) Common equities (12.2 ) (15.7 ) (38.4 ) Short-term investments (0.2 ) (0.1 ) 0 Subtotal gross realized losses on security sales (64.7 ) (40.0 ) (49.9 ) Net realized gains (losses) on security sales Fixed maturities: U.S. government obligations (22.5 ) 22.2 16.6 State and local government obligations 10.4 14.4 7.5 Corporate and other debt securities 15.2 40.8 26.2 Residential mortgage-backed securities 23.4 2.3 4.1 Commercial mortgage-backed securities (0.4 ) 7.7 14.4 Other asset-backed securities (0.1 ) 0 0 Redeemable preferred stocks 2.1 14.3 0.1 Total fixed maturities 28.1 101.7 68.9 Equity securities: Nonredeemable preferred stocks 52.5 6.6 62.1 Common equities 30.8 45.6 12.0 Short-term investments (0.2 ) 0 0 Subtotal net realized gains (losses) on security sales 111.2 153.9 143.0 Other-than-temporary impairment losses Fixed maturities: Commercial mortgage-backed securities (0.4 ) 0 0 Redeemable preferred stocks 0 (25.3 ) 0 Total fixed maturities (0.4 ) (25.3 ) 0 Equity securities: Common equities (11.2 ) (0.3 ) (8.7 ) Subtotal investment other-than-temporary impairment losses (11.6 ) (25.6 ) (8.7 ) Other asset impairment (49.6 ) (59.7 ) 0 Subtotal other-than-temporary impairment losses (61.2 ) (85.3 ) (8.7 ) Other gains (losses) Hybrid securities (1.6 ) 2.1 (1.3 ) Derivative instruments 0 (20.0 ) (20.7 ) Litigation settlements 1.2 0.4 0.4 Subtotal other gains (losses) (0.4 ) (17.5 ) (21.6 ) Total net realized gains (losses) on securities $ 49.6 $ 51.1 $ 112.7
Components of Net Investment Income	The components of net investment income for the years ended December 31, were: (millions) 2017 2016 2015 Fixed maturities: U.S. government obligations $ 72.7 $ 18.2 $ 28.3 State and local government obligations 51.5 52.3 60.7 Foreign government obligations 0.3 0.4 0.4 Corporate debt securities 125.2 110.7 102.4 Residential mortgage-backed securities 34.7 47.3 54.3 Commercial mortgage-backed securities 79.6 81.6 74.6 Other asset-backed securities 47.1 28.0 22.0 Redeemable preferred stocks 11.8 14.9 15.0 Total fixed maturities 422.9 353.4 357.7 Equity securities: Nonredeemable preferred stocks 44.1 48.6 43.7 Common equities 58.3 57.2 51.0 Short-term investments 37.8 19.7 2.2 Investment income 563.1 478.9 454.6 Investment expenses (23.9 ) (22.4 ) (22.8 ) Net investment income $ 539.2 $ 456.5 $ 431.8
Derivative Instruments	The following table shows the status of our derivative instruments at December 31, 2017 and 2016 , and for the years ended December 31, 2017 , 2016 , and 2015 : (millions) Balance Sheet 2 Comprehensive Income Statement Notional Value 1 Assets (Liabilities) Fair Value Pretax Net Realized Gains (Losses) Years ended December 31, December 31, December 31, Derivatives designated as: 2017 2016 2015 Purpose Classification 2017 2016 2017 2016 2015 Hedging instruments Closed: Ineffective cash flow hedge $ 31 $ 370 $ 18 Manage NA $ 0 $ 0 $ 0 $ (1.3 ) $ 0.2 Non-hedging instruments Assets: Interest rate swaps 0 0 750 Manage Investments - fixed 0 0 0 0 (23.4 ) Closed: Interest rate swaps 0 750 0 Manage NA 0 0 0 (19.0 ) 0 U.S. Treasury Note futures 0 135 691 Manage NA 0 0 0 0.3 2.5 Total NA NA NA $ 0 $ 0 $ 0 $ (20.0 ) $ (20.7 ) 1 The amounts represent the value held at year end for open positions and the maximum amount held during the year for closed positions. 2 To the extent we held both derivative assets and liabilities with the same counterparty that were subject to an enforceable master netting arrangement, we reported them on a gross basis on our balance sheets, consistent with our historical presentation. NA = Not Applicable

Fair Value (Tables)

Fair Value (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Composition of Investment Portfolio by Major Security Type	The composition of the investment portfolio by major security type and our outstanding debt was: Fair Value (millions) Level 1 Level 2 Level 3 Total Cost December 31, 2017 Fixed maturities: U.S. government obligations $ 6,645.9 $ 0 $ 0 $ 6,645.9 $ 6,688.8 State and local government obligations 0 2,297.1 0 2,297.1 2,285.6 Foreign government obligations 0 0 0 0 0 Corporate debt securities 0 4,997.7 0 4,997.7 4,997.2 Subtotal 6,645.9 7,294.8 0 13,940.7 13,971.6 Asset-backed securities: Residential mortgage-backed 0 836.7 0 836.7 828.8 Commercial mortgage-backed 0 2,758.6 0 2,758.6 2,760.1 Other asset-backed 0 2,454.7 0 2,454.7 2,454.5 Subtotal asset-backed securities 0 6,050.0 0 6,050.0 6,043.4 Redeemable preferred stocks: Financials 0 64.1 0 64.1 61.3 Utilities 0 11.4 0 11.4 10.1 Industrials 0 135.5 0 135.5 123.5 Subtotal redeemable preferred stocks 0 211.0 0 211.0 194.9 Total fixed maturities 6,645.9 13,555.8 0 20,201.7 20,209.9 Equity securities: Nonredeemable preferred stocks: Financials 80.6 718.2 0 798.8 693.6 Industrials 0 0 5.0 5.0 5.0 Subtotal nonredeemable preferred stocks 80.6 718.2 5.0 803.8 698.6 Common equities: Common stocks 3,399.5 0 0 3,399.5 1,498.7 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 3,399.5 0 0.3 3,399.8 1,499.0 Total fixed maturities and equity securities 10,126.0 14,274.0 5.3 24,405.3 22,407.5 Short-term investments 1,824.4 1,045.0 0 2,869.4 2,869.4 Total portfolio $ 11,950.4 $ 15,319.0 $ 5.3 $ 27,274.7 $ 25,276.9 Debt $ 0 $ 3,606.5 $ 37.1 $ 3,643.6 $ 3,306.3 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost December 31, 2016 Fixed maturities: U.S. government obligations $ 2,870.1 $ 0 $ 0 $ 2,870.1 $ 2,899.2 State and local government obligations 0 2,502.6 0 2,502.6 2,509.5 Foreign government obligations 24.5 0 0 24.5 24.5 Corporate debt securities 0 4,550.9 0 4,550.9 4,557.8 Subtotal 2,894.6 7,053.5 0 9,948.1 9,991.0 Asset-backed securities: Residential mortgage-backed 0 1,499.3 0 1,499.3 1,489.7 Commercial mortgage-backed 0 2,253.1 0.3 2,253.4 2,266.9 Other asset-backed 0 2,351.1 0 2,351.1 2,350.7 Subtotal asset-backed securities 0 6,103.5 0.3 6,103.8 6,107.3 Redeemable preferred stocks: Financials 0 59.5 0 59.5 59.8 Utilities 0 30.9 0 30.9 30.5 Industrials 0 101.5 0 101.5 98.5 Subtotal redeemable preferred stocks 0 191.9 0 191.9 188.8 Total fixed maturities 2,894.6 13,348.9 0.3 16,243.8 16,287.1 Equity securities: Nonredeemable preferred stocks: Financials 138.1 715.4 0 853.5 734.2 Industrials 0 0 0 0 0 Subtotal nonredeemable preferred stocks 138.1 715.4 0 853.5 734.2 Common equities: Common stocks 2,812.0 0 0 2,812.0 1,437.1 Other risk investments 0 0 0.4 0.4 0.4 Subtotal common equities 2,812.0 0 0.4 2,812.4 1,437.5 Total fixed maturities and equity securities 5,844.7 14,064.3 0.7 19,909.7 18,458.8 Short-term investments 3,009.3 563.6 0 3,572.9 3,572.9 Total portfolio $ 8,854.0 $ 14,627.9 $ 0.7 $ 23,482.6 $ 22,031.7 Debt $ 0 $ 3,188.5 $ 127.3 $ 3,315.8 $ 3,148.2
Summary of Changes in Fair Value Associated With Level 3 Assets	The following tables provide a summary of changes in fair value associated with Level 3 assets for the years ended December 31, 2017 and 2016 : Level 3 Fair Value (millions) Fair Value at Dec. 31, 2016 Calls/ Maturities/ Paydowns Purchases Sales Net Realized (Gain)/Loss on Sales Change in Valuation Net Transfers In (Out) Fair Value at Dec. 31, 2017 Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 0.3 $ (0.3 ) $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 Total fixed maturities 0.3 (0.3 ) 0 0 0 0 0 0 Equity securities: Nonredeemable preferred stocks: Industrials 0 0 5.0 0 0 0 0 5.0 Common equities: Other risk investments 0.4 (0.1 ) 0 0 0 0 0 0.3 Total Level 3 securities $ 0.7 $ (0.4 ) $ 5.0 $ 0 $ 0 $ 0 $ 0 $ 5.3 Level 3 Fair Value (millions) Fair Value at Dec. 31, 2015 Calls/ Maturities/ Paydowns Purchases Sales Net Realized (Gain)/Loss on Sales Change in Valuation Net Transfers In (Out) Fair Value at Dec. 31, 2016 Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 9.9 $ (9.6 ) $ 0 $ 0 $ 0 $ 0 $ 0 $ 0.3 Total fixed maturities 9.9 (9.6 ) 0 0 0 0 0 0.3 Equity securities: Nonredeemable preferred stocks: Industrials 0 0 0 0 0 0 0 0 Common equities: Other risk investments 0.3 0 0 0 0 0.1 0 0.4 Total Level 3 securities $ 10.2 $ (9.6 ) $ 0 $ 0 $ 0 $ 0.1 $ 0 $ 0.7
Summary of Quantitative Information about Level 3 Fair Value Measurements	The following tables provide a summary of the quantitative information about Level 3 fair value measurements for our applicable securities at December 31 : Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at Dec. 31, 2017 Valuation Technique Unobservable Input Unobservable Input Assumption Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 0 NA NA NA Total fixed maturities 0 Equity securities: Nonredeemable preferred stocks: Industrials 1 5.0 Internal price Unadjusted purchase price 3.9 Subtotal Level 3 securities 5.0 Third-party pricing exemption securities 2 0.3 Total Level 3 securities $ 5.3 1 The security was internally-priced since it is privately held and it was valued at December 31, 2017 using the purchase price. 2 The fair values for these securities were obtained from non-binding external sources where unobservable inputs are not reasonably available to us. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at Dec. 31, 2016 Valuation Technique Unobservable Input Unobservable Input Assumption Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 0.3 External vendor Prepayment rate 1 0 Total fixed maturities 0.3 Equity securities: Nonredeemable preferred stocks: Industrials 0 NA NA NA Subtotal Level 3 securities 0.3 Third-party pricing exemption securities 2 0.4 Total Level 3 securities $ 0.7 1 Assumes that one security has 0% of the principal amount of the underlying loans that will be paid off prematurely in each year. 2 The fair values for these securities were obtained from non-binding external sources where unobservable inputs are not reasonably available to us.

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Table	Debt at December 31, consisted of: 2017 2016 (millions) Carrying Value Fair Value Carrying Value Fair Value 3.75% Senior Notes due 2021 (issued: $500.0, August 2011) $ 498.8 $ 520.7 $ 498.4 $ 528.8 2.45% Senior Notes due 2027 (issued: $500.0, August 2016) 496.1 477.9 495.8 464.6 6 5/8% Senior Notes due 2029 (issued: $300.0, March 1999) 296.1 382.3 295.9 380.1 6.25% Senior Notes due 2032 (issued: $400.0, November 2002) 395.3 516.9 395.2 499.0 4.35% Senior Notes due 2044 (issued: $350.0, April 2014) 346.5 388.7 346.4 362.3 3.70% Senior Notes due 2045 (issued: $400.0, January 2015) 395.2 402.9 395.1 372.5 4.125% Senior Notes due 2047 (issued: $850.0, April 2017) 841.2 917.1 0 0 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 (issued: $1,000.0, June 2007; outstanding: $0 and $594.6) 0 0 594.1 581.2 Other debt instruments 37.1 37.1 127.3 127.3 Total $ 3,306.3 $ 3,643.6 $ 3,148.2 $ 3,315.8
Aggregate Principal Payments on Debt Outstanding	Aggregate required principal payments on debt outstanding at December 31, 2017, are as follows: (millions) Year Payments 2018 $ 25.0 2019 12.1 2020 0 2021 500.0 2022 0 Thereafter 2,800.0 Total $ 3,337.1
Cash Flow Hedging [Member]
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges Included in Accumulated Other Comprehensive Income	The following table shows the original gain (loss) recognized at debt issuance and the unamortized balance at December 31, 2017 , on a pretax basis: (millions) Unrealized Gain (Loss) at Debt Issuance Unamortized Balance at December 31, 2017 3.75% Senior Notes $ (5.1 ) $ (2.1 ) 6 5/8% Senior Notes (4.2 ) (2.7 ) 6.25% Senior Notes 5.1 3.6 4.35% Senior Notes (1.6 ) (1.5 ) 3.70% Senior Notes (12.9 ) (12.2 ) 4.125% Senior Notes (8.0 ) (7.9 )
Other Debt Instruments [Member]
Schedule of Carrying Values and Estimated Fair Values of Debt Instruments [Table Text Block]	The other debt instruments reported in the table above represent ARX indebtedness and consist of: December 31, 2017 December 31, 2016 Type of debt instrument Number of Instruments Carrying Value Number of Instruments Carrying Stated Maturity Date(s) Term loans 2 $ 37.1 2 $ 62.1 December 2018 and 2019 Junior subordinated notes 0 0 2 41.2 NA Senior notes 0 0 4 24.0 NA Total $ 37.1 $ 127.3 NA - Not applicable as the notes were redeemed during 2017.

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Components of Income Tax Provision	(millions) 2017 2016 2015 Current tax provision Federal $ 680.9 $ 469.6 $ 655.3 State 12.8 12.7 14.7 Deferred tax expense (benefit) Federal (149.4 ) (66.3 ) (47.7 ) State (3.5 ) (2.5 ) (11.2 ) Total income tax provision $ 540.8 $ 413.5 $ 611.1
Reconciliation of Provision (Benefit) for Income Taxes Reported in Consolidated Statements of Income with Tax at Statutory Rate	($ in millions) 2017 2016 2015 Income before income taxes $ 2,138.9 $ 1,470.7 $ 1,911.6 Tax at statutory federal rate $ 748.6 35 % $ 514.8 35 % $ 669.1 35 % Tax effect of: Net deferred tax liability revaluation 1 (99.5 ) (5 ) 0 0 0 0 Tax credits (52.4 ) (2 ) (62.2 ) (4 ) (1.9 ) 0 Stock-based compensation 2 (25.1 ) (1 ) 0 0 0 0 Dividends received deduction (20.7 ) (1 ) (22.6 ) (2 ) (19.8 ) (1 ) Exempt interest income (16.9 ) (1 ) (15.7 ) (1 ) (17.8 ) (1 ) Tax-deductible dividends (9.7 ) 0 (6.1 ) 0 (7.9 ) 0 Nondeductible compensation expense 3 10.1 0 0.5 0 0.3 0 State income taxes, net of federal taxes 6.0 0 6.6 0 2.3 0 Other items, net 0.4 0 (1.8 ) 0 0.6 0 Non-taxable gain 4 0 0 0 0 (13.8 ) (1 ) Total income tax provision $ 540.8 25 % $ 413.5 28 % $ 611.1 32 % 1 Pursuant to the recently enacted legislation commonly known as the Tax Cuts and Jobs Act of 2017 (the “Tax Act”); see further discussion below. 2 Represents excess tax benefits associated with share-based payments awards. Prior to 2017, these excess tax benefits were recorded directly to additional paid-in capital under the then existing accounting guidance. 3 Increase in 2017 primarily reflects changes in compensation that qualifies for deduction under the Tax Act. 4 Represents the tax effect of holding period gains on the 5% interest in ARX we owned prior to acquisition of a controlling interest on April 1, 2015.
Components of Net Deferred Tax Assets	t December 31, 2017 and 2016 . As noted above, the federal deferred tax assets and liabilities at December 31, 2017, have been revalued to reflect the new 21% federal corporate income tax rate under the Tax Act. (millions) 2017 2016 Federal deferred tax assets: Unearned premiums reserve $ 368.9 $ 515.6 Non-deductible accruals 166.0 259.9 Loss and loss adjustment expense reserves 48.4 76.5 Hedges on forecasted transactions 4.8 5.1 Investment basis differences 0 31.3 Other 7.3 8.9 Federal deferred tax liabilities: Net unrealized gains on securities (419.5 ) (507.2 ) Deferred acquisition costs (163.9 ) (227.9 ) Property and equipment (75.5 ) (120.2 ) Intangible assets (66.6 ) (145.3 ) Prepaid expenses (7.1 ) (9.2 ) Investment basis differences (5.5 ) 0 Deferred gain on extinguishment of debt (0.4 ) (1.5 ) Other (6.1 ) (7.7 ) Net federal deferred tax liability (149.2 ) (121.7 ) Net state deferred tax asset 14.2 10.4 Net deferred tax liability $ (135.0 ) $ (111.3 )

