The Progressive Corporation (PGR) 10-Q 2 Nov 17 2017 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information	9 Months Ended
	Sep. 30, 2017shares
Document Document And Entity Information [Abstract]
Entity Registrant Name	PROGRESSIVE CORP/OH/
Entity Central Index Key	80,661
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3
Amendment Flag	false
Trading Symbol	PGR
Entity Common Stock, Shares Outstanding	581,634,009

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) shares in Millions, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Revenues
Net premiums earned		$ 6,544	$ 5,723.4	$ 18,884	$ 16,602.6
Period over prior year period percent change in net premiums earned		14.00%		14.00%
Investment income		$ 142.9	119.3	$ 410.9	352.7
Period over prior year period percent change in investment income		20.00%		17.00%
Net realized gains (losses) on securities:
Net impairment losses recognized in earnings		$ (43)	(61.6)	$ (57.8)	(61.8)
Period Over Prior Year Period, Percent Increase (Decrease), Net Impairment Losses Recognized In Earnings		(30.00%)		(6.00%)
Net realized gains (losses) on securities		$ 18.3	40.9	$ 117.1	90.8
Period over prior year period percent change in net realized gains (losses) on securities		(55.00%)		29.00%
Total net realized gains (losses) on securities		$ (24.7)	(20.7)	$ 59.3	29
Period over prior year period percent change in total net realized gains (losses) on securities		19.00%		104.00%
Fees and other revenues		$ 96.3	86.8	$ 270.3	248.2
Period over prior year period percent change in fees and other revenues		11.00%		9.00%
Service revenues		$ 33.3	26.2	$ 94.5	77.7
Period over prior year period percent change in service revenues		27.00%		22.00%
Gains on extinguishment of debt		$ 0	0	$ 0.2	1.6
Percent Increase (Decrease) Gain Loss on Extingiushment of Debt				(88.00%)
Total revenues		$ 6,791.8	5,935	$ 19,719.2	17,311.8
Period over prior year period percent change in total revenues		14.00%		14.00%
Expenses
Losses and loss adjustment expenses		$ 5,050.5	4,398.2	$ 13,928.8	12,554.6
Period over prior year period percent change in losses and loss adjustment expenses		15.00%		11.00%
Policy acquisition costs		$ 540.1	475.4	$ 1,557.2	1,374.6
Period over prior year period percent change in policy acquisition costs		14.00%		13.00%
Other underwriting expenses		$ 877.7	739.6	$ 2,568.3	2,262.2
Period over prior year period percent change in other underwriting expenses		19.00%		14.00%
Investment expenses		$ 5.8	4.8	$ 18	14.9
Period over prior year period percent change in investment expenses		21.00%		21.00%
Service expenses		$ 28.9	23.2	$ 81.8	68.5
Period over prior year period percent change in service expenses		25.00%		19.00%
Interest expense		$ 37.4	35.3	$ 117.6	103.8
Period over prior year period percent change in interest expense		6.00%		13.00%
Total expenses		$ 6,540.4	5,676.5	$ 18,271.7	16,378.6
Period over prior year period percent change in total expenses		15.00%		12.00%
Net Income
Income before income taxes		$ 251.4	258.5	$ 1,447.5	933.2
Period over prior year period percent change in income before income taxes		(3.00%)		55.00%
Provision for income taxes		$ 36.6	53	$ 429.7	274.1
Period over prior year period percent change in provision for income taxes		(31.00%)		57.00%
Net income		$ 214.8	205.5	$ 1,017.8	659.1
Period over prior year period percent change in net income		5.00%		54.00%
Net (income) loss attributable to noncontrolling interest (NCI)		$ 9.2	(6.8)	$ (1.9)	(11.3)
Period over prior year period percent change in net (income) loss attributable to noncontrolling interest		(235.00%)		(83.00%)
Net income attributable to Progressive		$ 224	198.7	$ 1,015.9	647.8
Period over prior year period percent change in net income attributable to Progressive		13.00%		57.00%
Other Comprehensive Income (Loss)
Total net unrealized gains (losses) on securities		$ 75.5	87.1	$ 300.5	248.2
Period over prior year period percent change in total net unrealized gains (losses) on securities		(13.00%)		21.00%
Net unrealized losses on forecasted transactions		$ 0.1	(0.3)	$ (5.6)	(0.9)
Period Over Prior Year Period, Percent Increase (Decrease), Unrealized Gain (Loss) On Forecasted Transactions		(133.00%)
Foreign currency translation adjustment		$ 0.6	0.3	$ 0.8	0.7
Period over prior year period percent change in foreign currency translation adjustment		100.00%		14.00%
Other comprehensive income		$ 76.2	87.1	$ 295.7	248
Period over prior year period percent change in other comprehensive income		(13.00%)		19.00%
Other comprehensive (income) loss attributable to NCI		$ (0.7)	1.2	$ (2.9)	(2.3)
Period over prior year period change in Comprehensive (income) loss attributable to NCI		(158.00%)		26.00%
Comprehensive income attributable to Progressive		$ 299.5	$ 287	$ 1,308.7	$ 893.5
Period over prior year period percent change in comprehensive income		4.00%		46.00%
Computation of Per Share Earnings Attributable to Progressive
Average shares outstanding - Basic		581.3	581.5	580.7	582.4
Period over prior year period percent change in average shares outstanding - basic		0.00%		0.00%
Net effect of dilutive stock-based compensation		4.3	3.1	5	3.2
Period over prior year period percent change in net effect of dilutive stock-based compensation		39.00%		56.00%
Total average equivalent shares - Diluted		585.6	584.6	585.7	585.6
Period over prior year period percent change in total equivalent shares - diluted		0.00%		0.00%
Basic: Earnings per share		$ 0.39	$ 0.34	$ 1.75	$ 1.11
Period over prior year period percent change in basic: earnings per share		13.00%		57.00%
Diluted: Earnings per share		$ 0.38	0.34	$ 1.73	1.11
Period over prior year period percent change in diluted: earnings per share		13.00%		57.00%
Dividends declared per share	[1]	$ 0	$ 0	$ 0	$ 0
[1]	Progressive maintains an annual dividend program. See Note 9 – Dividends for further discussion.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Investments - Available-for-sale, at fair value:
Fixed maturities (amortized cost: $18,583.1, $13,708.0, and $16,287.1)		$ 18,660	$ 16,243.8	$ 13,916.9
Equity securities:
Nonredeemable preferred stocks (cost: $700.6, $726.6, and $734.2)		813.7	853.5	860.1
Common equities (cost: $1,485.5, $1,576.1, and $1,437.5)		3,209.5	2,812.4	2,868
Short-term investments (amortized cost: $4,311.5, $5,876.2, and $3,572.9)		4,311.5	3,572.9	5,876.2
Total investments		26,994.7	23,482.6	23,521.2
Cash		224.9	211.5	143.4
Restricted cash1	[1]	31.4	14.9	22.3
Accrued investment income		113	103.9	97.3
Premiums receivable, net of allowance for doubtful accounts of $189.3, $176.8, and $186.8		5,519.9	4,509.2	4,743.6
Reinsurance recoverables, including $76.4, $67.5, and $83.8 on paid losses and loss adjustment expenses		2,701.1	1,884.8	1,901.7
Prepaid reinsurance premiums		211.7	170.5	174.4
Deferred acquisition costs		782.6	651.2	675.4
Property and equipment, net of accumulated depreciation of $908.4, $830.1, and $845.8		1,129.4	1,177.1	1,115
Goodwill		452.7	449.4	449.4
Intangible assets, net of accumulated amortization of $157.7, $94.0, and $109.5		384.6	432.8	448.3
Other assets		386.6	339.6	328.8
Total assets		38,932.6	33,427.5	33,620.8
Liabilities
Unearned premiums		9,005.3	7,468.3	7,792.4
Loss and loss adjustment expense reserves		13,353.3	11,368	11,228.2
Net deferred income taxes		201.5	111.3	156.9
Dividends payable		0	395.4	0
Accounts payable, accrued expenses, and other liabilities		3,272.7	2,495.5	2,722.2
Debt	[2]	3,312.2	3,148.2	3,153.9
Total liabilities		29,145	24,986.7	25,053.6
Redeemable noncontrolling interest (NCI)3	[3]	498.2	483.7	472.5
Common shares, $1.00 par value (authorized 900.0; issued 797.5, including treasury shares of 215.9, 216.7, and 217.6)		581.6	579.9	580.8
Paid-in capital		1,365.1	1,303.4	1,284.7
Retained earnings		6,116.5	5,140.4	5,183.1
Accumulated other comprehensive income:
Net unrealized gains (losses) on securities		1,240.1	939.6	1,057.2
Net unrealized losses on forecasted transactions		(15)	(9.4)	(9.1)
Foreign currency translation adjustment		(0.3)	(1.1)	(0.8)
Accumulated other comprehensive (income) loss attributable to NCI		1.4	4.3	(1.2)
Total accumulated other comprehensive income attributable to Progressive		1,226.2	933.4	1,046.1
Total shareholders’ equity		9,289.4	7,957.1	8,094.7
Total liabilities, redeemable NCI, and shareholders’ equity		$ 38,932.6	$ 33,427.5	$ 33,620.8
[1]	See Note 7 – Supplemental Cash Flow Information for further discussion.