Loss And Loss Adjustment Expe34

Loss And Loss Adjustment Expense Reserves (Tables)	12 Months Ended
	Dec. 31, 2017
Insurance [Abstract]
Activity in Loss and Loss Adjustment Expense Reserves	Activity in the loss and loss adjustment expense reserves is summarized as follows: (millions) 2017 2016 2015 Balance at January 1 $ 11,368.0 $ 10,039.0 $ 8,857.4 Less reinsurance recoverables on unpaid losses 1,801.0 1,442.7 1,185.9 Net balance at January 1 9,567.0 8,596.3 7,671.5 Net loss and loss adjustment reserves (disposed) acquired 1 0 (2.5 ) 222.4 Total beginning reserves 9,567.0 8,593.8 7,893.9 Incurred related to: Current year 18,782.1 16,967.1 14,657.1 Prior years 25.9 (87.5 ) (315.1 ) Total incurred 18,808.0 16,879.6 14,342.0 Paid related to: Current year 12,201.5 11,149.0 9,577.3 Prior years 5,256.7 4,757.4 4,062.3 Total paid 17,458.2 15,906.4 13,639.6 Net balance at December 31 10,916.8 9,567.0 8,596.3 Plus reinsurance recoverables on unpaid losses 2,170.1 1,801.0 1,442.7 Balance at December 31 $ 13,086.9 $ 11,368.0 $ 10,039.0 1 During 2016, $2.5 million net reserves were disposed by ARX in an exchange transaction (see Note 16 – Goodwill and Intangible Assets for further discussion). During 2015, $222.4 million net reserves were acquired in the ARX acquisition.
Short-duration Insurance Contracts, Claims Development	The following tables show incurred and paid claims development, net of reinsurance, by accident year. Only 2017 is audited; all prior years are considered required supplementary information and, therefore, are unaudited. Expected development on our case reserves is excluded from the IBNR reserves on our vehicle businesses, as discussed above. For the Property business, the IBNR reserves include expected case development based on the methodology used in establishing the case reserves for that segment. The cumulative number of incurred claims are based on accident coverages (e.g., bodily injury, collision, comprehensive, personal injury protection, property damage) related to opened claims. Coverage counts related to claims closed without payment are excluded from the cumulative number of incurred claims. Personal Lines - Agency - Liability ($ in millions) As of Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance December 31, 2017 For the years ended December 31, Total of IBNR Liabilities Plus Expected Development on Reported Claims Cumulative Number of Incurred Claim Counts Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 3,506.0 $ 3,520.9 $ 3,518.2 $ 3,528.0 $ 3,484.2 $ 0 696,624 2014 3,702.1 3,627.7 3,633.2 3,654.4 50.9 701,787 2015 3,774.9 3,773.8 3,798.8 59.2 704,990 2016 4,082.9 4,130.0 158.7 738,563 2017 4,474.8 679.6 765,553 Total $ 19,542.2 Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance For the years ended December 31, Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 1,684.0 $ 2,794.1 $ 3,173.1 $ 3,362.9 $ 3,437.6 2014 1,809.0 2,868.1 3,284.5 3,482.3 2015 1,793.1 2,976.0 3,416.5 2016 1,941.6 3,231.5 2017 2,074.0 Total $ 15,641.9 All outstanding liabilities before 2013, net of reinsurance 1 55.2 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 3,955.5 1 Required supplementary information (unaudited) Personal Lines - Agency - Physical Damage ($ in millions) As of Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance December 31, 2017 For the years ended December 31, Total of IBNR Liabilities Plus Expected Development on Reported Claims Cumulative Number of Incurred Claim Counts Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 2,014.0 $ 2,001.4 $ 2,000.1 $ 1,999.4 $ 2,001.1 $ 0 1,347,920 2014 2,107.5 2,090.3 2,089.9 2,088.0 (3.3 ) 1,374,666 2015 2,136.8 2,137.2 2,134.4 (2.6 ) 1,336,268 2016 2,423.4 2,398.9 (12.1 ) 1,398,371 2017 2,635.5 (101.8 ) 1,496,298 Total $ 11,257.9 Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance For the years ended December 31, Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 1,952.7 $ 2,003.9 $ 2,002.0 $ 2,001.3 $ 2,000.8 2014 2,078.8 2,091.6 2,090.6 2,090.7 2015 2,106.2 2,138.1 2,134.4 2016 2,391.0 2,406.9 2017 2,599.8 Total $ 11,232.6 All outstanding liabilities before 2013, net of reinsurance 1 0.9 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 26.2 1 Required supplementary information (unaudited) Personal Lines - Direct - Liability ($ in millions) As of Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance December 31, 2017 For the years ended December 31, Total of IBNR Liabilities Plus Expected Development on Reported Claims Cumulative Number of Incurred Claim Counts Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 2,619.4 $ 2,621.8 $ 2,615.8 $ 2,649.8 $ 2,612.9 $ 0 550,166 2014 2,946.8 2,887.4 2,898.1 2,913.6 38.2 592,185 2015 3,330.5 3,328.3 3,354.2 48.6 659,102 2016 3,819.0 3,843.9 143.0 733,195 2017 4,209.5 598.6 761,280 Total $ 16,934.1 Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance For the years ended December 31, Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 1,252.0 $ 2,085.1 $ 2,375.5 $ 2,521.3 $ 2,577.8 2014 1,413.0 2,278.0 2,624.2 2,780.0 2015 1,545.2 2,615.0 3,021.0 2016 1,780.6 2,991.1 2017 1,912.6 Total $ 13,282.5 All outstanding liabilities before 2013, net of reinsurance 1 33.5 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 3,685.1 1 Required supplementary information (unaudited) Personal Lines - Direct - Physical Damage ($ in millions) As of Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance December 31, 2017 For the years ended December 31, Total of IBNR Liabilities Plus Expected Development on Reported Claims Cumulative Number of Incurred Claim Counts Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 1,653.7 $ 1,635.6 $ 1,634.6 $ 1,633.4 $ 1,635.0 $ 0 1,367,341 2014 1,889.3 1,862.2 1,861.7 1,859.2 (3.2 ) 1,471,016 2015 2,110.7 2,097.7 2,093.5 (3.0 ) 1,539,790 2016 2,521.0 2,475.4 (14.3 ) 1,675,405 2017 2,750.6 (145.3 ) 1,779,665 Total $ 10,813.7 Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance For the years ended December 31, Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 1,612.9 $ 1,638.2 $ 1,636.2 $ 1,635.3 $ 1,635.0 2014 1,874.6 1,864.1 1,862.7 1,861.8 2015 2,094.7 2,100.1 2,094.7 2016 2,505.0 2,485.8 2017 2,742.1 Total $ 10,819.4 All outstanding liabilities before 2013, net of reinsurance 1 0.1 Liabilities for claims and claim adjustment expenses, net of reinsurance $ (5.6 ) 1 Required supplementary information (unaudited) Commercial Lines - Liability ($ in millions) As of Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance December 31, 2017 For the years ended December 31, Total of IBNR Liabilities Plus Expected Development on Reported Claims Cumulative Number of Incurred Claim Counts Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 864.2 $ 864.6 $ 865.4 $ 865.0 $ 840.8 $ 0 78,075 2014 822.5 795.4 820.3 823.0 15.9 74,724 2015 897.6 911.1 914.8 23.7 77,807 2016 1,185.8 1,204.8 63.1 92,196 2017 1,374.1 219.2 94,992 Total $ 5,157.5 Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance For the years ended December 31, Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 233.3 $ 509.4 $ 659.1 $ 752.5 $ 802.6 2014 234.0 438.7 610.0 715.3 2015 238.4 501.5 675.0 2016 298.6 639.9 2017 325.8 Total $ 3,158.6 All outstanding liabilities before 2013, net of reinsurance 1 28.5 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 2,027.4 1 Required supplementary information (unaudited) Commercial Lines - Physical Damage ($ in millions) As of Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance December 31, 2017 For the years ended December 31, Total of IBNR Liabilities Plus Expected Development on Reported Claims Cumulative Number of Incurred Claim Counts Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 256.2 $ 254.2 $ 254.5 $ 253.7 $ 254.2 $ 0 62,673 2014 240.3 239.7 238.6 238.0 (0.5 ) 59,622 2015 274.4 274.1 273.5 (0.1 ) 62,593 2016 379.6 379.8 (0.9 ) 74,217 2017 415.4 (5.7 ) 77,767 Total $ 1,560.9 Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance For the years ended December 31, Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 239.4 $ 253.1 $ 253.9 $ 253.7 $ 253.8 2014 224.6 238.3 237.7 237.9 2015 248.5 271.9 272.0 2016 336.7 376.9 2017 369.0 Total $ 1,509.6 All outstanding liabilities before 2013, net of reinsurance 1 0.4 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 51.7 1 Required supplementary information (unaudited) Property Business ($ in millions) As of Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance December 31, 2017 For the years ended December 31, Total of IBNR Liabilities Plus Expected Development on Reported Claims Cumulative Number of Incurred Claim Counts Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 307.3 $ 283.3 $ 254.3 $ 254.1 $ 253.9 $ 2.5 30,600 2014 415.5 389.1 379.7 376.3 3.6 40,984 2015 460.0 416.5 403.6 11.5 41,957 2016 568.6 541.2 24.8 53,323 2017 672.8 99.7 64,487 Total $ 2,247.8 Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance For the years ended December 31, Accident Year 2013 1 2014 1 2015 1 2016 1 2017 2013 $ 185.1 $ 234.2 $ 244.9 $ 249.5 $ 250.5 2014 269.2 351.5 365.9 370.3 2015 280.3 372.8 383.5 2016 415.2 498.2 2017 506.7 Total $ 2,009.2 All outstanding liabilities before 2013, net of reinsurance 1 7.2 Liabilities for claims and claim adjustment expenses, net of reinsurance $ 245.8 1 Required supplementary information (unaudited)
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability	The following table reconciles the net incurred and paid claims development tables to the liability for claims and claim adjustment expenses: (millions) 2017 2016 Net outstanding liabilities Personal Lines Agency, Liability $ 3,955.5 $ 3,555.9 Agency, Physical Damage 26.2 30.1 Direct, Liability 3,685.1 3,219.1 Direct, Physical Damage (5.6 ) 11.2 Commercial Lines Liability 2,027.4 1,675.3 Physical Damage 51.7 46.6 Property 245.8 224.7 Other business 43.7 37.1 Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance 10,029.8 8,800.0 Reinsurance recoverable on unpaid claims Personal Lines Agency, Liability 769.8 724.5 Agency, Physical Damage 0 0 Direct, Liability 838.1 724.3 Direct, Physical Damage 0 0 Commercial Lines Liability 105.1 59.5 Physical Damage 0.1 0 Property 243.8 132.7 Other business 200.9 154.9 Total reinsurance recoverable on unpaid claims $ 2,157.8 $ 1,795.9 Unallocated claims adjustment expense related to: Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance 887.0 767.0 Reinsurance recoverable on unpaid claims 12.3 5.1 Total gross liability for unpaid claims and claim adjustment expense $ 13,086.9 $ 11,368.0
Short-duration Insurance Contracts, Schedule of Historical Claims Duration	The following table shows the average historical claims duration as of December 31, 2017 : (Required Supplementary Information - Unaudited) Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance Years 1 2 3 4 5 Personal Lines Agency, Liability 47.6% 30.8% 11.3% 5.4% 2.1% Agency, Physical Damage 98.9% 1.3% (0.1)% 0% 0% Direct, Liability 46.7% 31.3% 11.7% 5.5% 2.2% Direct, Physical Damage 100.1% 0% (0.2)% (0.1)% 0% Commercial Lines Liability 25.8% 28.7% 19.2% 11.9% 6.0% Physical Damage 90.9% 7.9% 0% 0% 0% Property 73.7% 19.5% 3.5% 1.4% 0.4%

Reinsurance (Tables)

Reinsurance (Tables)	12 Months Ended
	Dec. 31, 2017
Reinsurance Disclosures [Abstract]
Effect of Reinsurance on Premiums Written and Earned	The effect of reinsurance on premiums written and earned for the years ended December 31, was as follows: 2017 2016 2015 (millions) Written Earned Written Earned Written Earned Direct premiums $ 27,860.7 $ 26,425.7 $ 23,941.9 $ 23,111.2 $ 21,086.5 $ 20,454.1 Ceded premiums: Regulated plans (505.9 ) (479.6 ) (439.4 ) (425.1 ) (358.0 ) (362.6 ) Non-Regulated plans (222.7 ) (216.2 ) (149.0 ) (212.1 ) (164.5 ) (192.4 ) Total ceded premiums (728.6 ) (695.8 ) (588.4 ) (637.2 ) (522.5 ) (555.0 ) Net premiums $ 27,132.1 $ 25,729.9 $ 23,353.5 $ 22,474.0 $ 20,564.0 $ 19,899.1
Prepaid Reinsurance Premiums and Reinsurance Recoverables	Our prepaid reinsurance premiums and reinsurance recoverables were comprised of the following at December 31: Prepaid Reinsurance Premiums Reinsurance Recoverables ($ in millions) 2017 2016 2017 2016 Regulated plans: MCCA $ 44.3 22 % $ 36.5 21 % $ 1,611.5 71 % $ 1,452.7 77 % CAIP 50.0 25 40.2 24 218.0 10 170.6 9 NCRF 31.5 15 27.5 16 74.2 3 67.0 4 NFIP 53.8 26 49.1 29 148.8 7 78.5 4 Other 0.3 0 0.1 0 8.2 0 3.5 0 Total Regulated plans 179.9 88 153.4 90 2,060.7 91 1,772.3 94 Non-Regulated plans: Property 9.1 5 3.2 2 138.6 6 62.9 3 Other 14.3 7 13.9 8 74.1 3 49.6 3 Total Non-Regulated plans 23.4 12 17.1 10 212.7 9 112.5 6 Total $ 203.3 100 % $ 170.5 100 % $ 2,273.4 100 % $ 1,884.8 100 %

Employee Benefit Plans (Tables)

Employee Benefit Plans (Tables)	12 Months Ended
	Dec. 31, 2017
Amounts Charged to Income for Employees Incentive Compensation Plans	The amounts charged to income for Progressive and ARX incentive compensation plans for the years ended December 31, were: 2017 2016 2015 (millions) Pretax After Tax Pretax After Tax Pretax After Tax Non-equity incentive plans – cash $ 461.3 $ 299.8 $ 386.8 $ 251.4 $ 337.7 $ 219.5 Equity incentive plans: Equity awards 92.9 60.4 80.9 52.6 64.5 41.9 Liability awards 2.5 1.6 4.3 2.8 1.7 1.1
Assets Held in Deferral Plan Irrevocable Grantor Trust Account	The Deferral Plan Irrevocable Grantor Trust account held the following assets at December 31: (millions) 2017 2016 Progressive common shares 1 $ 128.2 $ 122.2 Other investment funds 2 167.0 136.9 Total $ 295.2 $ 259.1 1 Included 4.4 million and 4.7 million common shares as of December 31, 2017 and 2016 , respectively, to be distributed in common shares. 2 Amount is included in other assets on the balance sheet.
Employee
Summary of Restricted Stock Activity	A summary of all employee restricted equity award activity during the years ended December 31, follows: 2017 2016 2015 Restricted Equity Awards Number of Shares 1 Weighted Average Grant Date Fair Value Number of Shares 1 Weighted Average Grant Date Fair Value Number of Shares 1 Weighted Average Grant Date Fair Value Beginning of year 6,951,373 $ 26.18 7,725,227 $ 23.37 9,051,564 $ 21.27 Add (deduct): Granted 2 2,383,475 32.01 1,870,660 31.54 2,489,976 25.20 Vested (3,220,671 ) 22.53 (2,422,700 ) 21.50 (3,682,644 ) 19.53 Forfeited (255,329 ) 28.03 (221,814 ) 24.64 (133,669 ) 21.63 End of year 3,4 5,858,848 $ 30.47 6,951,373 $ 26.18 7,725,227 $ 23.37 1 Includes restricted stock units; 2015 also includes restricted stock. Upon vesting, all units will be converted on a one-for-one basis into Progressive common shares funded from existing treasury shares. All performance-based awards are included at their target amounts. 2 We reinvest dividend equivalents on restricted stock units. For 2017 , 2016 , and 2015 , the number of units “granted” shown in the table above includes 157,396 , 165,045 , and 196,947 of dividend equivalent units, respectively, at a weighted average grant date fair value of $0 , since the dividends were factored into the grant date fair value of the original grant. 3 At December 31, 2017 , the number of shares included 1,513,779 performance-based units at their target amounts. We expect 3,244,111 units to vest based upon our current estimates of the likelihood of achieving the pre-determined performance goals applicable to each award. 4 At December 31, 2017 , the total unrecognized compensation cost related to unvested equity awards was $87.1 million , which includes performance-based awards at their currently estimated vesting value. This compensation expense will be recognized into the income statement over the weighted average vesting period of 2.2 years.
Director
Summary of Restricted Stock Activity	A summary of all directors’ restricted stock activity during the years ended December 31, follows: 2017 2016 2015 Restricted Stock Number of Shares Weighted Average Grant Date Fair Value Number of Shares Weighted Average Grant Date Fair Value Number of Shares Weighted Average Grant Date Fair Value Beginning of year 55,839 $ 33.24 89,427 $ 27.23 81,579 $ 25.45 Add (deduct): Granted 53,284 40.54 55,839 33.24 89,427 27.23 Vested (55,839 ) 33.24 (89,427 ) 27.23 (81,579 ) 25.45 End of year 53,284 $ 40.54 55,839 $ 33.24 89,427 $ 27.23
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding and Exercisable [Table Text Block]	A summary of all ARX employee stock option activity since acquisition, follows: 2017 2016 2015 Options Outstanding Number of Shares Weighted Average Number of Shares Weighted Average Number of Shares Weighted Average Beginning of year 24,995 $ 526.46 24,995 $ 526.46 NA NA At acquisition date 4/1/2015 NA NA NA NA 26,000 $ 513.72 Add (deduct): Exercised 1 (2,445 ) 174.65 0 0 (1,005 ) 197.01 End of year 22,550 $ 564.60 24,995 $ 526.46 24,995 $ 526.46 Exercisable, end of year 17,950 $ 517.75 16,995 $ 438.77 12,995 $ 386.69 NA = Not Applicable 1 At the time of exercise in 2017 and 2015, the value earned by the option holders was $2.9 million and $1.1 million , respectively. 2017 2016 2015 Non-Vested Options Outstanding Number of Shares Weighted Average Number of Shares Weighted Average Number of Shares Weighted Average Beginning of year 8,000 $ 712.74 12,000 $ 677.81 NA NA At acquisition date 4/1/2015 NA NA NA NA 14,800 $ 675.55 Add (deduct): Vested (3,400 ) 665.79 (4,000 ) 607.95 (2,800 ) 665.85 End of year 1 4,600 $ 747.45 8,000 $ 712.74 12,000 $ 677.81 NA = Not Applicable 1 At December 31, 2017, 2016, and 2015, the remaining unrecognized compensation cost related to unvested options was $0.7 million , $1.6 million , and $2.9 million , respectively, and the remaining weighted average vesting period on the unvested awards was 1.01 years, 1.36 years, and 1.72 years, respectively.

Segment Information (Tables)

Segment Information (Tables)	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Reconciliation of Revenue and Operating Income from Segments to Consolidated	Following are the operating results for the years ended December 31: 2017 2016 2015 (millions) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Personal Lines Agency $ 11,177.6 $ 839.6 $ 9,791.7 $ 492.8 $ 9,108.6 $ 713.2 Direct 10,769.6 683.7 9,396.5 412.2 8,185.9 403.4 Total Personal Lines 1 21,947.2 1,523.3 19,188.2 905.0 17,294.5 1,116.6 Commercial Lines 2,793.9 214.1 2,421.3 155.2 1,995.9 318.3 Property 2 988.8 (50.3 ) 864.5 32.5 609.1 61.3 Other indemnity 3 0 (0.2 ) 0 (1.6 ) (0.4 ) (1.0 ) Total underwriting operations 25,729.9 1,686.9 22,474.0 1,091.1 19,899.1 1,495.2 Fees and other revenues 4 370.6 NA 332.5 NA 302.0 NA Service businesses 126.8 17.3 103.3 11.3 86.3 8.8 Investments 5 612.7 588.8 530.0 507.6 567.3 544.5 Other gains (losses) (1.0 ) (1.0 ) 1.6 1.6 (0.9 ) (0.9 ) Interest expense NA (153.1 ) NA (140.9 ) NA (136.0 ) Consolidated total $ 26,839.0 $ 2,138.9 $ 23,441.4 $ 1,470.7 $ 20,853.8 $ 1,911.6 NA = Not Applicable 1 Personal auto insurance accounted for 93% of the total Personal Lines segment net premiums earned in 2017 , compared to 92% in 2016 and 2015 ; insurance for our special lines products (e.g., motorcycles, ATVs, RVs, watercraft, and snowmobiles) accounted for the balance of the Personal Lines net premiums earned. 2 We began reporting our Property business as a segment on April 1, 2015, upon acquisition of a controlling interest in ARX; therefore, the year ended 2015 only includes results for nine months and is not comparable to results reported for 2017 or 2016. During 2017 , 2016 , and 2015, pretax profit (loss) also includes $66.2 million , $62.1 million , and $46.8 million , respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX. Although this expense is included in our Property segment, it is not reported in the consolidated results of ARX and, therefore, will not affect the value of the net income attributable to the noncontrolling interest. 3 Our professional liability group recognized $0.4 million of reinstatement premiums paid to our reinsurers pursuant to their reinsurance contracts during 2015. This premium reduction was reflected in our companywide total results. 4 Pretax profit (loss) for fees and other revenues are allocated to operating segments. 5 Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit is net of investment expenses.
Underwriting Margins and Combined Ratios for our Underwriting Operations	Following are the underwriting margins/combined ratios for our underwriting operations for the years ended December 31: 2017 2016 2015 Underwriting Margin Combined Ratio Underwriting Margin Combined Ratio Underwriting Margin Combined Ratio Personal Lines Agency 7.5 % 92.5 5.0 % 95.0 7.8 % 92.2 Direct 6.3 93.7 4.4 95.6 4.9 95.1 Total Personal Lines 6.9 93.1 4.7 95.3 6.5 93.5 Commercial Lines 7.7 92.3 6.4 93.6 15.9 84.1 Property 1 (5.1 ) 105.1 3.8 96.2 10.1 89.9 Total underwriting operations 6.6 93.4 4.9 95.1 7.5 92.5 1 We began reporting our Property business as a segment on April 1, 2015, when we acquired a controlling interest in ARX; therefore, the year ended 2015 only includes results for nine months and is not comparable to results reported for 2017 or 2016. Included in 2017 , 2016 , and 2015 is 6.7 points, 7.2 points, and 7.7 points, respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX.

Other Comprehensive Income (L38

Other Comprehensive Income (Loss) (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Components of Other Comprehensive Income (Loss)	The components of other comprehensive income (loss), including reclassification adjustments by income statement line item, for the years ended December 31, were as follows: Components of Changes in Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income Total tax (provision) benefit After tax total accumulated other comprehensive income Total net unrealized gains (losses) on securities Net unrealized gains on forecasted transactions Foreign currency translation adjustment Loss attributable to NCI Total at December 31, 2016 $ 1,439.5 $ (506.1 ) $ 933.4 $ 939.6 $ (9.4 ) $ (1.1 ) $ 4.3 Other comprehensive income (loss) before reclassifications: Investment securities 636.9 (224.0 ) 412.9 412.9 0 0 0 Net non-credit related OTTI losses, adjusted for valuation changes 0 0 0 0 0 0 0 Forecasted transactions (8.0 ) 2.8 (5.2 ) 0 (5.2 ) 0 0 Foreign currency translation adjustment 0.4 (0.1 ) 0.3 0 0 0.3 0 Loss attributable to noncontrolling interest (NCI) (3.5 ) 1.2 (2.3 ) 0 0 0 (2.3 ) Total other comprehensive income (loss) before reclassifications 625.8 (220.1 ) 405.7 412.9 (5.2 ) 0.3 (2.3 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (14.9 ) 5.3 (9.6 ) (9.6 ) 0 0 0 Net realized gains (losses) on securities 103.3 (36.2 ) 67.1 67.1 0 0 0 Other gains (losses) 1 (1.2 ) 0.4 (0.8 ) 0 0 (0.8 ) 0 Interest expense 0.3 (0.1 ) 0.2 0 0.2 0 0 Total reclassification adjustment for amounts realized in net income 87.5 (30.6 ) 56.9 57.5 0.2 (0.8 ) 0 Total other comprehensive income (loss) 538.3 (189.5 ) 348.8 355.4 (5.4 ) 1.1 (2.3 ) Total at December 31, 2017 $ 1,977.8 $ (695.6 ) $ 1,282.2 $ 1,295.0 $ (14.8 ) $ 0 $ 2.0 1 During 2017, we ceased writing insurance in Australia resulting in a loss of $1.2 million relating to the foreign currency translation adjustment. The loss is netted against a gain on extinguishment of debt (see Note 4 – Debt) in other gains (losses) on our consolidated statements of comprehensive income for the year end December 31, 2017. Components of Changes in Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income Total tax (provision) benefit After tax total accumulated other comprehensive income Total net unrealized gains (losses) on securities Net unrealized gains on forecasted transactions Foreign currency translation adjustment Loss attributable to NCI Total at December 31, 2015 $ 1,234.5 $ (434.1 ) $ 800.4 $ 809.0 $ (8.2 ) $ (1.5 ) $ 1.1 Other comprehensive income (loss) before reclassifications: Investment securities 320.5 (112.6 ) 207.9 207.9 0 0 0 Net non-credit related OTTI losses, adjusted for valuation changes (0.1 ) 0.1 0 0 0 0 0 Forecasted transactions 0 0 0 0 0 0 0 Foreign currency translation adjustment 0.6 (0.2 ) 0.4 0 0 0.4 0 Loss attributable to noncontrolling interest (NCI) 5.1 (1.9 ) 3.2 0 0 0 3.2 Total other comprehensive income (loss) before reclassifications 326.1 (114.6 ) 211.5 207.9 0 0.4 3.2 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (27.1 ) 9.5 (17.6 ) (17.6 ) 0 0 0 Net realized gains (losses) on securities 146.3 (51.4 ) 94.9 94.9 0 0 0 Other gains (losses) 0 0 0 0 0 0 0 Interest expense 1.9 (0.7 ) 1.2 0 1.2 0 0 Total reclassification adjustment for amounts realized in net income 121.1 (42.6 ) 78.5 77.3 1.2 0 0 Total other comprehensive income (loss) 205.0 (72.0 ) 133.0 130.6 (1.2 ) 0.4 3.2 Total at December 31, 2016 $ 1,439.5 $ (506.1 ) $ 933.4 $ 939.6 $ (9.4 ) $ (1.1 ) $ 4.3 Components of Changes in Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income Total tax (provision) benefit After tax total accumulated other comprehensive income Total net unrealized gains (losses) on securities Net unrealized gains on forecasted transactions Foreign currency translation adjustment Loss attributable to NCI Total at December 31, 2014 $ 1,574.0 $ (550.9 ) $ 1,023.1 $ 1,021.9 $ 1.5 $ (0.3 ) $ 0 Other comprehensive income (loss) before reclassifications: Investment securities (198.7 ) 67.5 (131.2 ) (131.2 ) 0 0 0 Net non-credit related OTTI losses, adjusted for valuation changes 0 0 0 0 0 0 0 Forecasted transactions (12.9 ) 4.5 (8.4 ) 0 (8.4 ) 0 0 Foreign currency translation adjustment (1.8 ) 0.6 (1.2 ) 0 0 (1.2 ) 0 Loss attributable to noncontrolling interest (NCI) 1.6 (0.5 ) 1.1 0 0 0 1.1 Total other comprehensive income (loss) before reclassifications (211.8 ) 72.1 (139.7 ) (131.2 ) (8.4 ) (1.2 ) 1.1 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (23.8 ) 8.4 (15.4 ) (15.4 ) 0 0 0 Net realized gains (losses) on securities 149.7 (52.5 ) 97.2 97.1 0.1 0 0 Other gains (losses) 0 0 0 0 0 0 0 Interest expense 1.8 (0.6 ) 1.2 0 1.2 0 0 Total reclassification adjustment for amounts realized in net income 127.7 (44.7 ) 83.0 81.7 1.3 0 0 Total other comprehensive income (loss) (339.5 ) 116.8 (222.7 ) (212.9 ) (9.7 ) (1.2 ) 1.1 Total at December 31, 2015 $ 1,234.5 $ (434.1 ) $ 800.4 $ 809.0 $ (8.2 ) $ (1.5 ) $ 1.1

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Minimum Commitments under Noncancelable Operating Lease Agreements	The minimum commitments under these agreements at December 31, 2017 , were as follows: (millions) Commitments 2018 $ 53.6 2019 44.2 2020 28.5 2021 16.9 2022 8.0 Thereafter 1.1 Total $ 152.3
Expense Incurred for Leases	The expense we incurred for the leases disclosed above, as well as other operating leases that may be cancelable or have terms less than one year, was: (millions) Expense 2017 $ 77.2 2016 72.9 2015 66.6

Dividends (Tables)

Dividends (Tables)	12 Months Ended
	Dec. 31, 2017
Dividends [Abstract]
Dividends	Following is a summary of our shareholder dividends that were declared in the last three years: (millions, except per share amounts) Amount Dividend Type Declared Paid Per Total 1 Annual – Variable December 2017 February 2018 $ 1.1247 $ 655.1 Annual – Variable December 2016 February 2017 0.6808 395.4 Annual – Variable December 2015 February 2016 0.8882 519.2 1 Based on an estimate of shares outstanding as of the record date. For the dividends declared in December 2016 and 2015, we paid $395.4 million and $519.0 million , respectively.

Redeemable Noncontrolling Int41

Redeemable Noncontrolling Interest (Tables)	12 Months Ended
	Dec. 31, 2017
Noncontrolling Interest [Abstract]
Redeemable Noncontrolling Interest [Table Text Block]	The changes in the components of redeemable NCI during the year ended December 31, 2017, 2016, and 2015, were: (millions) December 31, 2017 December 31, 2016 December 31, 2015 Balance, Beginning of period $ 483.7 $ 464.9 $ 0 Fair value at date of acquisition 0 0 411.5 Net income attributable to NCI 5.9 26.2 32.9 Other comprehensive income (loss) attributable to NCI 2.3 (3.2 ) (1.1 ) Exercise of employee stock options 3.4 0 0 Purchase of shares from NCI 0 0 (12.6 ) Change in redemption value of NCI 8.4 (4.2 ) 34.2 Balance, End of period $ 503.7 $ 483.7 $ 464.9

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Net Carrying Amount of Other Intangible Assets	The following table is a summary of the net carrying amount of other intangible assets as of December 31, 2017 and 2016 : (millions) December 31, 2017 December 31, 2016 Intangible assets subject to amortization $ 354.2 $ 420.4 Indefinite-lived intangible assets 1 12.4 12.4 Total $ 366.6 $ 432.8 1 Indefinite-lived intangible assets are comprised of state insurance and agent licenses. State insurance licenses were previously subject to amortization under superseded accounting guidance and have $0.6 million of accumulated amortization for both periods presented.
Intangible Assets Subject to Amortization	Intangible assets subject to amortization consisted of the following: (millions) December 31, 2017 December 31, 2016 Category Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Policies in force $ 256.2 $ 100.7 $ 155.5 $ 256.2 $ 64.1 $ 192.1 Agency relationships 159.2 31.3 127.9 159.2 19.9 139.3 Software rights 79.1 29.4 49.7 79.1 18.8 60.3 Trade name 34.8 13.7 21.1 34.8 6.1 28.7 Total $ 529.3 $ 175.1 $ 354.2 $ 529.3 $ 108.9 $ 420.4
Amortization on Intangible Assets	The estimated aggregate amortization on these intangible assets for each of the next five years as of December 31, 2017, is as follows: (millions) Year Amortization Expense 2018 $ 71.9 2019 66.4 2020 56.9 2021 56.6 2022 29.2

SCHEDULE I - Summary Of Inves43

SCHEDULE I - Summary Of Investments - Other Than Investments in Related Parties Summary Of Investments - Other than Investments In Related Parties (Detail) $ in Millions	Dec. 31, 2017USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	$ 25,276.9
Fair Value	27,274.7
Amount At Which Shown In The Balance Sheet	27,274.7
Fixed maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	20,209.9
Fair Value	20,201.7
Amount At Which Shown In The Balance Sheet	20,201.7
Fixed maturities | Bonds | US Government and Government Agencies and Authorities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	6,688.8
Fair Value	6,645.9
Amount At Which Shown In The Balance Sheet	6,645.9
Fixed maturities | Bonds | State and local government obligations
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,285.6
Fair Value	2,297.1
Amount At Which Shown In The Balance Sheet	2,297.1
Fixed maturities | Bonds | Public Utility, Bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	192.5
Fair Value	193.6
Amount At Which Shown In The Balance Sheet	193.6
Fixed maturities | Bonds | Corporate And Other Debt Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	4,804.7
Fair Value	4,804.1
Amount At Which Shown In The Balance Sheet	4,804.1
Fixed maturities | Bonds | Asset-backed Securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	6,043.4
Fair Value	6,050
Amount At Which Shown In The Balance Sheet	6,050
Fixed maturities | Redeemable preferred stocks
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	194.9
Fair Value	211
Amount At Which Shown In The Balance Sheet	211
Equity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,197.6
Fair Value	4,203.6
Amount At Which Shown In The Balance Sheet	4,203.6
Equity securities | Common Stocks, by Industry | Utilities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	106.8
Fair Value	177.5
Amount At Which Shown In The Balance Sheet	177.5
Equity securities | Common Stocks, by Industry | Financials
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	285.8
Fair Value	685
Amount At Which Shown In The Balance Sheet	685
Equity securities | Common Stocks, by Industry | Industrials
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	1,106.4
Fair Value	2,537.3
Amount At Which Shown In The Balance Sheet	2,537.3
Equity securities | Nonredeemable preferred stocks
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	698.6
Fair Value	803.8
Amount At Which Shown In The Balance Sheet	803.8
Short-term investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Cost	2,869.4
Fair Value	2,869.4
Amount At Which Shown In The Balance Sheet	$ 2,869.4

SCHEDULE II - Condensed Finan44

SCHEDULE II - Condensed Financial Information Of Registrant Condensed Statements Of Comprehensive Income (Parent Company) (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues
Total revenues		$ 26,839	$ 23,441.4	$ 20,853.8
Expenses
Interest expense		153.1	140.9	136
Total expenses		24,700.1	21,970.7	18,942.2
Income before income taxes		2,138.9	1,470.7	1,911.6
Benefit for income taxes		540.8	413.5	611.1
Net income attributable to Progressive		1,592.2	1,031	1,267.6
Comprehensive income attributable to Progressive		1,941	1,164	1,044.9
Parent Company
Revenues
Dividends from subsidiaries		867.3	375.5	852.5
Undistributed income (loss) from subsidiaries		866.3	741.9	500
Equity in net income of subsidiaries	[1]	1,733.6	1,117.4	1,352.5
Intercompany investment income	[1]	11.3	5.5	3.9
Gains (losses) on extinguishment of debt		0.2	1.6	(0.9)
Total revenues		1,745.1	1,124.5	1,355.5
Expenses
Interest expense		151.1	140.4	136.1
Deferred compensation	[2]	23.2	5.3	5.3
Other operating costs and expenses		4.6	4.2	5.4
Total expenses		178.9	149.9	146.8
Income before income taxes		1,566.2	974.6	1,208.7
Benefit for income taxes		26	56.4	58.9
Net income attributable to Progressive		1,592.2	1,031	1,267.6
Other comprehensive income (loss)		348.8	133	(222.7)
Comprehensive income attributable to Progressive		$ 1,941	$ 1,164	$ 1,044.9
[1]	Eliminated in consolidation.
[2]	See Note 4 – Employee Benefit Plans in these condensed financial statements.

SCHEDULE II - Condensed Finan45

SCHEDULE II - Condensed Financial Information Of Registrant Condensed Balance Sheets (Parent Company) (Detail) - USD ($) $ in Millions		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2014
Assets
Other assets		$ 412.9		$ 339.6
Total assets		38,701.2		33,427.5
Liabilities and Shareholders' Equity
Accounts payable, accrued expenses, and other liabilities	[1]	2,825.9		2,495.5
Dividends payable		655.1		395.4		$ 519.2
Total liabilities		28,912.7		24,986.7
Redeemable noncontrolling interest (NCI)		503.7	[2]	483.7	[2]	464.9	$ 0
Common shares, $1.00 par value (authorized 900.0; issued 797.5 including treasury shares of 215.8 and 217.6)		581.7		579.9
Paid-in capital		1,389.2		1,303.4
Retained earnings		6,031.7		5,140.4
Total accumulated other comprehensive income attributable to Progressive		1,282.2		933.4		800.4	$ 1,023.1
Total shareholders’ equity		9,284.8		7,957.1		$ 7,289.4
Total liabilities, redeemable NCI, and shareholders’ equity		38,701.2		33,427.5
Parent Company
Assets
Investment in affiliate		5		5
Investment in subsidiaries	[3]	11,721.3		10,280.9
Receivable from investment subsidiary	[3]	1,466.1		1,121.9
Intercompany receivable	[3]	578.6		443.3
Net deferred income taxes		67.1		97.1
Other assets		167.3		137.3
Total assets		14,005.4		12,085.5
Liabilities and Shareholders' Equity
Accounts payable, accrued expenses, and other liabilities		292.6		228.4
Dividends payable		655.1		395.4
Debt		3,269.2		3,020.9
Total liabilities		4,216.9		3,644.7
Redeemable noncontrolling interest (NCI)		503.7		483.7
Common shares, $1.00 par value (authorized 900.0; issued 797.5 including treasury shares of 215.8 and 217.6)		581.7		579.9
Paid-in capital		1,389.2		1,303.4
Retained earnings		6,031.7		5,140.4
Total accumulated other comprehensive income attributable to Progressive		1,282.2		933.4
Total shareholders’ equity		9,284.8		7,957.1
Total liabilities, redeemable NCI, and shareholders’ equity		$ 14,005.4		$ 12,085.5
[1]	See Note 12 – Litigation and Note 13 – Commitments and Contingencies for further discussion.
[2]	See Note 15 – Redeemable Noncontrolling Interest for further discussion.
[3]	Eliminated in consolidation.

SCHEDULE II - Condensed Finan46

SCHEDULE II - Condensed Financial Information Of Registrant Condensed Balance Sheets (Parent Company) (Parenthetical) (Detail) - $ / shares	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Common Shares, par value (USD per share)	$ 1	$ 1
Common Shares, authorized (shares)	900,000,000	900,000,000
Common Shares, issued (shares)	797,500,000	797,500,000
Common Shares, treasury shares (shares)	215,800,000	217,600,000
Parent Company
Condensed Financial Statements, Captions [Line Items]
Common Shares, par value (USD per share)	$ 1	$ 1
Common Shares, authorized (shares)	900,000,000	900,000,000
Common Shares, issued (shares)	797,500,000	797,500,000
Common Shares, treasury shares (shares)	215,800,000	217,600,000

SCHEDULE II - Condensed Finan47

SCHEDULE II - Condensed Financial Information Of Registrant Condensed Statements Of Cash Flows (Parent Company) (Detail) - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Feb. 28, 2017	Feb. 29, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Flows From Operating Activities:
Net income attributable to Progressive			$ 1,592.2	$ 1,031	$ 1,267.6
Adjustments to reconcile net income attributable to Progressive to net cash provided by operating activities:
Amortization of equity-based compensation			95.4	85.2	66.2
Changes in:
Accounts payable, accrued expenses, and other liabilities			400	308.9	37.9
Income taxes			(172.6)	(55.7)	(107.2)
Other, net			(134.8)	(90.2)	(60.2)
Net cash provided by operating activities			3,756.8	2,732.7	2,292.9
Cash Flows From Investing Activities:
Net cash used in investing activities			(3,406.7)	(2,480.7)	(1,923.9)
Cash Flows From Financing Activities:
Net proceeds from debt issuance			841.1	495.6	382
Reacquisitions of debt			(635.6)	(18.2)	(19.3)
Dividends paid to shareholders	$ (395.4)	$ (519)
Acquisition of treasury shares for restricted stock tax liabilities			(57.6)	(25.1)	(30.6)
Acquisition of treasury shares acquired in open market			(4.9)	(167.4)	(177.9)
Tax benefit from vesting of equity-based compensation			0	9.2	16.8
Net cash used in financing activities			(300.9)	(250.4)	(252.8)
Change in cash, cash equivalents, and restricted cash			48.9	2	116
Cash, cash equivalents, and restricted cash - Beginning of year			226.4	224.4	108.4
Cash, cash equivalents, and restricted cash - End of year			275.3	226.4	224.4
Parent Company
Cash Flows From Operating Activities:
Net income attributable to Progressive			1,592.2	1,031	1,267.6
Adjustments to reconcile net income attributable to Progressive to net cash provided by operating activities:
Undistributed (income) loss from subsidiaries			(866.3)	(741.9)	(500)
Amortization of equity-based compensation			2.1	2.2	2.4
(Gains) losses on extinguishment of debt			(0.2)	(1.6)	0.9
Changes in:
Intercompany receivable			(71.3)	(37.3)	7
Accounts payable, accrued expenses, and other liabilities			53.6	24.2	(46.2)
Income taxes			37.3	(5)	12.3
Other, net			(22.6)	(13.3)	(3.1)
Net cash provided by operating activities			724.8	258.3	740.9
Cash Flows From Investing Activities:
Additional investments in equity securities of consolidated subsidiaries			(86.7)	(112)	(40.2)
(Paid to) received from investment subsidiary			(344.2)	78.6	409.1
Net cash used in investing activities			(449.6)	(33.4)	(521.2)
Cash Flows From Financing Activities:
Net proceeds from debt issuance			841.1	495.6	394.9
Reacquisitions of debt			(594.4)	(18.2)	(19.3)
Dividends paid to shareholders			(395.4)	(519)	(403.6)
Acquisition of treasury shares for restricted stock tax liabilities			(57.6)	(25.1)	(30.6)
Acquisition of treasury shares acquired in open market			(4.9)	(167.4)	(177.9)
Loan to ARX Holding Corp.			(64)	0	0
Tax benefit from vesting of equity-based compensation			0	9.2	16.8
Net cash used in financing activities			(275.2)	(224.9)	(219.7)
Change in cash, cash equivalents, and restricted cash			0	0	0
Cash, cash equivalents, and restricted cash - Beginning of year			0	0	0
Cash, cash equivalents, and restricted cash - End of year			0	0	0
Insurance Company | Parent Company
Cash Flows From Investing Activities:
Acquisition of business			(18.7)	0	0
ARX Holding Corp. | Parent Company
Cash Flows From Investing Activities:
Acquisition of business			$ 0	$ 0	$ (890.1)

SCHEDULE II - Condensed Finan48

SCHEDULE II - Condensed Financial Information Of Registrant Condensed Financial Information Of Registrant - Additional Information (Detail) - USD ($) $ in Billions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Apr. 01, 2015
Significant Accounting Policies [Line Items]
Securities Held In Consolidated Non-Insurance Subsidiary	$ 1.6	$ 1.3
ARX Holding Corp.
Significant Accounting Policies [Line Items]
Equity interest percentage	69.00%	69.20%	69.20%	5.00%
Parent Company
Significant Accounting Policies [Line Items]
Securities Held In Consolidated Non-Insurance Subsidiary	$ 1.6	$ 1.3