[2]	Consists of both short-term and long-term debt. See Note 4 – Debt for further discussion.
[3]	See Note 12 – Redeemable Noncontrolling Interest for further discussion.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Statement of Financial Position [Abstract]
Fixed maturities, amortized cost	$ 18,583.1	$ 16,287.1	$ 13,708
Nonredeemable preferred stocks, cost	700.6	734.2	726.6
Common equities, cost	1,485.5	1,437.5	1,576.1
Short-term investments, amortized cost	4,311.5	3,572.9	5,876.2
Premiums receivable, allowance for doubtful accounts	189.3	186.8	176.8
Reinsurance recoverables, paid losses and loss adjustment expenses	76.4	83.8	67.5
Intangible Assets, Accumulated Amortization	157.7	109.5	94
Property and equipment, accumulated depreciation	$ 908.4	$ 845.8	$ 830.1
Common Shares, par value (USD per share)	$ 1	$ 1	$ 1
Common Shares, authorized (shares)	900,000,000	900,000,000	900,000,000
Common Shares, issued (shares)	797,500,000	797,500,000	797,500,000
Common Shares, treasury shares (shares)	215,900,000	217,600,000	216,700,000

Consolidated Statements of Chan

Consolidated Statements of Changes in Shareholders' Equity Statement - USD ($) $ in Millions	Total	Common Shares, $1.00 Par Value	Paid-in Capital	Retained Earnings	AOCI Attributable to Parent [Member]	Serial Preferred Shares [Member]	Voting Preference Shares [Member]
Balance, Beginning of period at Dec. 31, 2015		$ 583.6	$ 1,218.8	$ 4,686.6	$ 800.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Progressive	$ 647.8			647.8
Tax benefit from vesting of equity-based compensation			8
Treasury shares purchased		(5.2)	(11.4)	(147)
Cash dividends declared on common shares				0.2
Net restricted equity awards issued/vested		2.4	(2.4)
Amortization of equity-based compensation			64.6
Reinvested dividends on restricted stock units			1.1	(1.1)
Adjustment to carrying amount of redeemable noncontrolling interest	(6)		6
Attributable to noncontrolling interest	(2.3)				(2.3)
Other comprehensive income					248
Other, net				(3.4)
Balance, End of period at Sep. 30, 2016	8,094.7	580.8	1,284.7	5,183.1	1,046.1
Balance, Beginning of period at Dec. 31, 2015		583.6	1,218.8	4,686.6	800.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment to carrying amount of redeemable noncontrolling interest	(4.2)
Attributable to noncontrolling interest	3.2
Balance, End of period at Dec. 31, 2016	$ 7,957.1	579.9	1,303.4	5,140.4	933.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock, shares authorized (in shares)						20,000,000	5,000,000
Preferred stock, shares issued (in shares)						0	0
Preferred stock, shares outstanding (in shares)						0
Common Shares, par value (USD per share)	$ 1
Common Shares, par value (USD per share)	$ 1
Net income attributable to Progressive	$ 1,015.9			1,015.9
Tax benefit from vesting of equity-based compensation			0
Treasury shares purchased		(1.5)	(3.4)	(57.2)
Cash dividends declared on common shares				0
Net restricted equity awards issued/vested		3.2	(3.2)
Amortization of equity-based compensation			74.3
Reinvested dividends on restricted stock units			0.3	(0.3)
Adjustment to carrying amount of redeemable noncontrolling interest	6.3		(6.3)
Attributable to noncontrolling interest	(2.9)				(2.9)
Other comprehensive income					295.7
Other, net				17.7
Balance, End of period at Sep. 30, 2017	$ 9,289.4	$ 581.6	$ 1,365.1	$ 6,116.5	$ 1,226.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock, shares authorized (in shares)						20,000,000	5,000,000
Preferred stock, shares issued (in shares)						0	0
Preferred stock, shares outstanding (in shares)						0
Common Shares, par value (USD per share)	$ 1

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016
Statement of Cash Flows [Abstract]
Net income		$ 1,017.8	$ 659.1
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation		126.7	94.6
Amortization of intangible assets		48.2	46.6
Net amortization of fixed-income securities		67.5	57.1
Amortization of equity-based compensation		76.6	64.6
Net realized (gains) losses on securities		(59.3)	(29)
Net (gains) losses on disposition of property and equipment		5.3	4.1
(Gains) losses on extinguishment of debt		(0.2)	(1.6)
Net loss on exchange transaction		0	4.5
Changes in:
Premiums receivable		(1,010.3)	(752.8)
Reinsurance recoverables		(816.1)	(405.1)
Prepaid reinsurance premiums		(41.2)	44.9
Deferred acquisition costs		(131.4)	(128)
Income taxes		(111.2)	(121.2)
Unearned premiums		1,536.7	1,154.6
Loss and loss adjustment expense reserves		1,985.1	1,183.2
Accounts payable, accrued expenses, and other liabilities		622.1	464.8
Restricted cash		(16.5)	(22)
Other, net		(93.2)	(42.6)
Net cash provided by operating activities		3,206.6	2,275.8
Purchases:
Fixed maturities		(9,623.6)	(7,364.6)
Equity securities		(155.2)	(367.1)
Sales:
Fixed maturities		3,431.7	4,746.1
Equity securities		150.6	278.2
Maturities, paydowns, calls, and other:
Fixed maturities		3,872.5	4,189
Equity securities		50	0
Net sales (purchases) of short-term investments		(721.8)	(3,665.2)
Net unsettled security transactions		210.5	208.4
Purchases of property and equipment		(109.7)	(162.1)
Sales of property and equipment		13.8	4.5
Net cash disposed in exchange transaction1	[1]	0	(7.7)
Acquisition of an insurance company, net of cash acquired		(18.1)	0
Net cash used in investing activities		(2,899.3)	(2,140.5)
Cash Flows From Financing Activities
Proceeds from exercise of equity options		0.5	0
Tax benefit from vesting of equity-based compensation		0	8
Net proceeds from debt issuance		841.1	495.6
Payments of debt		(42.8)	(19.2)
Redemption/reacquisition of subordinated debt		(635.6)	(18.2)
Dividends paid to shareholders		(395.4)	(519)
Acquisition of treasury shares for restricted stock tax liabilities		(57.2)	(24.7)
Acquisition of treasury shares acquired in open market		(4.9)	(138.9)
Net cash used in financing activities		(294.3)	(216.4)
Effect of exchange rate changes on cash		0.4	0.4
Increase (decrease) in cash		13.4	(80.7)
Cash, January 1		211.5	224.1
Cash, September 30		$ 224.9	$ 143.4
[1]	See Note 7 – Supplemental Cash Flow Information for further discussion.