SCHEDULE II - Condensed Finan49

SCHEDULE II - Condensed Financial Information Of Registrant Progressive Corporation Cash Paid (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income taxes	$ 715.6	$ 459.4	$ 701.8
Interest	146.3	139.2	132
Parent Company
Income taxes	669.7	450.2	625
Interest	$ 142.2	$ 134.2	$ 128.2

SCHEDULE II - Condensed Finan50

SCHEDULE II - Condensed Financial Information Of Registrant Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Benefit for income taxes	$ 540.8	$ 413.5	$ 611.1
ARX and subsidiaries
Income Taxes [Line Items]
Noncontrolling interest, ownership percentage (less than)	80.00%
Adjustments for New Accounting Pronouncement
Income Taxes [Line Items]
Benefit for income taxes	$ 44.7

SCHEDULE III - Supplementary 51

SCHEDULE III - Supplementary Insurance Information Supplementary Insurance Information (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment [Line Items]
Deferred policy acquisition costs	[1]	$ 780.5	$ 651.2	$ 564.1
Future policy benefits, losses, claims, and loss expenses	[1]	13,086.9	11,368	10,039
Unearned premiums	[1]	8,903.5	7,468.3	6,621.8
Other policy claims and benefits payable	[1]	0	0	0
Premium revenue		25,729.9	22,474	19,899.1
Net investment income	[1],[2]	539.2	456.5	431.8
Benefits, claims, losses, and settlement expenses		18,808	16,879.6	14,342
Amortization of deferred policy acquisition costs		2,124.9	1,863.8	1,651.8
Other operating expenses		3,480.7	2,972	2,712.1
Net Premiums Written		27,132.1	23,353.5	20,564
Personal Lines
Supplementary Insurance Information, by Segment [Line Items]
Premium revenue		21,947.2	19,188.2	17,294.5
Benefits, claims, losses, and settlement expenses		16,141.4	14,591.1	12,748.7
Amortization of deferred policy acquisition costs		1,656.4	1,446.6	1,331.3
Other operating expenses		2,954.8	2,549.2	2,379.9
Net Premiums Written		22,928.4	19,819.5	17,703.6
Commercial Lines
Supplementary Insurance Information, by Segment [Line Items]
Premium revenue		2,793.9	2,421.3	1,995.9
Benefits, claims, losses, and settlement expenses		1,966.4	1,741	1,244.5
Amortization of deferred policy acquisition costs		309.3	266.7	219.4
Other operating expenses		335.3	285.4	232.6
Net Premiums Written		3,112.7	2,598.3	2,171.2
Property, Liability and Casualty Insurance Product Line
Supplementary Insurance Information, by Segment [Line Items]
Premium revenue		988.8	864.5	609.1
Benefits, claims, losses, and settlement expenses		700.2	546.1	349
Amortization of deferred policy acquisition costs		159.2	150.5	101.1
Other operating expenses		190.4	137.2	98.8
Net Premiums Written		1,091	935.7	689.6
Other Indemnity
Supplementary Insurance Information, by Segment [Line Items]
Premium revenue		0	0	(0.4)
Benefits, claims, losses, and settlement expenses		0	1.4	(0.2)
Amortization of deferred policy acquisition costs		0	0	0
Other operating expenses		0.2	0.2	0.8
Net Premiums Written		$ 0	$ 0	$ (0.4)
[1]	Progressive does not allocate assets, liabilities, or investment income to operating segments.
[2]	Excludes total net realized gains (losses) on securities.

SCHEDULE IV - Reinsurance Reins

SCHEDULE IV - Reinsurance Reinsurance (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	$ 26,425.7	$ 23,111.2	$ 20,454.1
Ceded to Other Companies	695.8	637.2	555
Property, Liability and Casualty Insurance Product Line
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross Amount	26,425.7	23,111.2	20,454.1
Ceded to Other Companies	695.8	637.2	555
Assumed From Other Companies	0	0	0
Net Amount	$ 25,729.9	$ 22,474	$ 19,899.1
Percentage of Amount Assumed to Net	0.00%	0.00%	0.00%

Reporting and Accounting Poli53

Reporting and Accounting Policies - Advertising Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Advertising costs	$ 1,005.4	$ 756.2	$ 748.3

Reporting and Accounting Poli54

Reporting and Accounting Policies - Capitalized Interest (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Capitalized interest	$ 2.8	$ 2.9	$ 2.4

Reporting and Accounting Poli55

Reporting and Accounting Policies - Compensation Expense Recognized for Equity-Based Compensation (Details) - Stock compensation plan - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pretax expense	$ 95.4	$ 85.2	$ 66.2
Tax benefit1	$ 33.4	$ 29.8	$ 23.2

Reporting and Accounting Poli56

Reporting and Accounting Policies - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Income taxes	$ 715.6	$ 459.4	$ 701.8
Interest	$ 146.3	$ 139.2	$ 132

Reporting and Accounting Poli57

Reporting and Accounting Policies - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Apr. 01, 2015
Property, Plant and Equipment [Line Items]
Change in Amortized Cost, Prospective Yield Adjustment, Quarterly Cash Flow Update	$ 0
Derivative Assets (Liabilities), at Fair Value, Net	0	$ 0
Held for sale property	$ 5,300,000	$ 8,700,000
Accelerated vesting percentage	50.00%
Tax at statutory federal rate	35.00%	35.00%	35.00%
Open reverse repurchase commitments	$ 0	$ 0
Assets	38,701,200,000	33,427,500,000
Liabilities	28,912,700,000	24,986,700,000
Benefit for income taxes	$ (540,800,000)	$ (413,500,000)	$ (611,100,000)
Computer equipment and laptops [Member]
Property, Plant and Equipment [Line Items]
Property and equipment useful life	3 years
Land and building [Member]
Property, Plant and Equipment [Line Items]
Percentage of total property and equipment	66.00%	65.00%
Software licenses [Member] | Lower Limit
Property, Plant and Equipment [Line Items]
Property and equipment useful life	1 year
Software licenses [Member] | Upper Limit
Property, Plant and Equipment [Line Items]
Property and equipment useful life	5 years
Capitalized software [Member] | Lower Limit
Property, Plant and Equipment [Line Items]
Property and equipment useful life	3 years
Capitalized software [Member] | Upper Limit
Property, Plant and Equipment [Line Items]
Property and equipment useful life	10 years
Buildings, improvements and integrated components [Member] | Lower Limit
Property, Plant and Equipment [Line Items]
Property and equipment useful life	7 years
Buildings, improvements and integrated components [Member] | Upper Limit
Property, Plant and Equipment [Line Items]
Property and equipment useful life	40 years
All other property and equipment [Member] | Lower Limit
Property, Plant and Equipment [Line Items]
Property and equipment useful life	3 years
All other property and equipment [Member] | Upper Limit
Property, Plant and Equipment [Line Items]
Property and equipment useful life	15 years
ARX Holding Corp.
Property, Plant and Equipment [Line Items]
Equity interest percentage	69.00%	69.20%	69.20%	5.00%
ARX Holding Corp. | Lower Limit
Property, Plant and Equipment [Line Items]
Intangible assets finite lives	2 years
ARX Holding Corp. | Upper Limit
Property, Plant and Equipment [Line Items]
Intangible assets finite lives	11 years
Accounting Standards Update 2016-01 [Member]
Property, Plant and Equipment [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 1,300,000,000
Adjustments for New Accounting Pronouncement
Property, Plant and Equipment [Line Items]
Benefit for income taxes	(44,700,000)
Adjustments for New Accounting Pronouncement
Property, Plant and Equipment [Line Items]
Benefit for income taxes	25,100,000
Cash and Cash Equivalents [Member] | ARX Holding Corp.
Property, Plant and Equipment [Line Items]
Open reverse repurchase commitments	247,200,000	$ 150,000,000	$ 174,800,000
Property Subject to Operating Lease [Member] | Pro Forma [Member] | Adjustments for New Accounting Pronouncement
Property, Plant and Equipment [Line Items]
Assets	140,000,000
Liabilities	$ 140,000,000
Tax Year 2018
Property, Plant and Equipment [Line Items]
Tax at statutory federal rate	21.00%

Investments Investment Portfoli

Investments Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 25,276.9	$ 22,031.7
Gross Unrealized Gains	2,097	1,588.9
Gross Unrealized Losses	(99.4)	(139.8)
Net Realized Gains (Losses)	0.2	1.8
Fair Value	$ 27,274.7	$ 23,482.6
% of Total Fair Value	100.00%	100.00%
Other Receivables from Broker-Dealers and Clearing Organizations	$ 5.8
Other Payables to Broker-Dealers and Clearing Organizations		$ 27.8
Securities Held In Consolidated Non-Insurance Subsidiary	1,600	1,300
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost	20,209.9	16,287.1
Gross Unrealized Gains	82	76.5
Gross Unrealized Losses	(90.4)	(121.6)
Net Realized Gains (Losses)	0.2	1.8
Fair Value	$ 20,201.7	$ 16,243.8
% of Total Fair Value	74.10%	69.20%
Fixed maturities | U.S. government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 6,688.8	$ 2,899.2
Gross Unrealized Gains	1.1	0
Gross Unrealized Losses	(44)	(29.1)
Net Realized Gains (Losses)	0	0
Fair Value	$ 6,645.9	$ 2,870.1
% of Total Fair Value	24.40%	12.20%
Fixed maturities | State and local government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 2,285.6	$ 2,509.5
Gross Unrealized Gains	20.7	13.8
Gross Unrealized Losses	(9.3)	(20.7)
Net Realized Gains (Losses)	0.1	0
Fair Value	$ 2,297.1	$ 2,502.6
% of Total Fair Value	8.40%	10.70%
Fixed maturities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 0	$ 24.5
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Net Realized Gains (Losses)	0	0
Fair Value	$ 0	$ 24.5
% of Total Fair Value	0.00%	0.10%
Fixed maturities | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 4,997.2	$ 4,557.8
Gross Unrealized Gains	14.8	17.3
Gross Unrealized Losses	(14.4)	(24.3)
Net Realized Gains (Losses)	0.1	0.1
Fair Value	$ 4,997.7	$ 4,550.9
% of Total Fair Value	18.30%	19.40%
Fixed maturities | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 828.8	$ 1,489.7
Gross Unrealized Gains	11.3	23.7
Gross Unrealized Losses	(3.4)	(15.6)
Net Realized Gains (Losses)	0	1.5
Fair Value	$ 836.7	$ 1,499.3
% of Total Fair Value	3.10%	6.40%
Fixed maturities | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 2,760.1	$ 2,266.9
Gross Unrealized Gains	11.8	12
Gross Unrealized Losses	(13.3)	(25.5)
Net Realized Gains (Losses)	0	0
Fair Value	$ 2,758.6	$ 2,253.4
% of Total Fair Value	10.10%	9.60%
Fixed maturities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 2,454.5	$ 2,350.7
Gross Unrealized Gains	4.5	4.6
Gross Unrealized Losses	(4.5)	(4.4)
Net Realized Gains (Losses)	0.2	0.2
Fair Value	$ 2,454.7	$ 2,351.1
% of Total Fair Value	9.00%	10.00%
Fixed maturities | Redeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 194.9	$ 188.8
Gross Unrealized Gains	17.8	5.1
Gross Unrealized Losses	(1.5)	(2)
Net Realized Gains (Losses)	(0.2)	0
Fair Value	$ 211	$ 191.9
% of Total Fair Value	0.80%	0.80%
Equity securities | Nonredeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 698.6	$ 734.2
Gross Unrealized Gains	114	135.4
Gross Unrealized Losses	(8.8)	(16.1)
Net Realized Gains (Losses)	0	0
Fair Value	$ 803.8	$ 853.5
% of Total Fair Value	2.90%	3.60%
Equity securities | Common equities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,499	$ 1,437.5
Gross Unrealized Gains	1,901	1,377
Gross Unrealized Losses	(0.2)	(2.1)
Net Realized Gains (Losses)	0	0
Fair Value	$ 3,399.8	$ 2,812.4
% of Total Fair Value	12.50%	12.00%
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 2,869.4	$ 3,572.9
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Net Realized Gains (Losses)	0	0
Fair Value	$ 2,869.4	$ 3,572.9
% of Total Fair Value	10.50%	15.20%

Investments Hybrid Securities (

Investments Hybrid Securities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Hybrid Securities [Line Items]
Hybrid securities	$ 142.9	$ 219.5
Fixed maturities | State and local government obligations
Hybrid Securities [Line Items]
Hybrid securities	6.1	0
Fixed maturities | Corporate debt securities
Hybrid Securities [Line Items]
Hybrid securities	99.8	40.1
Fixed maturities | Residential mortgage-backed securities
Hybrid Securities [Line Items]
Hybrid securities	0	170.5
Fixed maturities | Other asset-backed securities
Hybrid Securities [Line Items]
Hybrid securities	6.7	8.9
Fixed maturities | Redeemable preferred stocks
Hybrid Securities [Line Items]
Hybrid securities	$ 30.3	$ 0

Investments Composition of Fixe

Investments Composition of Fixed Maturities by Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Available for sale, cost
Less than one year	$ 3,964.1
One to five years	12,706.5
Five to ten years	3,294.4
Ten years or greater	244.9
Total	20,209.9	$ 16,287.1
Available for sale, Fair value
Less than one year	3,980
One to five years	12,671.3
Five to ten years	3,306.9
Ten years or greater	243.5
Total	$ 20,201.7	$ 16,243.8

Investments Gross Unrealized lo

Investments Gross Unrealized losses by Major Security (Detail) $ in Millions	Dec. 31, 2017USD ($)security	Dec. 31, 2016USD ($)security
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	1,166	1,369
Total Fair Value	$ 13,980.2	$ 11,175.9
Gross Unrealized Losses	$ (99.4)	$ (139.8)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	689	1,157
Less than 12 Months Fair Value	$ 10,434.6	$ 9,627.3
Less than 12 Months Unrealized Losses	$ (58.3)	$ (106.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	477	212
12 Months or Greater Fair Value	$ 3,545.6	$ 1,548.6
12 Months or Greater Unrealized Losses	$ (41.1)	$ (33.1)
Fixed maturities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	1,143	1,281
Total Fair Value	$ 13,839	$ 10,824.2
Gross Unrealized Losses	$ (90.4)	$ (121.6)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	670	1,080
Less than 12 Months Fair Value	$ 10,364.7	$ 9,432.4
Less than 12 Months Unrealized Losses	$ (57.6)	$ (101.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	473	201
12 Months or Greater Fair Value	$ 3,474.3	$ 1,391.8
12 Months or Greater Unrealized Losses	$ (32.8)	$ (20.4)
Fixed maturities | U.S. government obligations
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	58	30
Total Fair Value	$ 5,817	$ 2,774
Gross Unrealized Losses	$ (44)	$ (29.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	41	30
Less than 12 Months Fair Value	$ 4,869.3	$ 2,774
Less than 12 Months Unrealized Losses	$ (34.6)	$ (29.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	17	0
12 Months or Greater Fair Value	$ 947.7	$ 0
12 Months or Greater Unrealized Losses	$ (9.4)	$ 0
Fixed maturities | State and local government obligations
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	358	618
Total Fair Value	$ 1,200.3	$ 1,497.9
Gross Unrealized Losses	$ (9.3)	$ (20.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	230	584
Less than 12 Months Fair Value	$ 737.6	$ 1,404.3
Less than 12 Months Unrealized Losses	$ (4.4)	$ (19.6)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	128	34
12 Months or Greater Fair Value	$ 462.7	$ 93.6
12 Months or Greater Unrealized Losses	$ (4.9)	$ (1.1)
Fixed maturities | Corporate debt securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	222	184
Total Fair Value	$ 2,979.4	$ 2,615.1
Gross Unrealized Losses	$ (14.4)	$ (24.3)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	171	175
Less than 12 Months Fair Value	$ 2,072.9	$ 2,559.9
Less than 12 Months Unrealized Losses	$ (9.1)	$ (24)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	51	9
12 Months or Greater Fair Value	$ 906.5	$ 55.2
12 Months or Greater Unrealized Losses	$ (5.3)	$ (0.3)
Fixed maturities | Residential mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	201	233
Total Fair Value	$ 300.9	$ 953.7
Gross Unrealized Losses	$ (3.4)	$ (15.6)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	30	117
Less than 12 Months Fair Value	$ 75.1	$ 209.7
Less than 12 Months Unrealized Losses	$ (0.2)	$ (1.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	171	116
12 Months or Greater Fair Value	$ 225.8	$ 744
12 Months or Greater Unrealized Losses	$ (3.2)	$ (13.9)
Fixed maturities | Commercial mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	105	111
Total Fair Value	$ 1,682.3	$ 1,347.3
Gross Unrealized Losses	$ (13.3)	$ (25.5)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	63	85
Less than 12 Months Fair Value	$ 1,221.2	$ 1,061.2
Less than 12 Months Unrealized Losses	$ (5.9)	$ (22.9)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	42	26
12 Months or Greater Fair Value	$ 461.1	$ 286.1
12 Months or Greater Unrealized Losses	$ (7.4)	$ (2.6)
Fixed maturities | Other asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	197	103
Total Fair Value	$ 1,837.3	$ 1,605.2
Gross Unrealized Losses	$ (4.5)	$ (4.4)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	134	89
Less than 12 Months Fair Value	$ 1,377.8	$ 1,423.3
Less than 12 Months Unrealized Losses	$ (3.3)	$ (3.9)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	63	14
12 Months or Greater Fair Value	$ 459.5	$ 181.9
12 Months or Greater Unrealized Losses	$ (1.2)	$ (0.5)
Fixed maturities | Redeemable preferred stocks
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	2	2
Total Fair Value	$ 21.8	$ 31
Gross Unrealized Losses	$ (1.5)	$ (2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	1	0
Less than 12 Months Fair Value	$ 10.8	$ 0
Less than 12 Months Unrealized Losses	$ (0.1)	$ 0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	1	2
12 Months or Greater Fair Value	$ 11	$ 31
12 Months or Greater Unrealized Losses	$ (1.4)	$ (2)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	23	88
Total Fair Value	$ 141.2	$ 351.7
Gross Unrealized Losses	$ (9)	$ (18.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	19	77
Less than 12 Months Fair Value	$ 69.9	$ 194.9
Less than 12 Months Unrealized Losses	$ (0.7)	$ (5.5)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	4	11
12 Months or Greater Fair Value	$ 71.3	$ 156.8
12 Months or Greater Unrealized Losses	$ (8.3)	$ (12.7)
Equity securities | Nonredeemable preferred stocks
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	4	13
Total Fair Value	$ 127.8	$ 329.6
Gross Unrealized Losses	$ (8.8)	$ (16.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	1	8
Less than 12 Months Fair Value	$ 56.5	$ 175.2
Less than 12 Months Unrealized Losses	$ (0.5)	$ (3.8)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	3	5
12 Months or Greater Fair Value	$ 71.3	$ 154.4
12 Months or Greater Unrealized Losses	$ (8.3)	$ (12.3)
Equity securities | Common equities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	19	75
Total Fair Value	$ 13.4	$ 22.1
Gross Unrealized Losses	$ (0.2)	$ (2.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	18	69
Less than 12 Months Fair Value	$ 13.4	$ 19.7
Less than 12 Months Unrealized Losses	$ (0.2)	$ (1.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	1	6
12 Months or Greater Fair Value	$ 0	$ 2.4
12 Months or Greater Unrealized Losses	$ 0	$ (0.4)

Investments Total Non-Credit Po

Investments Total Non-Credit Portion of Other-Than-Temporary Impairment Recorded in Accumulated Other Comprehensive Income, Reflecting Original Non-Credit Loss at Time Credit Impairment (Detail) - Fixed maturities - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Other Than Temporary Impairment Non Credit Losses Recognized In Accumulated Other Comprehensive Income [Line Items]
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Pre-Tax, Available-for-sale, Debt Securities	$ (20)	$ (43.9)
Residential mortgage-backed securities
Other Than Temporary Impairment Non Credit Losses Recognized In Accumulated Other Comprehensive Income [Line Items]
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Pre-Tax, Available-for-sale, Debt Securities	(19.7)	(43.3)
Commercial mortgage-backed securities
Other Than Temporary Impairment Non Credit Losses Recognized In Accumulated Other Comprehensive Income [Line Items]
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Pre-Tax, Available-for-sale, Debt Securities	$ (0.3)	$ (0.6)

Investments OTTI Credit Losses

Investments OTTI Credit Losses Recognized in Earnings (Detail) - Fixed maturities - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	$ 11,500,000	$ 12,800,000	$ 13,100,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	400,000	0	0
Reductions for securities sold/matured	(11,200,000)	0	(1,400,000)
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	(200,000)	(1,300,000)	1,100,000
Ending balance	500,000	11,500,000	12,800,000
Residential mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	11,100,000	12,400,000	12,700,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	0	0	0
Reductions for securities sold/matured	(10,900,000)	0	(1,400,000)
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	(200,000)	(1,300,000)	1,100,000
Ending balance	0	11,100,000	12,400,000
Commercial mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	400,000	400,000	400,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	400,000	0	0
Reductions for securities sold/matured	(300,000)	0	0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0	0	0
Ending balance	$ 500,000	$ 400,000	$ 400,000

Investments Components of Net R

Investments Components of Net Realized Gains (Losses) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	$ 175.9	$ 193.9	$ 192.9
Gross realized losses on securities sales	(64.7)	(40)	(49.9)
Net realized gains (losses) on securities sales	111.2	153.9	143
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales	(61.2)	(85.3)	(8.7)
Other gains (losses)	(0.4)	(17.5)	(21.6)
Total net realized gains (losses) on securities	49.6	51.1	112.7
Fixed maturities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	74.5	120.6	77.2
Gross realized losses on securities sales	(46.4)	(18.9)	(8.3)
Net realized gains (losses) on securities sales	28.1	101.7	68.9
Fixed maturities | U.S. government obligations
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	6.2	24.6	17.5
Gross realized losses on securities sales	(28.7)	(2.4)	(0.9)
Net realized gains (losses) on securities sales	(22.5)	22.2	16.6
Fixed maturities | State and local government obligations
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	10.5	16	7.8
Gross realized losses on securities sales	(0.1)	(1.6)	(0.3)
Net realized gains (losses) on securities sales	10.4	14.4	7.5
Fixed maturities | Corporate And Other Debt Securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	20.3	43.3	31.2
Gross realized losses on securities sales	(5.1)	(2.5)	(5)
Net realized gains (losses) on securities sales	15.2	40.8	26.2
Fixed maturities | Residential mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	23.8	2.5	4.9
Gross realized losses on securities sales	(0.4)	(0.2)	(0.8)
Net realized gains (losses) on securities sales	23.4	2.3	4.1
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	4.9	13.3	15.7
Gross realized losses on securities sales	(5.3)	(5.6)	(1.3)
Net realized gains (losses) on securities sales	(0.4)	7.7	14.4
Fixed maturities | Other asset-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.3	0	0
Gross realized losses on securities sales	(0.4)	0	0
Net realized gains (losses) on securities sales	(0.1)	0	0
Fixed maturities | Redeemable preferred stocks
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	8.5	20.9	0.1
Gross realized losses on securities sales	(6.4)	(6.6)	0
Net realized gains (losses) on securities sales	2.1	14.3	0.1
Equity securities | Nonredeemable preferred stocks
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	58.4	11.9	65.3
Gross realized losses on securities sales	(5.9)	(5.3)	(3.2)
Net realized gains (losses) on securities sales	52.5	6.6	62.1
Equity securities | Common equities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	43	61.3	50.4
Gross realized losses on securities sales	(12.2)	(15.7)	(38.4)
Net realized gains (losses) on securities sales	30.8	45.6	12
Short-term investments
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0	0.1	0
Gross realized losses on securities sales	(0.2)	(0.1)	0
Net realized gains (losses) on securities sales	(0.2)	0	0
Hybrid and Derivative Instruments and Litigation Settlements | Litigation Settlements
Gain (Loss) on Investments [Line Items]
Other gains (losses)	1.2	0.4	0.4
Hybrid and Derivative Instruments and Litigation Settlements | Hybrid Preferred Stock
Gain (Loss) on Investments [Line Items]
Other gains (losses)	(1.6)	2.1	(1.3)
Hybrid and Derivative Instruments and Litigation Settlements | Derivative Instruments
Gain (Loss) on Investments [Line Items]
Other gains (losses)	0	(20)	(20.7)
Investment [Domain]
Gain (Loss) on Investments [Line Items]
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales	(11.6)	(25.6)	(8.7)
Investment [Domain] | Fixed maturities
Gain (Loss) on Investments [Line Items]
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales	(0.4)	(25.3)	0
Investment [Domain] | Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales	(0.4)	0	0
Investment [Domain] | Fixed maturities | Redeemable preferred stocks
Gain (Loss) on Investments [Line Items]
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales	0	(25.3)	0
Investment [Domain] | Equity securities | Common equities
Gain (Loss) on Investments [Line Items]
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales	(11.2)	(0.3)	(8.7)
Other Assets [Member] | Other Assets [Member]
Gain (Loss) on Investments [Line Items]
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales	$ (49.6)	$ (59.7)	$ 0

Investments Components of Net I

Investments Components of Net Investment Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ 563.1	$ 478.9	$ 454.6
Investment expenses	(23.9)	(22.4)	(22.8)
Net investment income	539.2	456.5	431.8
Fixed maturities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	422.9	353.4	357.7
Fixed maturities | U.S. government obligations
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	72.7	18.2	28.3
Fixed maturities | State and local government obligations
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	51.5	52.3	60.7
Fixed maturities | Foreign government obligations
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	0.3	0.4	0.4
Fixed maturities | Corporate debt securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	125.2	110.7	102.4
Fixed maturities | Residential mortgage-backed securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	34.7	47.3	54.3
Fixed maturities | Commercial mortgage-backed securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	79.6	81.6	74.6
Fixed maturities | Other asset-backed securities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	47.1	28	22
Fixed maturities | Redeemable preferred stocks
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	11.8	14.9	15
Equity securities | Nonredeemable preferred stocks
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	44.1	48.6	43.7
Equity securities | Common equities
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	58.3	57.2	51
Short-term investments
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Investment income	$ 37.8	$ 19.7	$ 2.2

Investments Derivative Instrume

Investments Derivative Instruments (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Aug. 31, 2016
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	$ 0	$ 0
Gain (Loss) on Derivative Instruments, Net, Pretax	0	(20,000,000)	$ (20,700,000)
Designated as Hedging Instrument [Member] | Ineffective Cash Flow Hedge [Member]
Derivative [Line Items]
Derivative, Notional Amount	31,000,000	370,000,000	18,000,000	$ 350,000,000
Derivative Assets (Liabilities), at Fair Value, Net	0	0
Gain (Loss) on Derivative Instruments, Net, Pretax	0	(1,300,000)	200,000
Non-hedging Instruments | Interest Rate Swaps
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	0	0
Non-hedging Instruments | Interest Rate Swaps | Fixed maturities
Derivative [Line Items]
Derivative, Notional Amount	0	0	750,000,000
Derivative Assets (Liabilities), at Fair Value, Net	0	0
Gain (Loss) on Derivative Instruments, Net, Pretax	0	0	(23,400,000)
Non-hedging Instruments | Future [Member]
Derivative [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net	0	0
Closed Positions [Member] | Non-hedging Instruments | Interest Rate Swaps
Derivative [Line Items]
Derivative, Notional Amount	0	750,000,000	0
Derivative Assets (Liabilities), at Fair Value, Net	0	0
Gain (Loss) on Derivative Instruments, Net, Pretax	0	(19,000,000)	0
Closed Positions [Member] | Non-hedging Instruments | Future [Member]
Derivative [Line Items]
Derivative, Notional Amount	0	135,000,000	691,000,000
Derivative Assets (Liabilities), at Fair Value, Net	0	0
Gain (Loss) on Derivative Instruments, Net, Pretax	$ 0	$ 300,000	$ 2,500,000

Investments - Additional Inform

Investments - Additional Information (Detail)	1 Months Ended		12 Months Ended
	Apr. 30, 2017USD ($)	Aug. 31, 2016USD ($)	Dec. 31, 2017USD ($)security	Dec. 31, 2016USD ($)security	Dec. 31, 2015USD ($)
Schedule of Investments [Line Items]
Principal amount of bonds and certificates of deposit on deposit to meet state insurance regulatory and/or rating agency requirements			$ 222,600,000
Securities exceeding 10% of Shareholders' Equity			0	$ 0
Fair value of fixed-maturity securities that were non-income producing during the preceding 12 months			0
Short-term Investments			2,869,400,000	3,572,900,000
Securities Sold under Agreements to Repurchase			0	0
Open reverse repurchase commitments			0	0
Gross Unrealized Losses			$ 99,400,000	$ 139,800,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security			1,166	1,369
Trading securities			$ 0	$ 0
Net realized gains (losses) on securities			114,100,000	137,900,000	$ 136,500,000
Gain (Loss) on Derivative Instruments, Net, Pretax			0	(20,000,000)	(20,700,000)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax			300,000	1,900,000	1,800,000
Derivative Assets (Liabilities), at Fair Value, Net			0	0
4.125% Senior Notes due 2047
Schedule of Investments [Line Items]
Senior notes, issuance amount	$ 850,000,000		$ 850,000,000	0
Debt Instrument, Interest Rate, Stated Percentage	4.125%		4.125%
Unrealized Gain (Loss) on Interest Rate Cash Flow Hedges, Pretax, Accumulated Other Comprehensive Income (Loss)	$ (8,000,000)
2.45% Senior Notes Due 2027
Schedule of Investments [Line Items]
Senior notes, issuance amount		$ 500,000,000	$ 500,000,000	500,000,000
Debt Instrument, Interest Rate, Stated Percentage		2.45%	2.45%
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Schedule of Investments [Line Items]
Senior notes, issuance amount			$ 0	1,000,000,000
Debt Instrument, Interest Rate, Stated Percentage			6.70%
Debt Instrument, Repurchased Face Amount			$ 31,000,000	20,000,000	18,000,000
Equity securities
Schedule of Investments [Line Items]
Gross Unrealized Losses			$ 9,000,000	$ 18,200,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security			23	88
Equity securities | Common equities
Schedule of Investments [Line Items]
Gross Unrealized Losses			$ 200,000	$ 2,100,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security			19	75
Equity securities | Nonredeemable preferred stocks
Schedule of Investments [Line Items]
Gross Unrealized Losses			$ 8,800,000	$ 16,100,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security			4	13
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Percentage Decline			6.00%
Fixed maturities
Schedule of Investments [Line Items]
Gross Unrealized Losses			$ 90,400,000	$ 121,600,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security			1,143	1,281
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment			$ 400,000	$ 0	0
Fixed maturities | Corporate debt securities
Schedule of Investments [Line Items]
Gross Unrealized Losses			$ 14,400,000	$ 24,300,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security			222	184
Fixed maturities | Redeemable preferred stocks
Schedule of Investments [Line Items]
Gross Unrealized Losses			$ 1,500,000	$ 2,000,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security			2	2
Fixed maturities | Commercial mortgage-backed securities
Schedule of Investments [Line Items]
Gross Unrealized Losses			$ 13,300,000	$ 25,500,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security			105	111
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment			$ 400,000	$ 0	0
Fixed Income Securities
Schedule of Investments [Line Items]
Gross Unrealized Losses			99,200,000
Foreign Government Debt
Schedule of Investments [Line Items]
Short-term Investments			0	0
Interest Rate Swaps
Schedule of Investments [Line Items]
Cash collateral received from counterparty				4,900,000
Categories of Investments, Marketable Securities, Trading Securities
Schedule of Investments [Line Items]
Net realized gains (losses) on securities			$ 0	0	0
Russell One Thousand | Equity securities | Common equities
Schedule of Investments [Line Items]
Percentage of common stock portfolio			96.00%
Managed Equity Strategy | Equity securities | Common equities
Schedule of Investments [Line Items]
Percentage of common stock portfolio			4.00%
Designated as Hedging Instrument [Member] | Ineffective Cash Flow Hedge [Member]
Schedule of Investments [Line Items]
Derivative, Notional Amount		$ 350,000,000	$ 31,000,000	370,000,000	18,000,000
Gain (Loss) on Derivative Instruments, Net, Pretax			0	(1,300,000)	200,000
Derivative Assets (Liabilities), at Fair Value, Net			0	0
Designated as Hedging Instrument [Member] | Ineffective Cash Flow Hedge [Member] | 2.45% Senior Notes Due 2027
Schedule of Investments [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax		$ (1,400,000)	(1,400,000)
Non-hedging Instruments | Interest Rate Swaps
Schedule of Investments [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net			0	0
Non-hedging Instruments | Future [Member]
Schedule of Investments [Line Items]
Derivative Assets (Liabilities), at Fair Value, Net			0	0
Non-hedging Instruments | Fixed maturities | Interest Rate Swaps
Schedule of Investments [Line Items]
Derivative, Notional Amount			0	0	750,000,000
Gain (Loss) on Derivative Instruments, Net, Pretax			0	0	$ (23,400,000)
Derivative Assets (Liabilities), at Fair Value, Net			$ 0	$ 0

Fair Value Composition of Inves

Fair Value Composition of Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 27,274.7	$ 23,482.6
Long-term Debt, Fair Value	3,643.6	3,315.8
Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	20,201.7	16,243.8
Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,050	6,103.8
Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	13,940.7	9,948.1
Fixed maturities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,645.9	2,870.1
Fixed maturities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,645.9	2,870.1
Fixed maturities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,297.1	2,502.6
Fixed maturities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,297.1	2,502.6
Fixed maturities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	24.5
Fixed maturities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	24.5
Fixed maturities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	4,997.7	4,550.9
Fixed maturities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	4,997.7	4,550.9
Fixed maturities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	836.7	1,499.3
Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	836.7	1,499.3
Fixed maturities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,758.6	2,253.4
Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,758.6	2,253.4
Fixed maturities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,454.7	2,351.1
Fixed maturities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,454.7	2,351.1
Fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	211	191.9
Fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	64.1	59.5
Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	11.4	30.9
Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	135.5	101.5
Equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	803.8	853.5
Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	798.8	853.5
Equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5	0
Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,399.8	2,812.4
Equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,399.5	2,812
Equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0.3	0.4
Total Fixed Maturities and Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	24,405.3	19,909.7
Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,869.4	3,572.9
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	25,276.9	22,031.7
Debt	3,306.3	3,148.2
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	20,209.9	16,287.1
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,043.4	6,107.3
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	13,971.6	9,991
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,688.8	2,899.2
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,285.6	2,509.5
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	24.5
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	4,997.2	4,557.8
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	828.8	1,489.7
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,760.1	2,266.9
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,454.5	2,350.7
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	194.9	188.8
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	61.3	59.8
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	10.1	30.5
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	123.5	98.5
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	698.6	734.2
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	693.6	734.2
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5	0
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,499	1,437.5
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,498.7	1,437.1
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0.3	0.4
Carrying (Reported) Amount, Fair Value Disclosure | Total Fixed Maturities and Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	22,407.5	18,458.8
Carrying (Reported) Amount, Fair Value Disclosure | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,869.4	3,572.9
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	11,950.4	8,854
Long-term Debt, Fair Value	0	0
Fair Value, Inputs, Level 1 | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,645.9	2,894.6
Fair Value, Inputs, Level 1 | Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,645.9	2,894.6
Fair Value, Inputs, Level 1 | Fixed maturities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,645.9	2,870.1
Fair Value, Inputs, Level 1 | Fixed maturities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	24.5
Fair Value, Inputs, Level 1 | Fixed maturities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 1 | Equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	80.6	138.1
Fair Value, Inputs, Level 1 | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	80.6	138.1
Fair Value, Inputs, Level 1 | Equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 1 | Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,399.5	2,812
Fair Value, Inputs, Level 1 | Equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,399.5	2,812
Fair Value, Inputs, Level 1 | Equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 1 | Total Fixed Maturities and Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	10,126	5,844.7
Fair Value, Inputs, Level 1 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,824.4	3,009.3
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	15,319	14,627.9
Long-term Debt, Fair Value	3,606.5	3,188.5
Fair Value, Inputs, Level 2 | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	13,555.8	13,348.9
Fair Value, Inputs, Level 2 | Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,050	6,103.5
Fair Value, Inputs, Level 2 | Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,294.8	7,053.5
Fair Value, Inputs, Level 2 | Fixed maturities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 2 | Fixed maturities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,297.1	2,502.6
Fair Value, Inputs, Level 2 | Fixed maturities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 2 | Fixed maturities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	4,997.7	4,550.9
Fair Value, Inputs, Level 2 | Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	836.7	1,499.3
Fair Value, Inputs, Level 2 | Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,758.6	2,253.1
Fair Value, Inputs, Level 2 | Fixed maturities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,454.7	2,351.1
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	211	191.9
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	64.1	59.5
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	11.4	30.9
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	135.5	101.5
Fair Value, Inputs, Level 2 | Equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	718.2	715.4
Fair Value, Inputs, Level 2 | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	718.2	715.4
Fair Value, Inputs, Level 2 | Equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 2 | Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 2 | Equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 2 | Equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 2 | Total Fixed Maturities and Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	14,274	14,064.3
Fair Value, Inputs, Level 2 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,045	563.6
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5.3	0.7
Long-term Debt, Fair Value	37.1	127.3
Fair Value, Inputs, Level 3 | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0.3
Fair Value, Inputs, Level 3 | Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0.3
Fair Value, Inputs, Level 3 | Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0.3
Fair Value, Inputs, Level 3 | Fixed maturities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5	0
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5	0
Fair Value, Inputs, Level 3 | Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0.3	0.4
Fair Value, Inputs, Level 3 | Equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0
Fair Value, Inputs, Level 3 | Equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0.3	0.4
Fair Value, Inputs, Level 3 | Total Fixed Maturities and Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5.3	0.7
Fair Value, Inputs, Level 3 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 0	$ 0

Fair Value Summary of Changes i

Fair Value Summary of Changes in Fair Value Associated With Level 3 Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	$ 0.7	$ 10.2
Calls/ Maturities/ Paydowns	(0.4)	(9.6)
Purchases	5	0
Sales	0	0
Net Realized (Gain)/Loss on Sales	0	0
Change in Valuation	0	0.1
Net Transfers In (Out)	0	0
Ending Fair value	5.3	0.7
Fixed maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0.3	9.9
Calls/ Maturities/ Paydowns	(0.3)	(9.6)
Purchases	0	0
Sales	0	0
Net Realized (Gain)/Loss on Sales	0	0
Change in Valuation	0	0
Net Transfers In (Out)	0	0
Ending Fair value	0	0.3
Fixed maturities | Commercial mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0.3	9.9
Calls/ Maturities/ Paydowns	(0.3)	(9.6)
Purchases	0	0
Sales	0	0
Net Realized (Gain)/Loss on Sales	0	0
Change in Valuation	0	0
Net Transfers In (Out)	0	0
Ending Fair value	0	0.3
Equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0	0
Calls/ Maturities/ Paydowns	0	0
Purchases	5	0
Sales	0	0
Net Realized (Gain)/Loss on Sales	0	0
Change in Valuation	0	0
Net Transfers In (Out)	0	0
Ending Fair value	5	0
Equity securities | Common equities | Other risk investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0.4	0.3
Calls/ Maturities/ Paydowns	(0.1)	0
Purchases	0	0
Sales	0	0
Net Realized (Gain)/Loss on Sales	0	0
Change in Valuation	0	0.1
Net Transfers In (Out)	0	0
Ending Fair value	$ 0.3	$ 0.4

Fair Value Summary of Quantitat

Fair Value Summary of Quantitative Information about Level 3 Fair Value Measurements (Detail) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)security$ / shares	Dec. 31, 2016USD ($)security
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	$ 27,274.7	$ 23,482.6
Equity securities | Nonredeemable preferred stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	803.8	853.5
Fixed maturities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	20,201.7	16,243.8
Fixed maturities | Commercial mortgage-backed securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	2,758.6	2,253.4
Fair Value, Inputs, Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	5.3	0.7
Fair Value, Inputs, Level 3 | Third Party Pricing Exemption Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	0.3	0.4
Fair Value, Inputs, Level 3 | Total Quantitative Level Three Securities [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	$ 5	$ 0.3
Fair Value, Inputs, Level 3 | Internal Pricing
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Number of securities | security	1	0
Fair Value	$ 5
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	5	$ 0
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks | Internal Pricing
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	$ 5	0
Valuation Technique	Internal price
Fair Value Inputs, Original Issue Price | $ / shares	$ 3.9
Fair Value, Inputs, Level 3 | Fixed maturities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	$ 0	0.3
Fair Value, Inputs, Level 3 | Fixed maturities | External Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	0	$ 0.3
Fair Value, Inputs, Level 3 | Fixed maturities | Commercial mortgage-backed securities | External Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Number of securities | security		1
Fair Value	$ 0	$ 0.3
Fair Value Inputs, Prepayment Rate		0.00%
Valuation Technique		External vendor

Fair Value - Additional Informa

Fair Value - Additional Information (Detail) $ in Millions	Dec. 31, 2017USD ($)security	Dec. 31, 2016USD ($)security
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value of securities transferred out of Level 2 into Level 1	$ 0	$ 0
Fair Value	$ 27,274.7	$ 23,482.6
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	66.00%	52.00%
Fair Value	$ 11,950.4	$ 8,854
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	98.00%	99.00%
Dealer Quoted Prices, Percent of FV Hierarchy Level	2.00%	1.00%
Fair Value	$ 15,319	$ 14,627.9
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 5.3	$ 0.7
Internal Pricing | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of securities | security	1	0
Fair Value	$ 5
Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	20,201.7	$ 16,243.8
Fixed maturities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,645.9	2,894.6
Fixed maturities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	13,555.8	13,348.9
Fixed maturities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 0	$ 0.3

Debt (Detail)

Debt (Detail) $ in Millions		Dec. 31, 2017USD ($)instrument	Dec. 31, 2016USD ($)instrument
Debt Instrument [Line Items]
Fair Value		$ 3,643.6	$ 3,315.8
Debt, Long-term and Short-term, Combined Amount	[1]	3,306.3	3,148.2
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Carrying Value		498.8	498.4
Fair Value		520.7	528.8
2.45% Senior Notes Due 2027
Debt Instrument [Line Items]
Carrying Value		496.1	495.8
Fair Value		477.9	464.6
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Carrying Value		296.1	295.9
Fair Value		382.3	380.1
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Carrying Value		395.3	395.2
Fair Value		516.9	499
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Carrying Value		346.5	346.4
Fair Value		388.7	362.3
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Carrying Value		395.2	395.1
Fair Value		402.9	372.5
4.125% Senior Notes due 2047
Debt Instrument [Line Items]
Carrying Value		841.2	0
Fair Value		917.1	0
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Carrying Value		0	594.1
Fair Value		0	581.2
Other Debt Instruments [Member]
Debt Instrument [Line Items]
Debt, Long-term and Short-term, Combined Amount		$ 37.1	$ 127.3
Notes Payable, Other Payables [Member] | Other Debt Instruments [Member]
Debt Instrument [Line Items]
Debt Instrument, Number of Debt Instruments Acquired | instrument		2	2
Debt, Long-term and Short-term, Combined Amount		$ 37.1	$ 62.1
Trust Preferred Debt Securities [Member] | Other Debt Instruments [Member]
Debt Instrument [Line Items]
Carrying Value		$ 0	$ 41.2
Debt Instrument, Number of Debt Instruments Acquired | instrument		0	2
Senior Notes [Member] | Other Debt Instruments [Member]
Debt Instrument [Line Items]
Carrying Value		$ 0	$ 24
Debt Instrument, Number of Debt Instruments Acquired | instrument		0	4
[1]	Consists of both short-term and long-term debt. See Note 4 – Debt for further discussion.