Basis of Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation — 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 September 30, 2017 and 69.2% at September 30, 2016 and December 31, 2016 . The decrease reflects ARX employee stock options that were exercised during the first quarter 2017. All intercompany accounts and transactions are eliminated in consolidation. The consolidated financial statements reflect all normal recurring adjustments that, in the opinion of management, were necessary for a fair statement of the results for the interim periods presented. The results of operations for the period ended September 30, 2017 , are not necessarily indicative of the results expected for the full year. These consolidated financial statements and the notes thereto should be read in conjunction with Progressive’s audited financial statements and accompanying notes included in Exhibit 13 to our Annual Report on Form 10-K for the year ended December 31, 2016 ( “ 2016 Annual Report to Shareholders ” ). Other assets on the consolidated balance sheets include properties that are considered “ held for sale, ” if any. The fair value of these properties, less the estimated cost to sell them, was $5.3 million at September 30, 2017 , and $8.7 million at both September 30, 2016 and December 31, 2016.

Investments

Investments	9 Months Ended
	Sep. 30, 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 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. ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value September 30, 2017 Fixed maturities: U.S. government obligations $ 4,612.2 $ 3.0 $ (20.2 ) $ 0 $ 4,595.0 17.0 % State and local government obligations 2,332.2 35.0 (3.1 ) 0.1 2,364.2 8.7 Foreign government obligations 24.2 0 0 0 24.2 0.1 Corporate debt securities 5,195.7 32.7 (4.2 ) 1.4 5,225.6 19.4 Residential mortgage-backed securities 911.3 12.2 (3.2 ) 0 920.3 3.4 Agency residential pass-through obligations 35.7 0 (0.4 ) 0 35.3 0.1 Commercial mortgage-backed securities 2,763.7 17.0 (12.8 ) 0 2,767.9 10.3 Other asset-backed securities 2,485.6 6.4 (2.0 ) 0.2 2,490.2 9.2 Redeemable preferred stocks 222.5 16.4 (2.0 ) 0.4 237.3 0.9 Total fixed maturities 18,583.1 122.7 (47.9 ) 2.1 18,660.0 69.1 Equity securities: Nonredeemable preferred stocks 700.6 120.4 (7.3 ) 0 813.7 3.0 Common equities 1,485.5 1,729.5 (5.5 ) 0 3,209.5 11.9 Short-term investments 4,311.5 0 0 0 4,311.5 16.0 Total portfolio 1,2 $ 25,080.7 $ 1,972.6 $ (60.7 ) $ 2.1 $ 26,994.7 100.0 % ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value September 30, 2016 Fixed maturities: U.S. government obligations $ 726.0 $ 8.9 $ (0.3 ) $ 0 $ 734.6 3.1 % State and local government obligations 2,508.9 58.7 (1.6 ) 0 2,566.0 10.9 Foreign government obligations 25.4 0 0 0 25.4 0.1 Corporate debt securities 4,327.5 76.9 (1.8 ) 0.7 4,403.3 18.7 Residential mortgage-backed securities 1,582.6 25.3 (15.5 ) 1.8 1,594.2 6.8 Agency residential pass-through obligations 43.6 0.5 0 0 44.1 0.2 Commercial mortgage-backed securities 2,249.4 35.1 (6.1 ) 0 2,278.4 9.7 Other asset-backed securities 2,037.5 7.9 (0.3 ) 0.3 2,045.4 8.7 Redeemable preferred stocks 207.1 19.6 (1.2 ) 0 225.5 1.0 Total fixed maturities 13,708.0 232.9 (26.8 ) 2.8 13,916.9 59.2 Equity securities: Nonredeemable preferred stocks 726.6 146.2 (13.9 ) 1.2 860.1 3.6 Common equities 1,576.1 1,296.3 (4.4 ) 0 2,868.0 12.2 Short-term investments 5,876.2 0 0 0 5,876.2 25.0 Total portfolio 1,2 $ 21,886.9 $ 1,675.4 $ (45.1 ) $ 4.0 $ 23,521.2 100.0 % ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of 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 1,448.5 23.7 (15.0 ) 1.5 1,458.7 6.2 Agency residential pass-through obligations 41.2 0 (0.6 ) 0 40.6 0.2 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 open derivative positions; at September 30, 2017 and 2016 , and December 31, 2016 , we had $238.3 million , $185.3 million , and $27.8 million , respectively, included in “other liabilities.” 2 The total fair value of the portfolio at September 30, 2017 and 2016 , and December 31, 2016 , included $1.1 billion , $1.0 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. Short-Term Investments Our short-term investments may include commercial paper and other investments that are expected to mature within one year. We did not enter into any repurchase commitment transactions during the first nine months of 2017 or 2016 , and we had no open repurchase commitments at September 30, 2017 and 2016, or December 31, 2016 . Also included in short-term investments are reverse repurchase commitment transactions, where we loan cash to approved counterparties and receive U.S. Treasury Notes pledged as collateral against the cash borrowed. Our exposure to credit risk is limited due to the nature of the collateral (i.e., U.S. Treasury Notes) received. We have counterparty exposure on these trades in the event of a counterparty default to the extent the collateral security’s value is below the amount of cash we delivered to acquire the collateral. The short-term duration of the transactions (primarily overnight) reduces that exposure. We had no open reverse repurchase commitments at September 30, 2017 or December 31, 2016 , compared to $68.0 million open at September 30, 2016. We did not enter into any reverse repurchase commitments for the nine months ended September 30, 2017 . During the nine months ended September 30, 2016, our largest outstanding balance of reverse repurchase commitments was $265.0 million , which was open for one day. For the 28 days we invested in these transactions, the average daily balance of reverse repurchase commitments was $ 122.6 million . To the extent we enter into repurchase or reverse repurchase transactions, and consistent with past practice, we would elect not to offset these transactions and would report them on a gross basis on our balance sheets despite the option to elect to offset these transactions as long as they were with the same counterparty and subject to an enforceable master netting arrangement. Hybrid Securities Included in our fixed-maturity and equity securities are hybrid securities, which are reported at fair value: September 30, December 31, (millions) 2017 2016 Fixed maturities: State and local government obligations $ 2.4 $ 0 $ 0 Corporate debt securities 114.9 31.2 40.1 Residential mortgage-backed securities 0 175.3 170.5 Other asset-backed securities 7.1 9.5 8.9 Redeemable preferred stocks 35.5 0 0 Total fixed maturities 159.9 216.0 219.5 Equity securities: Nonredeemable preferred stocks 0 13.6 0 Total hybrid securities $ 159.9 $ 229.6 $ 219.5 The state and local government obligations in the table above were acquired at a premium and contain a contingently exercisable call feature that allows the issuer, at its discretion, to call the securities at par based on a provision that is unrelated to the economic characteristics of the issuer. Certain corporate debt securities are accounted for as hybrid securities since they were acquired at a premium and contain a change-in-control put option (derivative) that permits the investor, at its sole option if and when a change in control is triggered, to put the security back to the issuer at a 1% premium to par. Due to this change-in-control put option and the substantial market premium paid to acquire these securities, there is the potential that the election to put, upon the change in control, would result in an acceleration of the recognition of the remaining premium paid on these securities in our results of operations. The put feature limits the potential loss in value that could be experienced in the event a corporate action occurs that