Debt (Parenthetical) (Detail)

Debt (Parenthetical) (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Apr. 30, 2017	Aug. 31, 2016
Debt Instrument [Line Items]
Debt instrument, outstanding	$ 3,337,100,000
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Senior notes, issuance amount	$ 500,000,000	$ 500,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.75%
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Senior notes, issuance amount	$ 300,000,000	300,000,000
Debt Instrument, Interest Rate, Stated Percentage	6.625%
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Senior notes, issuance amount	$ 400,000,000	400,000,000
Debt Instrument, Interest Rate, Stated Percentage	6.25%
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Senior notes, issuance amount	$ 350,000,000	350,000,000
Debt Instrument, Interest Rate, Stated Percentage	4.35%
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Senior notes, issuance amount	$ 400,000,000	400,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.70%
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Senior notes, issuance amount	$ 0	1,000,000,000
Debt instrument, outstanding	$ 0	594,600,000
Debt Instrument, Interest Rate, Stated Percentage	6.70%
2.45% Senior Notes Due 2027
Debt Instrument [Line Items]
Senior notes, issuance amount	$ 500,000,000	500,000,000		$ 500,000,000
Debt Instrument, Interest Rate, Stated Percentage	2.45%			2.45%
4.125% Senior Notes due 2047
Debt Instrument [Line Items]
Senior notes, issuance amount	$ 850,000,000	0	$ 850,000,000
Debt Instrument, Interest Rate, Stated Percentage	4.125%		4.125%
Discretionary Line of Credit
Debt Instrument [Line Items]
Proceeds from Lines of Credit	$ 0	$ 0

Debt Unrealized Gains (Losses)

Debt Unrealized Gains (Losses) From Debt Hedges Included In Accumulated Other (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Apr. 06, 2017	Dec. 31, 2016	Jan. 26, 2015	Apr. 22, 2014	Aug. 17, 2011	Nov. 21, 2002	Mar. 01, 1999
Debt Instrument [Line Items]
Unrealized Gain (Loss)	$ (14.8)		$ (9.4)
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Unrealized Gain (Loss)	(2.1)					$ (5.1)
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Unrealized Gain (Loss)	(2.7)							$ (4.2)
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Unrealized Gain (Loss)	3.6						$ 5.1
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Unrealized Gain (Loss)	(1.5)				$ (1.6)
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Unrealized Gain (Loss)	(12.2)			$ (12.9)
4.125% Senior Notes due 2047
Debt Instrument [Line Items]
Unrealized Gain (Loss)	$ (7.9)	$ (8)

Debt Aggregate Principal Paymen

Debt Aggregate Principal Payments On Debt Outstanding (Detail) $ in Millions	Dec. 31, 2017USD ($)
Debt Disclosure [Abstract]
Debt Instrument, Periodic Payment, Principal in Next Twelve Months	$ 25
Long-term Debt, Maturities, Repayments of Principal in Year Two	12.1
Long-Term Debt, Maturities, Repayments of Principal in Year Three	0
Long-Term Debt, Maturities, Repayments of Principal in Year Four	500
Long-Term Debt, Maturities, Repayments of Principal in Year Five	0
Long-Term Debt, Maturities, Repayments of Principal after Year Five	2,800
Total	$ 3,337.1

Debt - Additional Information (

Debt - Additional Information (Detail)	1 Months Ended		9 Months Ended	12 Months Ended
	Apr. 30, 2017USD ($)	Aug. 31, 2016USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2017USD ($)	Mar. 31, 2017	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Debt Instrument [Line Items]
Early Repayment of Subordinated Debt				$ 635,600,000		$ 18,200,000	$ 19,300,000
Gain (Loss) on Derivative Instruments, Net, Pretax				0		(20,000,000)	(20,700,000)
Debt Instrument, Periodic Payment, Principal in Next Twelve Months			$ 25,000,000	$ 25,000,000
Debt Instrument, Number of Subsidiaries, one hundred percent of common stock as collateral			3	3
Discretionary Line of Credit
Debt Instrument [Line Items]
Debt Instrument, Term				30 days
Line of Credit Facility, Maximum Borrowing Capacity			$ 250,000,000	$ 250,000,000
Proceeds from Lines of Credit				0		0
Discretionary Line of Credit | Federal Funds Rate
Debt Instrument [Line Items]
Basis points			0.50%		0.50%
Terminated Discretionary Line of Credit [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 100,000,000
4.125% Senior Notes due 2047
Debt Instrument [Line Items]
Debt Instrument, Face Amount	$ 850,000,000		$ 850,000,000	$ 850,000,000		0
Debt Instrument, Interest Rate, Stated Percentage	4.125%		4.125%	4.125%
Proceeds from Debt, Net of Issuance Costs	$ 841,100,000
Debt Related Commitment Fees and Debt Issuance Costs	$ 1,500,000
2.45% Senior Notes Due 2027
Debt Instrument [Line Items]
Debt Instrument, Face Amount		$ 500,000,000	$ 500,000,000	$ 500,000,000		500,000,000
Debt Instrument, Interest Rate, Stated Percentage		2.45%	2.45%	2.45%
Proceeds from Debt, Net of Issuance Costs		$ 495,600,000
Debt Related Commitment Fees and Debt Issuance Costs		900,000
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Debt Instrument, Face Amount			$ 0	$ 0		1,000,000,000
Debt Instrument, Interest Rate, Stated Percentage			6.70%	6.70%
Early Repayment of Subordinated Debt				$ 30,900,000		19,800,000
Gains (losses) on extinguishment of debt				200,000		1,600,000
Trust Preferred Debt Securities and Senior Notes [Member] | Other Debt Instruments [Member]
Debt Instrument [Line Items]
Repayments of Debt				$ 65,200,000
Loans Payable [Member] | Fixed Rate, Five Year Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Term				5 years
London Interbank Offered Rate (LIBOR) [Member] | Notes Payable, Other Payables [Member] | Other Debt Instruments [Member]
Debt Instrument [Line Items]
Basis points				2.25%
Designated as Hedging Instrument [Member] | Ineffective Cash Flow Hedge [Member]
Debt Instrument [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax				$ 0		$ (1,300,000)	$ 200,000
Designated as Hedging Instrument [Member] | Ineffective Cash Flow Hedge [Member] | 2.45% Senior Notes Due 2027
Debt Instrument [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax		$ (1,400,000)		(1,400,000)
debt extinguishment [Member] | 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Early Repayment of Subordinated Debt				$ 563,700,000

Income Taxes Components of Inco

Income Taxes Components of Income Tax Provision (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Current Federal Tax Expense (Benefit)	$ 680.9	$ 469.6	$ 655.3
Current State and Local Tax Expense (Benefit)	12.8	12.7	14.7
Deferred Federal Income Tax Expense (Benefit)	(149.4)	(66.3)	(47.7)
Deferred State and Local Income Tax Expense (Benefit)	(3.5)	(2.5)	(11.2)
Total income tax provision	$ 540.8	$ 413.5	$ 611.1

Income Taxes Reconciliation of

Income Taxes Reconciliation of Provision (Benefit) for income Taxes Reported in Consolidated Statements Of Income with Tax at Statutory Rate (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Apr. 01, 2015
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income before income taxes	$ 2,138,900,000	$ 1,470,700,000	$ 1,911,600,000
Tax at statutory federal rate	748,600,000	514,800,000	669,100,000
Net deferred tax liability revaluation1	(99,500,000)	0	0
Tax credits	(52,400,000)	(62,200,000)	(1,900,000)
Stock-based compensation2	(25,100,000)	0	0
Dividends received deduction	(20,700,000)	(22,600,000)	(19,800,000)
Exempt interest income	(16,900,000)	(15,700,000)	(17,800,000)
Tax-deductible dividends	(9,700,000)	(6,100,000)	(7,900,000)
State income taxes, net of federal taxes	6,000,000	6,600,000	2,300,000
Other items, net	400,000	(1,800,000)	600,000
Non-taxable gain4	0	0	(13,800,000)
Total income tax provision	$ 540,800,000	$ 413,500,000	$ 611,100,000
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Tax at statutory federal rate	35.00%	35.00%	35.00%
Net deferred tax liability revaluation1	(5.00%)	0.00%	0.00%
Tax credits	(2.00%)	(4.00%)	(0.00%)
Stock-based compensation2	(1.00%)	0.00%	0.00%
Dividends received deduction	(1.00%)	(2.00%)	(1.00%)
Exempt interest income	(1.00%)	(1.00%)	(1.00%)
Tax-deductible dividends	(0.00%)	(0.00%)	(0.00%)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Amount	$ 10,100,000	$ 500,000	$ 300,000
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Percent	0.00%	0.00%	0.00%
State income taxes, net of federal taxes	0.00%	0.00%	0.00%
Other items, net	0.00%	0.00%	0.00%
Non-taxable gain4	0.00%	0.00%	(1.00%)
Total income tax provision	25.00%	28.00%	32.00%
ARX Holding Corp.
Business Acquisition [Line Items]
Ownership interest prior to acquisition				5.00%

Income Taxes Components of Net

Income Taxes Components of Net Deferred Tax Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Federal deferred tax assets:
Unearned premiums reserve	$ 368.9	$ 515.6
Non-deductible accruals	166	259.9
Loss and loss adjustment expense reserves	48.4	76.5
Hedges on forecasted transactions	4.8	5.1
Investment basis differences	0	31.3
Other	7.3	8.9
Federal deferred tax liabilities:
Net unrealized gains on securities	(419.5)	(507.2)
Deferred acquisition costs	(163.9)	(227.9)
Property and equipment	(75.5)	(120.2)
Intangible assets	(66.6)	(145.3)
Prepaid expenses	(7.1)	(9.2)
Investment basis differences	(5.5)	0
Deferred gain on extinguishment of debt	(0.4)	(1.5)
Other	(6.1)	(7.7)
Net federal deferred tax liability	(149.2)	(121.7)
Net state deferred tax asset	14.2	10.4
Net deferred tax liability	$ (135)	$ (111.3)

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Tax at statutory federal rate	35.00%	35.00%	35.00%
Tax cuts and jobs act, reduction in income tax expense related to revaluing deferred tax assets and liabilities	$ 99,500,000
Federal tax benefits resulting from investments in renewable energy tax credit funds	52,400,000	$ 62,200,000	$ 1,900,000
Tax cuts and jobs act, provisional tax amount related to deductibility of executive compensation expense	4,500,000
Deferred tax assets valuation allowance	0	0
Net taxes payable	23,800,000	41,200,000
Provision for income taxes	540,800,000	413,500,000	611,100,000
Interest and Penalties [Member]
Income Taxes [Line Items]
Provision for income taxes	$ 200,000	$ 100,000	$ 100,000
ARX and subsidiaries
Income Taxes [Line Items]
Noncontrolling interest, ownership percentage (less than)	80.00%
Net unrealized gains (losses) on securities
Income Taxes [Line Items]
Tax cuts and jobs act, reduction in income tax expense related to revaluing deferred tax liability associated with net unrealized gains on investment portfolio	$ 275,700,000
Other Deferred Tax Assets and Liabilities [Member]
Income Taxes [Line Items]
Tax cuts and jobs act, reduction in income tax expense related to other deferred tax assets (liabilities)	$ 176,200,000
Tax Year 2018
Income Taxes [Line Items]
Tax at statutory federal rate	21.00%

Loss And Loss Adjustment Expe81

Loss And Loss Adjustment Expense Reserves Activity in Loss and loss Adjustment Expense Reserves (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Balance at beginning of period	$ 11,368	$ 10,039	$ 8,857.4
Less reinsurance recoverables on unpaid losses	1,801	1,442.7	1,185.9
Net balance at beginning of period	9,567	8,596.3	7,671.5
Net loss and loss adjustment reserves acquired	0	(2.5)	222.4
Total beginning reserves		9,567	8,593.8	$ 7,893.9
Incurred related to:
Current year	18,782.1	16,967.1	14,657.1
Prior years	25.9	(87.5)	(315.1)
Total incurred	18,808	16,879.6	14,342
Paid related to:
Current year	12,201.5	11,149	9,577.3
Prior years	5,256.7	4,757.4	4,062.3
Total paid	17,458.2	15,906.4	13,639.6
Net balance at ending of period	10,916.8	9,567	8,596.3
Plus reinsurance recoverables on unpaid losses	2,170.1	1,801	1,442.7
Balance at ending of period	$ 13,086.9	$ 11,368	$ 10,039

Loss And Loss Adjustment Expe82

Loss And Loss Adjustment Expense Reserves Loss And Loss Adjustment Expense Reserves - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)data_subset	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Number of subsets of business data reviewed (over) | data_subset	400
Reserve development, prior years	$ 25.9	$ (87.5)	$ (315.1)
Short-duration Insurance Contracts, Accident Year 2016 and 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years	64
Short-duration Insurance Contracts, Accident Years 2014 and Prior [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years	(38)
2,015
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years		(56)
2,013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years		(51)
2,014
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years		19	(239)
Commercial Lines Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years	1
Property Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years	$ (37)
Auto
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Percentage of reserves	98.00%
Personal Auto | Personal Lines Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years	$ 70	(40)	$ (217)
Personal Lines - Liability | Personal Lines Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years		(54)
Property | Property Segment
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Reserve development, prior years		$ (52)