results in a change in control that materially diminishes the credit quality of the issuer. Exercises of the puts would result in a loss of $9.6 million as of September 30, 2017 , if all of the bonds experienced a simultaneous change in control and we elected to exercise all of our put options. We are under no obligation to exercise the put option we hold if a change in control occurs. The residential mortgage-backed securities accounted for as hybrid securities are obligations of the issuer with payments of principal based on the performance of a reference pool of loans. This embedded derivative results in the securities incorporating the risk of default from both the issuer and the related loan pool. These securities were sold during 2017. The other asset-backed security in the table above represents one hybrid security that was acquired at a deep discount to par due to a failing auction, and contains a put option that allows the investor to put that security back to the auction at par if the auction is restored. This embedded derivative had the potential to more than double our initial investment yield at acquisition. The redeemable preferred stock in the table above represents one hybrid security acquired in 2017 with a provision that requires the issuer, in the event of bankruptcy, to convert the security into an equivalent number of shares of a newly-issued perpetual preferred stock. This embedded derivative has the potential to result in the security converting from a debt instrument to an equity instrument. During 2016, we sold the nonredeemable preferred stocks referred to in the table above. These securities were perpetual preferred stocks with fixed-rate coupons that have call features, whereby the change in value of the call features was a component of the overall change in value of the preferred stocks. Fixed Maturities The composition of fixed maturities by maturity at September 30, 2017 , was: (millions) Cost Fair Value Less than one year $ 4,088.1 $ 4,105.7 One to five years 11,476.9 11,508.8 Five to ten years 2,873.9 2,901.3 Ten years or greater 144.2 144.2 Total $ 18,583.1 $ 18,660.0 Asset-backed securities are classified in the maturity distribution table based upon their projected cash flows. All other securities which 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 September 30, 2017 , we had $55.2 million of gross unrealized losses in our fixed-income securities (i.e., fixed-maturity securities and nonredeemable preferred stocks) and $5.5 million in our common equities. We currently do not intend to sell the fixed-income securities and determined that it is more likely that we will not be required to sell these securities for the period of time necessary to recover their cost bases. A review of our fixed-income securities indicated that the issuers were current with respect to their interest obligations and that there was no evidence of any deterioration of the current cash flow projections that would indicate we would not receive the remaining principal at maturity. For common equities, 95% of our common stock portfolio was indexed to the Russell 1000; as such, this portfolio may contain securities in a loss position for an extended period of time, subject to possible write-downs, as described below. We may retain these securities as long as the portfolio and index correlation remain similar. To the extent there is issuer-specific deterioration, we may write down the securities of that issuer. The remaining 5% of our common stocks were part of a managed equity strategy selected and administered by an external investment advisor. If our review of loss position securities were to indicate there was a fundamental, or market, impairment on these securities that was determined to be other-than-temporary, we would recognize a write-down in accordance with our stated policy. The following tables show the composition of gross unrealized losses by major security type and by the length of time that individual securities have been in a continuous unrealized loss position: Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses September 30, 2017 Fixed maturities: U.S. government obligations 54 $ 3,804.0 $ (20.2 ) 31 $ 2,302.2 $ (7.1 ) 23 $ 1,501.8 $ (13.1 ) State and local government obligations 169 610.4 (3.1 ) 40 141.0 (0.7 ) 129 469.4 (2.4 ) Corporate debt securities 113 1,523.8 (4.2 ) 59 636.0 (1.1 ) 54 887.8 (3.1 ) Residential mortgage-backed securities 126 308.8 (3.2 ) 14 37.4 (0.1 ) 112 271.4 (3.1 ) Agency residential pass-through obligations 65 32.7 (0.4 ) 12 1.8 0 53 30.9 (0.4 ) Commercial mortgage-backed securities 105 1,588.9 (12.8 ) 61 1,073.5 (4.6 ) 44 515.4 (8.2 ) Other asset-backed securities 148 1,393.2 (2.0 ) 103 920.8 (1.0 ) 45 472.4 (1.0 ) Redeemable preferred stocks 2 21.4 (2.0 ) 1 10.9 0 1 10.5 (2.0 ) Total fixed maturities 782 9,283.2 (47.9 ) 321 5,123.6 (14.6 ) 461 4,159.6 (33.3 ) Equity securities: Nonredeemable preferred stocks 3 72.4 (7.3 ) 0 0 0 3 72.4 (7.3 ) Common equities 67 52.0 (5.5 ) 62 49.3 (4.9 ) 5 2.7 (0.6 ) Total equity securities 70 124.4 (12.8 ) 62 49.3 (4.9 ) 8 75.1 (7.9 ) Total portfolio 852 $ 9,407.6 $ (60.7 ) 383 $ 5,172.9 $ (19.5 ) 469 $ 4,234.7 $ (41.2 ) Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses September 30, 2016 Fixed maturities: U.S. government obligations 4 $ 104.8 $ (0.3 ) 4 $ 104.8 $ (0.3 ) 0 $ 0 $ 0 State and local government obligations 151 512.0 (1.6 ) 117 409.6 (1.0 ) 34 102.4 (0.6 ) Corporate debt securities 52 598.3 (1.8 ) 42 541.5 (0.8 ) 10 56.8 (1.0 ) Residential mortgage-backed securities 138 915.8 (15.5 ) 26 108.8 (0.4 ) 112 807.0 (15.1 ) Agency residential pass-through obligations 15 4.2 0 7 1.9 0 8 2.3 0 Commercial mortgage-backed securities 56 610.6 (6.1 ) 17 194.7 (1.7 ) 39 415.9 (4.4 ) Other asset-backed securities 43 488.7 (0.3 ) 16 219.5 (0.1 ) 27 269.2 (0.2 ) Redeemable preferred stocks 2 31.8 (1.2 ) 0 0 0 2 31.8 (1.2 ) Total fixed maturities 461 3,266.2 (26.8 ) 229 1,580.8 (4.3 ) 232 1,685.4 (22.5 ) Equity securities: Nonredeemable preferred stocks 7 187.5 (13.9 ) 1 9.9 (0.2 ) 6 177.6 (13.7 ) Common equities 86 53.4 (4.4 ) 83 49.5 (4.2 ) 3 3.9 (0.2 ) Total equity securities 93 240.9 (18.3 ) 84 59.4 (4.4 ) 9 181.5 (13.9 ) Total portfolio 554 $ 3,507.1 $ (45.1 ) 313 $ 1,640.2 $ (8.7 ) 241 $ 1,866.9 $ (36.4 ) Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair 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 178 917.7 (15.0 ) 69 175.8 (1.1 ) 109 741.9 (13.9 ) Agency residential pass-through obligations 55 36.0 (0.6 ) 48 33.9 (0.6 ) 7 2.1 0 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 ) Since September 30, 2016 , the number of securities in our fixed-maturity portfolio with unrealized losses increased, primarily the result of rising interest rates during the latter part of 2016 . A narrowing of credit spreads for the first nine months of 2017 resulted in a decrease in the number of fixed-maturity securities with unrealized losses since December 31, 2016 . We had no material decreases in valuation as a result of credit rating downgrades on our fixed-maturity securities. All of the fixed-maturity securities in an unrealized loss position at September 30, 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 three securities, averaging approximately 9% of their total cost. The decrease in the number of securities is 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 we 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 the first nine months of 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, market 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): September 30, December 31, (millions) 2017 2016 Fixed