Loss And Loss Adjustment Expe83

Loss And Loss Adjustment Expense Reserves Incurred and Paid Claims Development with IBNR and Claim Counts (Details) $ in Millions	Dec. 31, 2017USD ($)claim	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014USD ($)	Dec. 31, 2013USD ($)
Claims Development [Line Items]
Liabilities for claims and claim adjustment expenses, net of reinsurance	$ 10,029.8	$ 8,800
Commercial Lines - Physical Damage | Commercial Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	1,560.9
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	1,509.6
All outstanding liabilities before 2013, net of reinsurance1	0.4
Liabilities for claims and claim adjustment expenses, net of reinsurance	51.7	46.6
Commercial Lines - Liability | Commercial Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	5,157.5
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	3,158.6
All outstanding liabilities before 2013, net of reinsurance1	28.5
Liabilities for claims and claim adjustment expenses, net of reinsurance	2,027.4	1,675.3
Property | Property Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	2,247.8
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	2,009.2
All outstanding liabilities before 2013, net of reinsurance1	7.2
Liabilities for claims and claim adjustment expenses, net of reinsurance	245.8	224.7
2013 | Commercial Lines - Physical Damage | Commercial Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	254.2	253.7	$ 254.5	$ 254.2	$ 256.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0
Cumulative Number of Incurred Claim Counts | claim	62,673
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 253.8	253.7	253.9	253.1	239.4
2013 | Commercial Lines - Liability | Commercial Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	840.8	865	865.4	864.6	864.2
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0
Cumulative Number of Incurred Claim Counts | claim	78,075
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 802.6	752.5	659.1	509.4	233.3
2013 | Property | Property Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	253.9	254.1	254.3	283.3	307.3
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 2.5
Cumulative Number of Incurred Claim Counts | claim	30,600
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 250.5	249.5	244.9	234.2	185.1
2014 | Commercial Lines - Physical Damage | Commercial Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	238	238.6	239.7	240.3
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.5)
Cumulative Number of Incurred Claim Counts | claim	59,622
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 237.9	237.7	238.3	224.6
2014 | Commercial Lines - Liability | Commercial Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	823	820.3	795.4	822.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 15.9
Cumulative Number of Incurred Claim Counts | claim	74,724
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 715.3	610	438.7	234
2014 | Property | Property Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	376.3	379.7	389.1	415.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 3.6
Cumulative Number of Incurred Claim Counts | claim	40,984
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 370.3	365.9	351.5	269.2
2015 | Commercial Lines - Physical Damage | Commercial Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	273.5	274.1	274.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.1)
Cumulative Number of Incurred Claim Counts | claim	62,593
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 272	271.9	248.5
2015 | Commercial Lines - Liability | Commercial Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	914.8	911.1	897.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 23.7
Cumulative Number of Incurred Claim Counts | claim	77,807
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 675	501.5	238.4
2015 | Property | Property Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	403.6	416.5	460
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 11.5
Cumulative Number of Incurred Claim Counts | claim	41,957
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 383.5	372.8	280.3
2016 | Commercial Lines - Physical Damage | Commercial Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	379.8	379.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (0.9)
Cumulative Number of Incurred Claim Counts | claim	74,217
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 376.9	336.7
2016 | Commercial Lines - Liability | Commercial Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	1,204.8	1,185.8
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 63.1
Cumulative Number of Incurred Claim Counts | claim	92,196
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 639.9	298.6
2016 | Property | Property Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	541.2	568.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 24.8
Cumulative Number of Incurred Claim Counts | claim	53,323
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 498.2	415.2
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Commercial Lines - Physical Damage | Commercial Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	415.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (5.7)
Cumulative Number of Incurred Claim Counts | claim	77,767
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 369
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Commercial Lines - Liability | Commercial Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	1,374.1
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 219.2
Cumulative Number of Incurred Claim Counts | claim	94,992
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 325.8
Short-duration Insurance Contracts, Accident Year 2017 [Member] | Property | Property Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	672.8
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 99.7
Cumulative Number of Incurred Claim Counts | claim	64,487
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 506.7
Agency Channel | Personal Lines - Liability | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	19,542.2
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	15,641.9
All outstanding liabilities before 2013, net of reinsurance1	55.2
Liabilities for claims and claim adjustment expenses, net of reinsurance	3,955.5	3,555.9
Agency Channel | Personal Lines - Physical Damage | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	11,257.9
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	11,232.6
All outstanding liabilities before 2013, net of reinsurance1	0.9
Liabilities for claims and claim adjustment expenses, net of reinsurance	26.2	30.1
Agency Channel | 2013 | Personal Lines - Liability | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	3,484.2	3,528	3,518.2	3,520.9	3,506
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0
Cumulative Number of Incurred Claim Counts | claim	696,624
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 3,437.6	3,362.9	3,173.1	2,794.1	1,684
Agency Channel | 2013 | Personal Lines - Physical Damage | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	2,001.1	1,999.4	2,000.1	2,001.4	2,014
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0
Cumulative Number of Incurred Claim Counts | claim	1,347,920
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 2,000.8	2,001.3	2,002	2,003.9	1,952.7
Agency Channel | 2014 | Personal Lines - Liability | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	3,654.4	3,633.2	3,627.7	3,702.1
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 50.9
Cumulative Number of Incurred Claim Counts | claim	701,787
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 3,482.3	3,284.5	2,868.1	1,809
Agency Channel | 2014 | Personal Lines - Physical Damage | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	2,088	2,089.9	2,090.3	2,107.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (3.3)
Cumulative Number of Incurred Claim Counts | claim	1,374,666
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 2,090.7	2,090.6	2,091.6	2,078.8
Agency Channel | 2015 | Personal Lines - Liability | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	3,798.8	3,773.8	3,774.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 59.2
Cumulative Number of Incurred Claim Counts | claim	704,990
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 3,416.5	2,976	1,793.1
Agency Channel | 2015 | Personal Lines - Physical Damage | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	2,134.4	2,137.2	2,136.8
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (2.6)
Cumulative Number of Incurred Claim Counts | claim	1,336,268
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 2,134.4	2,138.1	2,106.2
Agency Channel | 2016 | Personal Lines - Liability | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	4,130	4,082.9
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 158.7
Cumulative Number of Incurred Claim Counts | claim	738,563
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 3,231.5	1,941.6
Agency Channel | 2016 | Personal Lines - Physical Damage | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	2,398.9	2,423.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (12.1)
Cumulative Number of Incurred Claim Counts | claim	1,398,371
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 2,406.9	2,391
Agency Channel | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Personal Lines - Liability | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	4,474.8
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 679.6
Cumulative Number of Incurred Claim Counts | claim	765,553
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 2,074
Agency Channel | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Personal Lines - Physical Damage | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	2,635.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (101.8)
Cumulative Number of Incurred Claim Counts | claim	1,496,298
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 2,599.8
Direct Channel | Personal Lines - Liability | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	16,934.1
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	13,282.5
All outstanding liabilities before 2013, net of reinsurance1	33.5
Liabilities for claims and claim adjustment expenses, net of reinsurance	3,685.1	3,219.1
Direct Channel | Personal Lines - Physical Damage | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	10,813.7
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	10,819.4
All outstanding liabilities before 2013, net of reinsurance1	0.1
Liabilities for claims and claim adjustment expenses, net of reinsurance	(5.6)	11.2
Direct Channel | 2013 | Personal Lines - Liability | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	2,612.9	2,649.8	2,615.8	2,621.8	2,619.4
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0
Cumulative Number of Incurred Claim Counts | claim	550,166
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 2,577.8	2,521.3	2,375.5	2,085.1	1,252
Direct Channel | 2013 | Personal Lines - Physical Damage | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	1,635	1,633.4	1,634.6	1,635.6	1,653.7
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 0
Cumulative Number of Incurred Claim Counts | claim	1,367,341
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 1,635	1,635.3	1,636.2	1,638.2	$ 1,612.9
Direct Channel | 2014 | Personal Lines - Liability | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	2,913.6	2,898.1	2,887.4	2,946.8
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 38.2
Cumulative Number of Incurred Claim Counts | claim	592,185
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 2,780	2,624.2	2,278	1,413
Direct Channel | 2014 | Personal Lines - Physical Damage | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	1,859.2	1,861.7	1,862.2	1,889.3
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (3.2)
Cumulative Number of Incurred Claim Counts | claim	1,471,016
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 1,861.8	1,862.7	1,864.1	$ 1,874.6
Direct Channel | 2015 | Personal Lines - Liability | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	3,354.2	3,328.3	3,330.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 48.6
Cumulative Number of Incurred Claim Counts | claim	659,102
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 3,021	2,615	1,545.2
Direct Channel | 2015 | Personal Lines - Physical Damage | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	2,093.5	2,097.7	2,110.7
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (3)
Cumulative Number of Incurred Claim Counts | claim	1,539,790
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 2,094.7	2,100.1	$ 2,094.7
Direct Channel | 2016 | Personal Lines - Liability | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	3,843.9	3,819
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 143
Cumulative Number of Incurred Claim Counts | claim	733,195
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 2,991.1	1,780.6
Direct Channel | 2016 | Personal Lines - Physical Damage | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	2,475.4	2,521
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (14.3)
Cumulative Number of Incurred Claim Counts | claim	1,675,405
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 2,485.8	$ 2,505
Direct Channel | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Personal Lines - Liability | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	4,209.5
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ 598.6
Cumulative Number of Incurred Claim Counts | claim	761,280
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 1,912.6
Direct Channel | Short-duration Insurance Contracts, Accident Year 2017 [Member] | Personal Lines - Physical Damage | Personal Lines Segment
Claims Development [Line Items]
Incurred Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	2,750.6
Total of IBNR Liabilities Plus Expected Development on Reported Claims	$ (145.3)
Cumulative Number of Incurred Claim Counts | claim	1,779,665
Cumulative Paid Claims and Allocated Claim Adjustment Expenses, Net of Reinsurance	$ 2,742.1

Loss And Loss Adjustment Expe84

Loss And Loss Adjustment Expense Reserves Reconciliation of the Claims Development to the Liability for Claims and Claim Adjustment Expenses (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	$ 10,029.8	$ 8,800
Total reinsurance recoverable on unpaid claims	2,157.8	1,795.9
Unallocated claims adjustment expense related to:	887	767
Total gross liability for unpaid claims and claim adjustment expense	13,086.9	11,368	$ 10,039	$ 8,857.4
Other business
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	43.7	37.1
Total reinsurance recoverable on unpaid claims	200.9	154.9
Commercial Lines Segment | Commercial Lines - Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	2,027.4	1,675.3
Total reinsurance recoverable on unpaid claims	105.1	59.5
Commercial Lines Segment | Commercial Lines - Physical Damage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	51.7	46.6
Total reinsurance recoverable on unpaid claims	0.1	0
Property Segment | Property
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	245.8	224.7
Total reinsurance recoverable on unpaid claims	243.8	132.7
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item	12.3	5.1
Agency Channel | Personal Lines Segment | Personal Lines - Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	3,955.5	3,555.9
Total reinsurance recoverable on unpaid claims	769.8	724.5
Agency Channel | Personal Lines Segment | Personal Lines - Physical Damage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	26.2	30.1
Total reinsurance recoverable on unpaid claims	0	0
Direct Channel | Personal Lines Segment | Personal Lines - Liability
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	3,685.1	3,219.1
Total reinsurance recoverable on unpaid claims	838.1	724.3
Direct Channel | Personal Lines Segment | Personal Lines - Physical Damage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for unpaid claims and claim adjustment expenses, net of reinsurance	(5.6)	11.2
Total reinsurance recoverable on unpaid claims	$ 0	$ 0

Loss And Loss Adjustment Expe85

Loss And Loss Adjustment Expense Reserves Historical Claims Duration (Details)	Dec. 31, 2017
Personal Lines Segment | Agency Channel | Personal Lines - Liability
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	47.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	30.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	11.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	5.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	2.10%
Personal Lines Segment | Agency Channel | Personal Lines - Physical Damage
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	98.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	1.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	(0.10%)
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	0.00%
Personal Lines Segment | Direct Channel | Personal Lines - Liability
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	46.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	31.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	11.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	5.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	2.20%
Personal Lines Segment | Direct Channel | Personal Lines - Physical Damage
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	100.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	(0.20%)
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	(0.10%)
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	0.00%
Commercial Lines Segment | Commercial Lines - Liability
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	25.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	28.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	19.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	11.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	6.00%
Commercial Lines Segment | Commercial Lines - Physical Damage
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	90.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	7.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	0.00%
Property Segment | Property
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	73.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	19.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	3.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	1.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	0.40%

Reinsurance Effect of Reinsuran

Reinsurance Effect of Reinsurance on Premiums Written and Earned (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Direct premiums written	$ 27,860.7	$ 23,941.9	$ 21,086.5
Ceded written	(728.6)	(588.4)	(522.5)
Net premiums written	27,132.1	23,353.5	20,564
Direct premiums earned	26,425.7	23,111.2	20,454.1
Ceded earned	(695.8)	(637.2)	(555)
Net premiums earned	25,729.9	22,474	19,899.1
non regulated reinsurance plan [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded written	(222.7)	(149)	(164.5)
Ceded earned	(216.2)	(212.1)	(192.4)
Regulated reinsurance plan [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Ceded written	(505.9)	(439.4)	(358)
Ceded earned	$ (479.6)	$ (425.1)	$ (362.6)

Reinsurance Prepaid Reinsurance

Reinsurance Prepaid Reinsurance Premiums and Reinsurance Recoverables (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 203.3	$ 170.5
Percentage Of Total Prepaid Reinsurance Premiums	100.00%	100.00%
Reinsurance Recoverables, Including Reinsurance Premium Paid	$ 2,273.4	$ 1,884.8
Percentage Of Total Reinsurance Recoverables	100.00%	100.00%
State Reinsurance Plans [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 179.9	$ 153.4
Percentage Of Total Prepaid Reinsurance Premiums	88.00%	90.00%
Reinsurance Recoverables, Including Reinsurance Premium Paid	$ 2,060.7	$ 1,772.3
Percentage Of Total Reinsurance Recoverables	91.00%	94.00%
State Reinsurance Plans [Member] | Michigan Catastrophic Claims Association [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 44.3	$ 36.5
Percentage Of Total Prepaid Reinsurance Premiums	22.00%	21.00%
Reinsurance Recoverables, Including Reinsurance Premium Paid	$ 1,611.5	$ 1,452.7
Percentage Of Total Reinsurance Recoverables	71.00%	77.00%
State Reinsurance Plans [Member] | Commercial Auto Insurance Procedures Plans [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 50	$ 40.2
Percentage Of Total Prepaid Reinsurance Premiums	25.00%	24.00%
Reinsurance Recoverables, Including Reinsurance Premium Paid	$ 218	$ 170.6
Percentage Of Total Reinsurance Recoverables	10.00%	9.00%
State Reinsurance Plans [Member] | North Carolina Reinsurance Facility [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 31.5	$ 27.5
Percentage Of Total Prepaid Reinsurance Premiums	15.00%	16.00%
Reinsurance Recoverables, Including Reinsurance Premium Paid	$ 74.2	$ 67
Percentage Of Total Reinsurance Recoverables	3.00%	4.00%
State Reinsurance Plans [Member] | other reinsurance program [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 0.3	$ 0.1
Percentage Of Total Prepaid Reinsurance Premiums	0.00%	0.00%
Reinsurance Recoverables, Including Reinsurance Premium Paid	$ 8.2	$ 3.5
Percentage Of Total Reinsurance Recoverables	0.00%	0.00%
Federal Reinsurance Plans [Member] | National Flood Insurance Program [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 53.8	$ 49.1
Percentage Of Total Prepaid Reinsurance Premiums	26.00%	29.00%
Reinsurance Recoverables, Including Reinsurance Premium Paid	$ 148.8	$ 78.5
Percentage Of Total Reinsurance Recoverables	7.00%	4.00%
Non State Reinsurance Plans [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 23.4	$ 17.1
Percentage Of Total Prepaid Reinsurance Premiums	12.00%	10.00%
Reinsurance Recoverables, Including Reinsurance Premium Paid	$ 212.7	$ 112.5
Percentage Of Total Reinsurance Recoverables	9.00%	6.00%
Property reinsurance program [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 9.1	$ 3.2
Percentage Of Total Prepaid Reinsurance Premiums	5.00%	2.00%
Reinsurance Recoverables, Including Reinsurance Premium Paid	$ 138.6	$ 62.9
Percentage Of Total Reinsurance Recoverables	6.00%	3.00%
other reinsurance program [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Prepaid reinsurance premiums	$ 14.3	$ 13.9
Percentage Of Total Prepaid Reinsurance Premiums	7.00%	8.00%
Reinsurance Recoverables, Including Reinsurance Premium Paid	$ 74.1	$ 49.6
Percentage Of Total Reinsurance Recoverables	3.00%	3.00%

Reinsurance - Additional Inform

Reinsurance - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Prepaid Reinsurance Premiums [Abstract]
Policyholder Benefits and Claims Incurred, Ceded	$ 1,094.2	$ 764.4	$ 457.3

Statutory Financial Informati89

Statutory Financial Information - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statutory Financial Information Disclosures [Abstract]
Consolidated statutory policyholders' surplus	$ 9,664.4	$ 8,560
Statutory net income	1,416.2	$ 1,022.3	$ 1,333.1
Consolidated statutory policyholders' surplus, net admitted assets of insurance subsidiaries and affiliate that are required to meet minimum statutory surplus requirements in such entities' states of domicile	830.1
Aggregate cash dividends paid to the parent company by subsidiaries	872.8
Maximum aggregate dividend amount subsidiaries could pay without prior approval from regulatory authorities	$ 1,390.5

Employee Benefit Plans Amounts

Employee Benefit Plans Amounts Charged to Income for employees Incentive Compensation Plans (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Incentive Compensation Cash Award
Incentive Compensation Plans Expense [Line Items]
Pretax	$ 461.3	$ 386.8	$ 337.7
After Tax	299.8	251.4	219.5
Employee Stock [Member]
Incentive Compensation Plans Expense [Line Items]
Pretax	92.9	80.9	64.5
After Tax	60.4	52.6	41.9
Employee Stock Option
Incentive Compensation Plans Expense [Line Items]
Pretax	2.5	4.3	1.7
After Tax	$ 1.6	$ 2.8	$ 1.1

Employee Benefit Plans Summary

Employee Benefit Plans Summary of All Employee Restricted Equity Award Activity (Detail) - Employee Restricted Equity Awards - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Shares
Beginning of year	6,951,373	7,725,227	9,051,564
Add (deduct):
Granted	2,383,475	1,870,660	2,489,976
Vested	(3,220,671)	(2,422,700)	(3,682,644)
Forfeited	(255,329)	(221,814)	(133,669)
End of year	5,858,848	6,951,373	7,725,227
Weighted Average Grant Date Fair Value
Beginning of year	$ 26.18	$ 23.37	$ 21.27
Granted	32.01	31.54	25.20
Vested	22.53	21.50	19.53
Forfeited	28.03	24.64	21.63
End of year	$ 30.47	$ 26.18	$ 23.37
Unrecognized compensation cost related to unvested equity awards	$ 87.1
Period of recognition of compensation expense related to unvested equity awards	2 years 2 months
Dividend Equivalent Units
Add (deduct):
Granted	157,396	165,045	196,947
Weighted Average Grant Date Fair Value
Granted	$ 0	$ 0	$ 0
Performance Shares
Weighted Average Grant Date Fair Value
Target shares	1,513,779
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Expected To Vest Number	3,244,111

Employee Benefit Plans Summar92

Employee Benefit Plans Summary of all Directors' Restricted Stock Activity (Detail) - Non Employee Director Restricted Equity Awards - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Shares
Beginning of year	55,839	89,427	81,579
Add (deduct):
Granted	53,284	55,839	89,427
Vested	(55,839)	(89,427)	(81,579)
End of year	53,284	55,839	89,427
Weighted Average Grant Date Fair Value
Beginning of year	$ 33.24	$ 27.23	$ 25.45
Granted	40.54	33.24	27.23
Vested	33.24	27.23	25.45
End of year	$ 40.54	$ 33.24	$ 27.23

Employee Benefit Plans Assets H

Employee Benefit Plans Assets Held in Deferral Plan Irrevocable Grantor Trust Account (Detail) - USD ($) shares in Millions, $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assets held in Deferral Plan Irrevocable Grantor Trust account	$ 295.2	$ 259.1
Progressive common shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Deferred Compensation Arrangement with Individual, Shares Authorized for Issuance	4.4	4.7
Assets held in Deferral Plan Irrevocable Grantor Trust account	$ 128.2	$ 122.2
Other Investment Funds
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Assets held in Deferral Plan Irrevocable Grantor Trust account	$ 167	$ 136.9

Employee Benefit Plans - Additi

Employee Benefit Plans - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)Personshares	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employee Benefits Disclosure [Line Items]
Minimum employment period for benefits	30 days
Employee Stock Ownership Plan Shares held	24,600,000
Postemployment benefits liability | $	$ 19.5	$ 21.7
Compensation, Percentage	100.00%
Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Aggregate fair value of the restricted equity awards that vested during the period | $	$ 130.5	77	$ 105.4
Non Employee Director Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Vesting period of grants to date	11 months
Total pretax intrinsic value of options exercised and restricted stock vested | $	$ 2.2	$ 3	$ 2.2
Executive Deferred Compensation Plan
Employee Benefits Disclosure [Line Items]
Common shares reserved for issuance under executive deferred compensation plan	11,100,000
2010 Equity Incentive Plan | Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Shares authorized	18,000,000
2015 Equity Incentive Plan | Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Shares authorized	13,000,000
2003 Directors Equity Incentive Plan | Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Shares authorized	0
2003 Directors Equity Incentive Plan | Non Employee Director Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Shares authorized	0
2017 Directors Equity Incentive Plan | Non Employee Director Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Shares authorized	500,000
Performance Based Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Estimated Number Of Participants | Person	45
Performance Based Restricted Equity Awards | Insurance Operating Results | Lower Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Performance Based Restricted Equity Awards | Insurance Operating Results | Upper Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	250.00%	250.00%	250.00%	250.00%	250.00%	200.00%
Performance Based Restricted Equity Awards | Investment Results | Lower Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	0.00%	0.00%	0.00%	0.00%	0.00%	0.00%
Performance Based Restricted Equity Awards | Investment Results | Upper Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	200.00%	200.00%	200.00%	200.00%	200.00%	200.00%
Performance Based Restricted Equity Awards | Growth in percentage of auto policies bundled with other specified types of policies [Member] | Lower Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount			100.00%
Performance Based Restricted Equity Awards | Growth in percentage of auto policies bundled with other specified types of policies [Member] | Upper Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount			200.00%
Performance Based Restricted Equity Awards | Growth in percentage of auto policies bundled with other specified types of policies, performance objective not achieved
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount			0.00%
Performance Based Restricted Equity Awards | Unit growth in a particular customer segment [Member] | Lower Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	85.00%	85.00%
Performance Based Restricted Equity Awards | Unit growth in a particular customer segment [Member] | Upper Limit
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	150.00%	150.00%
Performance Based Restricted Equity Awards | Unit growth in a particular customer segment, performance objective not achieved
Employee Benefits Disclosure [Line Items]
Performance-based awards granted vesting percentage of the award amount	0.00%	0.00%
Defined Contribution Pension Plan 401k
Employee Benefits Disclosure [Line Items]
Matching contributions | $	$ 97.3	$ 86.8	$ 79.1
Defined Contribution Pension Plan 401k | Upper Limit
Employee Benefits Disclosure [Line Items]
Matching contribution to defined contribution pension plan	6.00%
Vesting tranche one | Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Vesting period of grants to date	3 years
Vesting tranche two | Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Vesting period of grants to date	4 years
Vesting tranche three | Employee Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Vesting period of grants to date	5 years
Cash [Member]
Employee Benefits Disclosure [Line Items]
Compensation, Percentage	40.00%
Non Employee Director Restricted Equity Awards
Employee Benefits Disclosure [Line Items]
Compensation, Percentage	60.00%

Employee Benefit Plans Summar95

Employee Benefit Plans Summary of ARX employee stock option plans (Details) - Employee Stock Option - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Apr. 01, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Number	22,550	24,995	24,995	26,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding, Weighted Average Exercise Price	$ 564.60	$ 526.46	$ 526.46	$ 513.72
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	(2,445)	0	(1,005)
Share-based Compensation Arrangements by Share-based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price	$ 174.65	$ 0	$ 197.01
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Exercisable, Number	17,950	16,995	12,995
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Weighted Average Exercise Price	$ 517.75	$ 438.77	$ 386.69
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value	$ 2.9		$ 1.1
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Number of Shares	4,600	8,000	12,000	14,800
Share-based Compensation Arrangement by Share-based Payment Award, Options, Nonvested, Weighted Average Grant Date Fair Value	$ 747.45	$ 712.74	$ 677.81	$ 675.55
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Number of Shares	(3,400)	(4,000)	(2,800)
Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested, Weighted Average Grant Date Fair Value	$ 665.79	$ 607.95	$ 665.85
Unrecognized compensation cost related to unvested equity awards	$ 0.7	$ 1.6	$ 2.9
Period of recognition of compensation expense related to unvested equity awards	1 year 5 days	1 year 4 months 10 days	1 year 8 months 20 days