maturities: Residential mortgage-backed securities $ (19.7 ) $ (43.3 ) $ (43.3 ) Commercial mortgage-backed securities (0.4 ) (0.6 ) (0.6 ) Total fixed maturities $ (20.1 ) $ (43.9 ) $ (43.9 ) The following tables provide rollforwards of the amounts related to credit losses recognized in earnings for the periods ended September 30, 2017 and 2016 , 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: Three Months Ended September 30, 2017 Mortgage-Backed (millions) Residential Commercial Total Balance at June 30, 2017 $ 0.2 $ 0.1 $ 0.3 Credit losses for which an OTTI was not previously recognized 0 0.4 0.4 Reductions for securities sold/matured 0 0 0 Change in recoveries of future cash flows expected to be collected 1 0 0 0 Balance at September 30, 2017 $ 0.2 $ 0.5 $ 0.7 Nine Months Ended September 30, 2017 Mortgage-Backed (millions) Residential Commercial Total Balance 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 0 0 Balance at September 30, 2017 $ 0.2 $ 0.5 $ 0.7 Three Months Ended September 30, 2016 Mortgage-Backed (millions) Residential Commercial Total Balance at June 30, 2016 $ 11.8 $ 0.4 $ 12.2 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 (0.3 ) 0 (0.3 ) Balance at September 30, 2016 $ 11.5 $ 0.4 $ 11.9 Nine Months Ended September 30, 2016 Mortgage-Backed (millions) Residential Commercial Total Balance 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 (0.9 ) 0 (0.9 ) Balance at September 30, 2016 $ 11.5 $ 0.4 $ 11.9 1 Reflects the current period change in the expected recovery of prior 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. We did not have any credit impairment write-downs for the nine months ended September 30, 2016 . Realized Gains (Losses) The components of net realized gains (losses) for the three and nine months ended September 30, were: Three Months Nine Months (millions) 2017 2016 2017 2016 Gross realized gains on security sales Fixed maturities: U.S. government obligations $ 0.9 $ 6.4 $ 5.8 $ 24.1 State and local government obligations 4.0 0.6 7.1 16.0 Corporate and other debt securities 5.1 15.6 16.5 38.1 Residential mortgage-backed securities 2.8 0.2 23.8 1.9 Agency residential pass-through obligations 0 0 0 0.1 Commercial mortgage-backed securities 0 5.5 2.4 12.0 Other asset-backed securities 0 0 0.3 0 Redeemable preferred stocks 7.7 0 8.0 0 Total fixed maturities 20.5 28.3 63.9 92.2 Equity securities: Nonredeemable preferred stocks 3.0 4.9 54.6 11.9 Common equities 5.7 14.4 23.0 43.3 Short-term investments 0 0.1 0 0.1 Subtotal gross realized gains on security sales 29.2 47.7 141.5 147.5 Gross realized losses on security sales Fixed maturities: U.S. government obligations (1.0 ) (0.8 ) (4.6 ) (1.2 ) State and local government obligations 0 0 (0.1 ) (1.6 ) Corporate and other debt securities (1.8 ) (0.2 ) (4.6 ) (1.9 ) Residential mortgage-backed securities (0.1 ) 0 (0.4 ) 0 Agency residential pass-through obligations 0 0 0 (0.2 ) Commercial mortgage-backed securities (0.5 ) 0 (3.6 ) (4.1 ) Redeemable preferred stocks (6.4 ) (6.5 ) (6.4 ) (6.5 ) Total fixed maturities (9.8 ) (7.5 ) (19.7 ) (15.5 ) Equity securities: Nonredeemable preferred stocks (0.1 ) (0.4 ) (5.9 ) (3.1 ) Common equities (0.2 ) (0.3 ) (0.3 ) (5.3 ) Subtotal gross realized losses on security sales (10.1 ) (8.2 ) (25.9 ) (23.9 ) Net realized gains (losses) on security sales Fixed maturities: U.S. government obligations (0.1 ) 5.6 1.2 22.9 State and local government obligations 4.0 0.6 7.0 14.4 Corporate and other debt securities 3.3 15.4 11.9 36.2 Residential mortgage-backed securities 2.7 0.2 23.4 1.9 Agency residential pass-through obligations 0 0 0 (0.1 ) Commercial mortgage-backed securities (0.5 ) 5.5 (1.2 ) 7.9 Other asset-backed securities 0 0 0.3 0 Redeemable preferred stocks 1.3 (6.5 ) 1.6 (6.5 ) Total fixed maturities 10.7 20.8 44.2 76.7 Equity securities: Nonredeemable preferred stocks 2.9 4.5 48.7 8.8 Common equities 5.5 14.1 22.7 38.0 Short-term investments 0 0.1 0 0.1 Subtotal net realized gains (losses) on security sales 19.1 39.5 115.6 123.6 Other-than-temporary impairment losses Fixed maturities: Commercial mortgage-backed securities (0.4 ) 0 (0.4 ) 0 Redeemable preferred stocks 0 (25.4 ) 0 (25.4 ) Total fixed maturities (0.4 ) (25.4 ) (0.4 ) (25.4 ) Equity securities: Common equities (8.9 ) (1.4 ) (12.5 ) (1.6 ) Subtotal investment other-than-temporary impairment losses (9.3 ) (26.8 ) (12.9 ) (27.0 ) Other asset impairment (33.7 ) (34.8 ) (44.9 ) (34.8 ) Subtotal other-than-temporary impairment losses (43.0 ) (61.6 ) (57.8 ) (61.8 ) Other gains (losses) Hybrid securities (0.9 ) 0.6 0.3 2.9 Derivative instruments 0 0.7 0 (35.8 ) Litigation settlements 0.1 0.1 1.2 0.1 Subtotal other gains (losses) (0.8 ) 1.4 1.5 (32.8 ) Total net realized gains (losses) on securities $ (24.7 ) $ (20.7 ) $ 59.3 $ 29.0 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. Our preferred stock portfolio reflects a large realized gain due primarily to one issue called by the issuer at par. This security was held at a deep discount due to previous other-than-temporary impairment write-downs taken during the market crisis of 2008. Subsequent to the write-down, the security experienced significant recovery and was trading near its anticipated call price. Upon call, we recognized the difference between the consideration received and our book value as a realized gain. 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 three and nine months ended September 30, were: Three Months Nine Months (millions) 2017 2016 2017 2016 Fixed maturities: U.S. government obligations $ 18.4 $ 3.3 $ 48.1 $ 12.4 State and local government obligations 12.9 12.6 39.3 39.3 Foreign government obligations 0.1 0.1 0.3 0.3 Corporate debt securities 32.2 27.3 93.1 81.9 Residential mortgage-backed securities 7.8 11.1 27.3 34.9 Agency residential pass-through obligations 0.2 0.3 0.6 0.9 Commercial mortgage-backed securities 20.0 22.4 57.1 62.6 Other asset-backed securities 12.1 6.8 33.8 18.7 Redeemable preferred stocks 2.8 3.8 9.1 11.5 Total fixed maturities 106.5 87.7 308.7 262.5 Equity securities: Nonredeemable preferred stocks 10.9 12.4 33.0 36.8 Common equities 14.9 13.9 42.8 42.0 Short-term investments 10.6 5.3 26.4 11.4 Investment income 142.9 119.3 410.9 352.7 Investment expenses (5.8 ) (4.8 ) (18.0 ) (14.9 ) Net investment income $ 137.1 $ 114.5 $ 392.9 $ 337.8 The amount of investment income (interest and dividends) we recognize varies from year to year based on the average assets held during the year and the book yields of the securities in our portfolio. The increase in income year-over-year for the three and nine months was mainly the result of an increase in assets, as yields remained relatively consistent. Trading Securities At September 30, 2017 and 2016 , and December 31, 2016 , we did not hold any trading securities and did not have any net realized gains (losses) on trading securities for the three and nine months ended September 30, 2017 and 2016 . Derivative Instruments The following table shows the status of our derivative instruments at September 30, 2017 and 2016 , and December 31, 2016 , and for the three and nine months ended September 30, 2017 and 2016 : (millions) Balance Sheet 2 Comprehensive Income Statement Assets (Liabilities) Pretax Net Realized Gains (Losses) Notional Value 1 Three Months Ended Nine Months Ended September 30, Dec. 31, September 30, Dec. 31, September 30, September 30, Derivatives designated as: 2017 2016 2016 Purpose Classification 2017 2016 2016 2017 2016 2017 2016 Hedging instrument Closed: Ineffective cash flow hedge $ 31 $ 370 $ 370 Manage interest rate risk NA $ 0 $ 0 $ 0 $ 0 $ (1.4 ) $ 0 $ (1.3 ) Non-hedging instruments Liabilities: Interest rate swaps 0 315 0 Manage portfolio duration Other liabilities 0 (13.0 ) 0 0 1.7 0 (17.8 ) Closed: Interest rate swaps 0 435 750 Manage portfolio duration NA 0 0 0 0 0.3 0 (17.0 ) U.S. Treasury Note futures 0 135 135 Manage NA 0 0 0 0 0.1 0 0.3 Total NA NA NA $ 0 $ (13.0 ) $ 0 $ 0 $ 0.7 $ 0 $ (35.8 ) NA= Not applicable 1 The amounts represent the value held at quarter and year end for open positions and the maximum amount held during the period for closed positions. 2 To the extent we hold both derivative assets and liabilities with the same counterparty that are subject to an enforceable master netting arrangement, we reported them on a gross basis on our balance sheets, consistent with our historical presentation. 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 and the remaining $20 million in 2016 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. 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 September 30, 2017 or December 31, 2016 . At September 30, 2016 , 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 September 30, 2016 , the balance of the cash collateral that we delivered to the applicable counterparties on the then open interest rate swaps was $15.1 million . We did not open any U.S. treasury futures during 2017 . During 2016 , we opened and closed treasury futures; no positions were outstanding at the end of the third quarter or year end.

Fair Value

Fair Value	9 Months Ended
	Sep. 30, 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, 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 September 30, 2017 Fixed maturities: U.S. government obligations $ 4,595.0 $ 0 $ 0 $ 4,595.0 $ 4,612.2 State and local government obligations 0 2,364.2 0 2,364.2 2,332.2 Foreign government obligations 24.2 0 0 24.2 24.2 Corporate debt securities 0 5,225.6 0 5,225.6 5,195.7 Subtotal 4,619.2 7,589.8 0 12,209.0 12,164.3 Asset-backed securities: Residential mortgage-backed 0 920.3 0 920.3 911.3 Agency residential pass-through obligations 0 35.3 0 35.3 35.7 Commercial mortgage-backed 0 2,767.9 0 2,767.9 2,763.7 Other asset-backed 0 2,490.2 0 2,490.2 2,485.6 Subtotal asset-backed securities 0 6,213.7 0 6,213.7 6,196.3 Redeemable preferred stocks: Financials 0 63.8 0 63.8 61.4 Utilities 0 31.9 0 31.9 30.5 Industrials 0 141.6 0 141.6 130.6 Subtotal redeemable preferred stocks 0 237.3 0 237.3 222.5 Total fixed maturities 4,619.2 14,040.8 0 18,660.0 18,583.1 Equity securities: Nonredeemable preferred stocks: Financials 82.2 726.5 0 808.7 695.6 Industrials 0 0 5.0 5.0 5.0 Subtotal nonredeemable preferred stocks 82.2 726.5 5.0 813.7 700.6 Common equities: Common stocks 3,209.2 0 0 3,209.2 1,485.2 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 3,209.2 0 0.3 3,209.5 1,485.5 Total fixed maturities and equity securities 7,910.6 14,767.3 5.3 22,683.2 20,769.2 Short-term investments 3,175.4 1,136.1 0 4,311.5 4,311.5 Total portfolio $ 11,086.0 $ 15,903.4 $ 5.3 $ 26,994.7 $ 25,080.7 Debt $ 0 $ 3,574.6 $ 43.3 $ 3,617.9 $ 3,312.2 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost September 30, 2016 Fixed maturities: U.S. government obligations $ 734.6 $ 0 $ 0 $ 734.6 $ 726.0 State and local government obligations 0 2,566.0 0 2,566.0 2,508.9 Foreign government obligations 25.4 0 0 25.4 25.4 Corporate debt securities 0 4,403.3 0 4,403.3 4,327.5 Subtotal 760.0 6,969.3 0 7,729.3 7,587.8 Asset-backed securities: Residential mortgage-backed 0 1,594.2 0 1,594.2 1,582.6 Agency residential pass-through obligations 0 44.1 0 44.1 43.6 Commercial mortgage-backed 0 2,277.8 0.6 2,278.4 2,249.4 Other asset-backed 0 2,045.4 0 2,045.4 2,037.5 Subtotal asset-backed securities 0 5,961.5 0.6 5,962.1 5,913.1 Redeemable preferred stocks: Financials 0 95.7 0 95.7 77.1 Utilities 0 30.4 0 30.4 30.6 Industrials 0 99.4 0 99.4 99.4 Subtotal redeemable preferred stocks 0 225.5 0 225.5 207.1 Total fixed maturities 760.0 13,156.3 0.6 13,916.9 13,708.0 Equity securities: Nonredeemable preferred stocks: Financials 154.6 705.5 0 860.1 726.6 Industrials 0 0 0 0 0 Subtotal nonredeemable preferred stocks 154.6 705.5 0 860.1 726.6 Common equities: Common stocks 2,867.6 0 0 2,867.6 1,575.7 Other risk investments 0 0 0.4 0.4 0.4 Subtotal common equities 2,867.6 0 0.4 2,868.0 1,576.1 Total fixed maturities and equity securities 3,782.2 13,861.8 1.0 17,645.0 16,010.7 Short-term investments 5,051.5 824.7 0 5,876.2 5,876.2 Total portfolio $ 8,833.7 $ 14,686.5 $ 1.0 $ 23,521.2 $ 21,886.9 Debt $ 0 $ 3,371.6 $ 133.6 $ 3,505.2 $ 3,153.9 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 1,458.7 0 1,458.7 1,448.5 Agency residential pass-through obligations 0 40.6 0 40.6 41.2 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. We did not have any transfers between Level 1 and Level 2 during 2017 or 2016 . We recognize transfers between levels at the end of the reporting period. Our short-term security holdings classified as Level 1 are highly liquid, actively marketed, and have a very short duration, primarily 30 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 September 30, 2017 , vendor-quoted prices represented 59% of our Level 1 classifications (excluding short-term investments), compared to 24% and 52% at September 30, 2016 and December 31, 2016 , respectively. 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 September 30, 2017 and 2016 , and December 31, 2016 , vendor-quoted prices comprised 98% , 99% , and 99% , respectively, of our Level 2 classifications (excluding short-term investments), while dealer-quoted prices represented 2% , 1% , and 1% , 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. When necessary, we 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 receive 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 prices to previous market valuation prices. 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 the 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 activity, 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 September 30, 2017 and 2016 , and December 31, 2016 , securities in our fixed-maturity portfolio listed as Level 3 were comprised substantially of securities that were either: (i) private placements, (ii) thinly held and/or traded securities, or (iii) non-investment-grade or non-rated securities with little liquidity. Based on these factors, it was difficult to independently verify observable market inputs that were used to generate the external valuations we received. Despite the lack of sufficient observable market information for our Level 3 securities, we believe the valuations received in conjunction with our procedures for evaluating third-party prices support the fair values reported in the financial statements. At September 30, 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 September 30, 2017 equals the cost at acquisition. We did not hold any internally-priced securities at September 30, 2016 or 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 prices received from external sources remained unadjusted. The following tables provide a summary of changes in fair value associated with Level 3 assets for the three and nine months ended September 30, 2017 and 2016 : Level 3 Fair Value Three Months Ended September 30, 2017 (millions) Fair Value at June 30, 2017 Calls/ Maturities/ Paydowns Purchases Sales Net Realized (Gain) Loss on Sales Change in Valuation Net Transfers In (Out) Fair Value at September 30, 2017 Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 Total fixed maturities 0 0 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 0.3 0 0 0 0 0 0 0.3 Total Level 3 securities $ 0.3 $ 0 $ 5.0 $ 0 $ 0 $ 0 $ 0 $ 5.3 Level 3 Fair Value