Employee Benefit Plans Summar96

Employee Benefit Plans Summary of ARX Defined Contribution Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Pension Plan 401k
Defined Contribution Plan Disclosure [Line Items]
Matching contributions	$ 97.3	$ 86.8	$ 79.1

Segment Information Operating R

Segment Information Operating Results (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 26,839	$ 23,441.4	$ 20,853.8
Pretax Profit (Loss)	2,138.9	1,470.7	1,911.6
Personal Lines Segment | Agency Channel | Underwriting Operations
Segment Reporting Information [Line Items]
Revenues	11,177.6	9,791.7	9,108.6
Pretax Profit (Loss)	839.6	492.8	713.2
Personal Lines Segment | Direct Channel | Underwriting Operations
Segment Reporting Information [Line Items]
Revenues	10,769.6	9,396.5	8,185.9
Pretax Profit (Loss)	683.7	412.2	403.4
Operating Segments | Underwriting Operations
Segment Reporting Information [Line Items]
Revenues	25,729.9	22,474	19,899.1
Pretax Profit (Loss)	1,686.9	1,091.1	1,495.2
Operating Segments | Personal Lines Segment | Underwriting Operations
Segment Reporting Information [Line Items]
Revenues	21,947.2	19,188.2	17,294.5
Pretax Profit (Loss)	1,523.3	905	1,116.6
Operating Segments | Commercial Lines Segment | Underwriting Operations
Segment Reporting Information [Line Items]
Revenues	2,793.9	2,421.3	1,995.9
Pretax Profit (Loss)	214.1	155.2	318.3
Operating Segments | Property Segment | Underwriting Operations
Segment Reporting Information [Line Items]
Revenues	988.8	864.5	609.1
Pretax Profit (Loss)	(50.3)	32.5	61.3
Operating Segments | Other Indemnity | Underwriting Operations
Segment Reporting Information [Line Items]
Revenues	0	0	(0.4)
Pretax Profit (Loss)	(0.2)	(1.6)	(1)
Segment Reconciling Items | Fees And Other Revenues
Segment Reporting Information [Line Items]
Revenues	370.6	332.5	302
Segment Reconciling Items | Service Businesses
Segment Reporting Information [Line Items]
Revenues	126.8	103.3	86.3
Pretax Profit (Loss)	17.3	11.3	8.8
Segment Reconciling Items | Investments
Segment Reporting Information [Line Items]
Revenues	612.7	530	567.3
Pretax Profit (Loss)	588.8	507.6	544.5
Segment Reconciling Items | Other gains (losses)
Segment Reporting Information [Line Items]
Revenues	(1)	1.6	(0.9)
Pretax Profit (Loss)	(1)	1.6	(0.9)
Segment Reconciling Items | Interest Expense
Segment Reporting Information [Line Items]
Pretax Profit (Loss)	$ (153.1)	$ (140.9)	$ (136)

Segment Information Operating98

Segment Information Operating Results (Parenthetical) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Amortization of intangible assets	$ 66.2	$ 62.1	$ 46.8
Net Premiums Written	$ (27,132.1)	$ (23,353.5)	$ (20,564)
Personal Lines Segment | Underwriting Operations | Operating Segments
Segment Reporting Information [Line Items]
Personal auto insurance percentage of the total personal lines segment net premiums earned	93.00%	92.00%	92.00%
Property Segment | Underwriting Operations | Operating Segments
Segment Reporting Information [Line Items]
Amortization of intangible assets	$ 66.2	$ 62.1	$ 46.8
Other Indemnity
Segment Reporting Information [Line Items]
Net Premiums Written	$ 0	$ 0	$ 0.4

Segment Information Underwritin

Segment Information Underwriting Margins and Combined Ratios for our Underwriting Operations (Details) - Underwriting Operations	9 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Personal Lines Segment | Agency Channel
Segment Reporting Information [Line Items]
Underwriting Margin		7.50%	5.00%	7.80%
Combined Ratio		92.50%	95.00%	92.20%
Personal Lines Segment | Direct Channel
Segment Reporting Information [Line Items]
Underwriting Margin		6.30%	4.40%	4.90%
Combined Ratio		93.70%	95.60%	95.10%
Operating Segments
Segment Reporting Information [Line Items]
Underwriting Margin		6.60%	4.90%	7.50%
Combined Ratio		93.40%	95.10%	92.50%
Operating Segments | Personal Lines Segment
Segment Reporting Information [Line Items]
Underwriting Margin		6.90%	4.70%	6.50%
Combined Ratio		93.10%	95.30%	93.50%
Operating Segments | Commercial Lines Segment
Segment Reporting Information [Line Items]
Underwriting Margin		7.70%	6.40%	15.90%
Combined Ratio		92.30%	93.60%	84.10%
Operating Segments | Property Segment
Segment Reporting Information [Line Items]
Underwriting Margin		(5.10%)	3.80%	10.10%
Combined Ratio		105.10%	96.20%	89.90%
Combined ratio amortization points	0.077	0.067	0.072

Segment Information - Additiona

Segment Information - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)stateEntity	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Segment Reporting Information [Line Items]
Depreciation | $	$ 169.9	$ 137.4	$ 103.7
Operating Segments | Personal Lines Segment | Agency Channel | Lower Limit
Segment Reporting Information [Line Items]
Independent insurance agencies and brokerages | Entity	35,000
Residential Insurance [Member] | Property Segment
Segment Reporting Information [Line Items]
Number of states	41
National Flood Insurance Program [Member] | Property Segment
Segment Reporting Information [Line Items]
Number of states	43

Other Comprehensive Income (101

Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
AOCI before Tax, Attributable to Parent, beginning balance		$ 1,439.5	$ 1,234.5	$ 1,574
Unrealized Holding Gain (Loss) on Securities Arising During the Period, before tax		636.9	320.5	(198.7)
Net Non-credit Related OTTI Losses, Adjusted for Valuation Changes, before tax		0	(0.1)	0
Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax		(8)	0	(12.9)
Foreign Currently Translation Adjustment, before tax		0.4	0.6	(1.8)
Portion Attributable to Noncontrolling Interest, before tax		(3.5)	5.1	1.6
Total other comprehensive income (loss) before reclassifications, before tax		625.8	326.1	(211.8)
Total reclassification adjustment for amounts realized in net income, before tax		87.5	121.1	127.7
Total other comprehensive income, before tax		538.3	205	(339.5)
AOCI before Tax, Attributable to Parent, ending balance	$ 1,234.5	1,977.8	1,439.5	1,234.5
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
AOCI Tax, Attributable to Parent, beginning balance		(506.1)	(434.1)	(550.9)
Unrealized Holding Gain (Loss) on Securities Arising During the Period, tax		(224)	(112.6)	67.5
Net Non-credit Related OTTI Losses, Adjusted for Valuation Changes, tax		0	0.1	0
Unrealized Gain (Loss) on Derivatives Arising During Period, Tax		2.8	0	4.5
Foreign currency translation adjustment, tax		(0.1)	(0.2)	0.6
Portion Attributable to Noncontrolling Interest, tax		1.2	(1.9)	(0.5)
Total Other Comprehensive Income (Loss), before Reclassification Adjustments, Tax		(220.1)	(114.6)	72.1
Total reclassification adjustment for amounts realized in net income, Total tax (provisions) benefit		(30.6)	(42.6)	(44.7)
Total other comprehensive income, Total tax (provision) benefit		(189.5)	(72)	116.8
AOCI Tax, Attributable to Parent, ending balance	(434.1)	(695.6)	(506.1)	(434.1)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance		933.4	800.4	1,023.1
Unrealized Holding Gain (Loss) on Securities Arising During Period, after tax		412.9	207.9	(131.2)
Net Non-credit Related OTTI Losses, Adjusted for Valuation Changes, after tax		0	0	0
Unrealized Gain (Loss) on Derivatives Arising During Period, after tax		(5.2)	0	(8.4)
Foreign currency translation adjustment, after tax		0.3	0.4	(1.2)
Other comprehensive (income) loss attributable to NCI	1.1	(2.3)	3.2	1.1
Total other comprehensive income (loss), before reclassifications, after tax		405.7	211.5	(139.7)
Total reclassification adjustment for amounts realized in net income, Net of tax		56.9	78.5	83
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent		348.8	133	(222.7)
After tax total accumulated other comprehensive income, ending balance	800.4	1,282.2	933.4	800.4
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, before Tax		(14.9)	(27.1)	(23.8)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Tax		5.3	9.5	8.4
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax		(9.6)	(17.6)	(15.4)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax		103.3	146.3	149.7
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax		(36.2)	(51.4)	(52.5)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax		67.1	94.9	97.2
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, before Tax		(1.2)	0	0
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, Tax		0.4	0	0
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, Net of Tax		(0.8)	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax		0.3	1.9	1.8
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax		(0.1)	(0.7)	(0.6)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax		0.2	1.2	1.2
Net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance		939.6	809	1,021.9
Unrealized Holding Gain (Loss) on Securities Arising During Period, after tax		412.9	207.9	(131.2)
Net Non-credit Related OTTI Losses, Adjusted for Valuation Changes, after tax		0	0	0
Unrealized Gain (Loss) on Derivatives Arising During Period, after tax		0	0	0
Foreign currency translation adjustment, after tax		0	0	0
Other comprehensive (income) loss attributable to NCI		0	0	0
Total other comprehensive income (loss), before reclassifications, after tax		412.9	207.9	(131.2)
Total reclassification adjustment for amounts realized in net income, Net of tax		57.5	77.3	81.7
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent		355.4	130.6	(212.9)
After tax total accumulated other comprehensive income, ending balance	809	1,295	939.6	809
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax		(9.6)	(17.6)	(15.4)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax		67.1	94.9	97.1
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, Net of Tax		0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax		0	0	0
Net unrealized gains on forecasted transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance		(9.4)	(8.2)	1.5
Unrealized Holding Gain (Loss) on Securities Arising During Period, after tax		0	0	0
Net Non-credit Related OTTI Losses, Adjusted for Valuation Changes, after tax		0	0	0
Unrealized Gain (Loss) on Derivatives Arising During Period, after tax		(5.2)	0	(8.4)
Foreign currency translation adjustment, after tax		0	0	0
Other comprehensive (income) loss attributable to NCI		0	0	0
Total other comprehensive income (loss), before reclassifications, after tax		(5.2)	0	(8.4)
Total reclassification adjustment for amounts realized in net income, Net of tax		0.2	1.2	1.3
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent		(5.4)	(1.2)	(9.7)
After tax total accumulated other comprehensive income, ending balance	(8.2)	(14.8)	(9.4)	(8.2)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax		0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax		0	0	0.1
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, Net of Tax		0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax		0.2	1.2	1.2
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance		(1.1)	(1.5)	(0.3)
Unrealized Holding Gain (Loss) on Securities Arising During Period, after tax		0	0	0
Net Non-credit Related OTTI Losses, Adjusted for Valuation Changes, after tax		0	0	0
Unrealized Gain (Loss) on Derivatives Arising During Period, after tax		0	0	0
Foreign currency translation adjustment, after tax		0.3	0.4	(1.2)
Other comprehensive (income) loss attributable to NCI		0	0	0
Total other comprehensive income (loss), before reclassifications, after tax		0.3	0.4	(1.2)
Total reclassification adjustment for amounts realized in net income, Net of tax		(0.8)	0	0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent		1.1	0.4	(1.2)
After tax total accumulated other comprehensive income, ending balance	(1.5)	0	(1.1)	(1.5)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax		0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax		0	0	0
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, Net of Tax		(0.8)	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax		0	0	0
Noncontrolling Interest
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance		4.3	1.1	0
Unrealized Holding Gain (Loss) on Securities Arising During Period, after tax		0	0	0
Net Non-credit Related OTTI Losses, Adjusted for Valuation Changes, after tax		0	0	0
Unrealized Gain (Loss) on Derivatives Arising During Period, after tax		0	0	0
Foreign currency translation adjustment, after tax		0	0	0
Other comprehensive (income) loss attributable to NCI		(2.3)	3.2	1.1
Total other comprehensive income (loss), before reclassifications, after tax		(2.3)	3.2	1.1
Total reclassification adjustment for amounts realized in net income, Net of tax		0	0	0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent		(2.3)	3.2	1.1
After tax total accumulated other comprehensive income, ending balance	$ 1.1	2	4.3	1.1
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax		0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax		0	0	0
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Reclassification Adjustment from AOCI, Realized upon Sale or Liquidation, Net of Tax		0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax		$ 0	$ 0	$ 0

Other Comprehensive Income (102

Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement Additional Information (Detail) $ in Millions	Dec. 31, 2017USD ($)
Statement of Comprehensive Income [Abstract]
Net unrealized gains on forecasted transactions, expected to reclassify into income within the next 12 months	$ 1

Commitments and Contingencies M

Commitments and Contingencies Minimum Commitments under Noncancelable Operating Lease Agreements (Detail) $ in Millions	Dec. 31, 2017USD ($)
Commitments and Contingencies Disclosure [Abstract]
Operating Leases, Future Minimum Payments Due, Next Twelve Months	$ 53.6
Operating Leases, Future Minimum Payments, Due in Two Years	44.2
Operating Leases, Future Minimum Payments, Due in Three Years	28.5
Operating Leases, Future Minimum Payments, Due in Four Years	16.9
Operating Leases, Future Minimum Payments, Due in Five Years	8
Operating Leases, Future Minimum Payments, Due Thereafter	1.1
Operating Leases, Future Minimum Payments Due	$ 152.3

Commitments and Contingencies E

Commitments and Contingencies Expenses Incurred for Leases (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Abstract]
Expense	$ 77.2	$ 72.9	$ 66.6

Commitments and Contingencies -

Commitments and Contingencies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
reinsurance contract obligation [Line Items]
Minimum commitment under noncancelable purchase obligations	$ 546.3
unrecorded unconditional purchase obligations reinsurance [Member] | Lower Limit
reinsurance contract obligation [Line Items]
Reinsurance Contract Terms	1 year
Other Commitment	$ 90.7
unrecorded unconditional purchase obligations reinsurance [Member] | Upper Limit
reinsurance contract obligation [Line Items]
Reinsurance Contract Terms	3 years

Dividends Dividends (Details)

Dividends Dividends (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
	Feb. 28, 2018	Feb. 28, 2017	Feb. 29, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Dividends payable				$ 655.1	$ 395.4	$ 519.2
Payments of Ordinary Dividends, Common Stock		$ 395.4	$ 519
Annual variable dividend
Common Stock, Dividends, Per Share, Cash Paid		$ 0.6808	$ 0.8882
Subsequent Event | Annual variable dividend
Common Stock, Dividends, Per Share, Cash Paid	$ 1.1247

Dividends-Additional Informatio

Dividends-Additional Information (Details)	12 Months Ended
	Dec. 31, 2017USD ($)	Dec. 31, 2016	Dec. 31, 2015
Dividends Payable [Line Items]
Target percentage used to determine annual variable dividend	33.33%	33.33%	33.33%
Tax at statutory federal rate	35.00%	35.00%	35.00%
Gainshare factor used to determine annual variable dividend range, minimum	0
Gainshare factor used to determine annual variable dividend range, maximum	2
Gainshare factor used to determine annual variable dividend	1.79	1.67	1.60
Minimum Payments of Ordinary Dividends, Common Stock	$ 0
Tax Year 2018
Dividends Payable [Line Items]
Tax at statutory federal rate	21.00%

Redeemable Noncontrolling In108

Redeemable Noncontrolling Interest (Details) $ in Millions	9 Months Ended	12 Months Ended
	Dec. 31, 2015USD ($)	Dec. 31, 2017USD ($)voteshares		Dec. 31, 2016USD ($)		Dec. 31, 2015USD ($)	Apr. 01, 2021installment	Apr. 01, 2018installment	Apr. 01, 2015USD ($)installment	Dec. 31, 2014USD ($)
Redeemable Noncontrolling Interest [Line Items]
Number of Installments Related to Stockholders Agreement | installment									2
Number of stockholder votes to approve actions | vote		2
Redeemable Noncontrolling Interest, Equity, Carrying Amount	$ 464.9	$ 503.7	[1]	$ 483.7	[1]	$ 464.9				$ 0
Fair value at date of acquisition		0		0					$ 411.5
Net income attributable to NCI	32.9	5.9		26.2		32.9
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Noncontrolling Interest	(1.1)	2.3		(3.2)		$ (1.1)
Stock Issued During Period, Value, Stock Options Exercised Attributable to Redeemable Noncontrolling Interest	0	3.4		0
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests	(12.6)	0		0
Adjustment to carrying amount of redeemable noncontrolling interest	$ 34.2	$ 8.4		$ (4.2)
Forecast
Redeemable Noncontrolling Interest [Line Items]
Number of Installments Related to Stockholders Agreement | installment							1	1
ARX Holding Corp.
Redeemable Noncontrolling Interest [Line Items]
Equity interest percentage	69.20%	69.00%		69.20%		69.20%			5.00%
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number | shares		22,550
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value		$ 32.5
ARX Holding Corp. | Forecast
Redeemable Noncontrolling Interest [Line Items]
Equity interest percentage							100.00%	80.00%
Lower Limit
Redeemable Noncontrolling Interest [Line Items]
Multiple for determining purchase price of shares		1
Upper Limit
Redeemable Noncontrolling Interest [Line Items]
Multiple for determining purchase price of shares		2
[1]	See Note 15 – Redeemable Noncontrolling Interest for further discussion.

Goodwill and Intangible Asse109

Goodwill and Intangible Assets Goodwill (Details) - USD ($)	6 Months Ended		12 Months Ended
	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill [Line Items]
Goodwill	$ 452,700,000		$ 452,700,000	$ 449,400,000
Goodwill, Impaired, Accumulated Impairment Loss	$ 0		0
Goodwill, Acquired During Period			3,300,000	1,800,000
Amortization of intangible assets			$ 66,200,000	$ 62,100,000	$ 46,800,000
Weighted Average [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Remaining Amortization Period			4 years 10 months 27 days
Trade name
Goodwill [Line Items]
Intangible assets finite lives	2 years	10 years
Residential Property Insurance Company
Goodwill [Line Items]
ARX Acquisition Percent in Residential Insurance Subsidiary				100.00%
Commercial Property Insurance Company | Disposal Group, Disposed of by Sale, Not Discontinued Operations
Goodwill [Line Items]
ARX Disposal Percent in Commercial Insurance Subsidiary				100.00%

Goodwill and Intangible Asse110

Goodwill and Intangible Assets - Net Carrying Amount of Other Intangible Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Accumulated Amortization	$ 175.7	$ 109.5
Indefinite-lived intangible assets	12.4	12.4
Total	366.6	432.8
Finite-Lived Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Accumulated Amortization	175.1	108.9
Intangible assets subject to amortization	354.2	420.4
Licenses
Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Accumulated Amortization	$ 0.6	$ 0.6

Goodwill and Intangible Asse111

Goodwill and Intangible Assets - Finite-lived Intangible Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	$ 175.7	$ 109.5
Policies in force
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	256.2	256.2
Accumulated Amortization	100.7	64.1
Net Carrying Amount	155.5	192.1
Agency relationships
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	159.2	159.2
Accumulated Amortization	31.3	19.9
Net Carrying Amount	127.9	139.3
Software rights
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	79.1	79.1
Accumulated Amortization	29.4	18.8
Net Carrying Amount	49.7	60.3
Trade name
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	34.8	34.8
Accumulated Amortization	13.7	6.1
Net Carrying Amount	21.1	28.7
Finite-Lived Intangible Assets
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	529.3	529.3
Accumulated Amortization	175.1	108.9
Net Carrying Amount	$ 354.2	$ 420.4

Goodwill and Intangible Asse112

Goodwill and Intangible Assets Goodwill and Intangible Assets - Amortization (Details) $ in Millions	Dec. 31, 2017USD ($)
Finite-Lived Intangible Assets, Net, Amortization Expense, Fiscal Year Maturity [Abstract]
Finite- Lived Intangible Assets, Amortization Expense, Next Twelve Months	$ 71.9
Finite-Lived Intangible Assets, Amortization Expense, Year Two	66.4
Finite-Lived Intangible Assets, Amortization Expense, Year Three	56.9
Finite-Lived Intangible Assets, Amortization Expense, Year Four	56.6
Finite-Lived Intangible Assets, Amortization Expense, Year Five	$ 29.2