Nine months ended September 30, 2017 (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 September 30, 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 Three Months Ended September 30, 2016 (millions) Fair Value at June 30, 2016 Calls/ Purchases Sales Net Realized (Gain) Loss on Sales Change in Net Fair Value at September 30, 2016 Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 9.2 $ (8.7 ) $ 0 $ 0 $ 0 $ 0.1 $ 0 $ 0.6 Total fixed maturities 9.2 (8.7 ) 0 0 0 0.1 0 0.6 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 $ 9.5 $ (8.7 ) $ 0 $ 0 $ 0 $ 0.2 $ 0 $ 1.0 Level 3 Fair Value Nine months ended September 30, 2016 (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 September 30, 2016 Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 9.9 $ (9.3 ) $ 0 $ 0 $ 0 $ 0 $ 0 $ 0.6 Total fixed maturities 9.9 (9.3 ) 0 0 0 0 0 0.6 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 $ (9.3 ) $ 0 $ 0 $ 0 $ 0.1 $ 0 $ 1.0 The following tables provide a summary of the quantitative information about Level 3 fair value measurements for our applicable securities at September 30, 2017 and 2016 , and December 31, 2016 : Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at September 30, 2017 Valuation Technique Unobservable Input Unobservable 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 Purchase price 3.9 Subtotal Level 3 securities 5.0 Pricing exemption securities 2 0.3 Total Level 3 securities $ 5.3 NA= Not applicable, since we did not hold any commercial mortgage-backed Level 3 securities at September 30, 2017 . 1 The security was internally-priced since it is privately held and it was valued at September 30, 2017 using the purchase price. 2 The fair values for these securities were determined with unobservable inputs not reasonably available to us. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at September 30, 2016 Valuation Technique Unobservable Input Unobservable Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 0.6 External vendor Prepayment rate 1 0 % Total fixed maturities 0.6 Equity securities: Nonredeemable preferred stocks: Industrials 0 NA NA NA Subtotal Level 3 securities 0.6 Pricing exemption securities 2 0.4 Total Level 3 securities $ 1.0 NA= Not applicable, since we did not hold any nonredeemable preferred stock Level 3 securities at September 30, 2016 . 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 determined with unobservable inputs 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 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 Pricing exemption securities 2 0.4 Total Level 3 securities $ 0.7 NA= Not applicable, since we did not hold any nonredeemable preferred stock Level 3 securities at December 31, 2016 . 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 determined with unobservable inputs 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	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Debt	Debt — Debt at each of the balance sheet periods consisted of: September 30, 2017 September 30, 2016 December 31, 2016 (millions) Carrying Value Fair Value Carrying Value Fair Value Carrying Value Fair Value 3.75% Senior Notes due 2021 $ 498.7 $ 525.7 $ 498.4 $ 544.2 $ 498.4 $ 528.8 2.45% Senior Notes due 2027 496.0 475.3 495.6 497.5 495.8 464.6 6 5/8% Senior Notes due 2029 296.0 388.3 295.8 402.6 295.9 380.1 6.25% Senior Notes due 2032 395.3 518.7 395.1 533.3 395.2 499.0 4.35% Senior Notes due 2044 346.5 379.8 346.4 403.0 346.4 362.3 3.70% Senior Notes due 2045 395.2 392.6 395.1 418.7 395.1 372.5 4.125% Senior Notes due 2047 841.2 894.2 0 0 0 0 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 0 0 593.9 572.3 594.1 581.2 Other debt instruments 43.3 43.3 133.6 133.6 127.3 127.3 Total $ 3,312.2 $ 3,617.9 $ 3,153.9 $ 3,505.2 $ 3,148.2 $ 3,315.8 The other debt instruments reported in the table above represent ARX indebtedness and consist of: September 30, 2017 September 30, 2016 December 31, 2016 ($ in millions) Type of debt instrument Number of Instruments Carrying Value Number of Instruments Carrying Number of Instruments Carrying Stated Maturity Date(s) Term loans 2 $ 43.3 2 $ 68.4 2 $ 62.1 December 2018 and 2019 Junior subordinated notes 1 0 0 2 41.2 2 41.2 June 2036 and 2037 Senior notes 1 0 0 4 24.0 4 24.0 Various 2 Total $ 43.3 $ 133.6 $ 127.3 1 The notes were redeemed during the third quarter 2017 (discussed below). 2 The senior notes original maturity dates were May 2033, April 2034, December 2034, and June 2035. The Progressive Corporation Debt During the second quarter of 2017, we issued $850 million of 4.125% Senior Notes due 2047 (the “ 4.125% Senior Notes”) in an underwritten public offering. We received proceeds, after deducting underwriter’s discounts, commissions and other issuance costs, of approximately $841.1 million . In addition, upon issuance of the 4.125% Senior Notes, we closed a forecasted debt issuance hedge, which was entered into to hedge against a possible rise in interest rates, and recognized an $8.0 million pretax loss as part of accumulated other comprehensive income (loss); the loss will be recognized as an adjustment to interest expense and amortized over the life of the 4.125% Senior Notes. During the second quarter 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 the first quarter of 2017 and the full year 2016, 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 the first quarter 2017 and $1.6 million in 2016. During the third quarter 2016, we issued $500 million of our 2.45% Senior Notes due 2027 in an underwritten public offering. We received proceeds, after deducting underwriter’s discounts, commissions, and other issuance costs, of approximately $495.6 million . Consistent with the other senior notes issued by Progressive, interest on the 4.125% Senior Notes and the 2.45% Senior Notes is payable semiannually and both notes are redeemable, in whole or in part, at any time. 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 substantially equal to their carrying values, based on the current rates offered for debt of similar maturities and interest rates. During the third quarter 2017, ARX redeemed their 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 was eliminated in consolidation. The Progressive Corporation Line of Credit During the second quarter 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 the second quarter 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. All advances under the line of credit are subject to PNC’s discretion. We had no borrowings under either line of credit during the first nine months of 2017 or throughout 2016.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes — At September 30, 2017 and 2016 , and December 31, 2016 , we determined that we did not need a valuation allowance on our gross deferred tax assets. 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. For the nine months ended September 30, 2017 , there have been no material changes in our uncertain tax positions. The effective tax rates for the three and nine months ended September 30, 2017, were 14.6% and 29.7% , respectively, compared to 20.5% and 29.4% for the same periods in 2016. On January 1, 2017, we began recording the excess tax benefits from stock-based compensation to the tax provision in accordance with the adoption of the new accounting standard update (see Note 14 – New Accounting Standards for additional discussion). Previously these benefits were recorded directly to paid-in capital. In addition, during the third quarter of both 2017 and 2016, we recorded the ratable portion of the tax benefits related to federal renewable energy tax credit fund investments that were entered into during the periods.

Loss and Loss Adjustment Expens

Loss and Loss Adjustment Expense Reserves	9 Months Ended
	Sep. 30, 2017
Loss and Loss Adjustment Expenses Reserves [Abstract]
Loss and Loss Adjustment Expenses Reserves	Loss and Loss Adjustment Expense Reserves — Activity in the loss and loss adjustment expense reserves during the nine month periods is summarized as follows: September 30, (millions) 2017 2016 Balance, Beginning of period $ 11,368.0 $ 10,039.0 Less reinsurance recoverables on unpaid losses 1,801.0 1,442.7 Net balance, Beginning of period 9,567.0 8,596.3 Net loss and loss adjustment expense reserves disposed 1 0 (2.5 ) Total beginning reserves 9,567.0 8,593.8 Incurred related to: Current year 13,886.3 12,577.5 Prior years 42.5 (22.9 ) Total incurred 13,928.8 12,554.6 Paid related to: Current year 8,379.4 7,768.2 Prior years 4,387.8 3,986.2 Total paid 12,767.2 11,754.4 Net balance, End of period 10,728.6 9,394.0 Plus reinsurance recoverables on unpaid losses 2,624.7 1,834.2 Balance, End of period $ 13,353.3 $ 11,228.2 1 During 2016, $2.5 million net reserves were disposed by ARX in an exchange transaction. We experienced unfavorable reserve development of $42.5 million and favorable reserve development of $22.9 million for the first nine months of 2017 and 2016, respectively, which is reflected as “Incurred related to prior years ” in the table above. Year-to-date 2017 • Approximately $51 million of unfavorable prior year reserve development was attributable to accident years 2016 and 2015. This unfavorable development was partially offset by $8 million of favorable development attributable to accident year 2014 and prior accident years. • Our personal auto businesses incurred $76 million of unfavorable loss and loss adjustment expense (LAE) reserve development for the first nine months of 2017, primarily in the Agency business, in part reflecting an increase in costs related to property damage and higher LAE costs. • Our Property business experienced $24 million in favorable development primarily due to the identification of prior year losses eligible to be ceded under our catastrophe bond reinsurance program and lower severity and frequency than anticipated for accident year 2016. • The remaining favorable development for the first nine months was attributable to both our special lines and commercial auto products. Year-to-date 2016 • Approximately $30 million of the favorable prior year reserve development was attributable to accident year 2015, partially offset by $26 million of unfavorable development attributable to accident year 2014; we had favorable development for 2013 and prior accident years. • Our Personal Lines and Property businesses incurred $9 million and $46 million , respectively, of favorable loss and LAE reserve development for the first nine months of 2016, partially offset by the unfavorable loss and LAE reserve development in our Commercial Lines business of $31 million . In our Property business, both the severity and frequency of late reported claims was less than anticipated. • Our personal auto product favorable development was in our Direct auto businesses. • 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, driven in part by 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.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	9 Months Ended
	Sep. 30, 2017
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	Supplemental Cash Flow Information — Cash includes only bank demand deposits. We paid the following in the respective periods: Nine Months Ended September 30, (millions) 2017 2016 Income taxes $ 538.7 $ 380.8 Interest 107.4 98.0 Restricted cash on our consolidated balance sheets 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. 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 decrease in cash that was reported in our consolidated statement of cash flows for the nine months ended September 30, 2016.

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2017
Segment Reporting [Abstract]
Segment Information	Segment Information — Our Personal Lines segment writes insurance for personal autos and recreational vehicles (our special lines products). 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. Our Property segment writes residential property insurance for homeowners, other property owners, and renters. Our other indemnity businesses manage our run-off businesses. Our service businesses provide insurance-related services, including processing Commercial Automobile Insurance Procedures/Plans (CAIP) business and serving as an agent for homeowners, general liability, and workers’ compensation insurance through our programs with ASI and unaffiliated insurance companies. All segment revenues are generated from external customers; all intercompany transactions, including those between Progressive and ASI, are eliminated in consolidation. Following are the operating results for the respective periods: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 (millions) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Personal Lines Agency $ 2,840.0 $ 69.5 $ 2,474.3 $ 98.6 $ 8,224.0 $ 524.7 $ 7,245.5 $ 354.5 Direct 2,734.8 128.7 2,391.2 84.4 7,908.5 466.7 6,946.7 237.5 Total Personal Lines 1 5,574.8 198.2 4,865.5 183.0 16,132.5 991.4 14,192.2 592.0 Commercial Lines 714.0 42.8 630.2 4.4 2,031.2 166.4 1,772.4 96.7 Property 2 255.2 (69.0 ) 227.7 10.4 720.3 (57.5 ) 638.0 (27.7 ) Other indemnity 0 0 0 (0.8 ) 0 (0.3 ) 0 (1.6 ) Total underwriting operations 6,544.0 172.0 5,723.4 197.0 18,884.0 1,100.0 16,602.6 659.4 Fees and other revenues 3 96.3 NA 86.8 NA 270.3 NA 248.2 NA Service businesses 33.3 4.4 26.2 3.0 94.5 12.7 77.7 9.2 Investments 4 118.2 112.4 98.6 93.8 470.2 452.2 381.7 366.8 Gains on extinguishment of debt 0 0 0 0 0.2 0.2 1.6 1.6 Interest expense NA (37.4 ) NA (35.3 ) NA (117.6 ) NA (103.8 ) Consolidated total $ 6,791.8 $ 251.4 $ 5,935.0 $ 258.5 $ 19,719.2 $ 1,447.5 $ 17,311.8 $ 933.2 NA = Not applicable 1 Personal auto insurance accounted for 93% of the total Personal Lines segment net premiums earned in the three and nine months ended September 30, 2017 , and 92% for the same periods in 2016 ; insurance for our special lines products (e.g., motorcycles, watercraft, and RVs) accounted for the balance of the Personal Lines net premiums earned. 2 For the three and nine months ended September 30, 2017 , pretax profit (loss) includes $17.2 million and $48.2 million , respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX and $15.5 million and $46.6 million for the same periods in 2016. 3 Pretax profit (loss) for fees and other revenues are attributable to operating segments. 4 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 and combined ratios for our underwriting operations for the respective periods: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Under-writing Combined Ratio Under-writing Combined Ratio Under-writing Margin Combined Ratio Under-writing Margin Combined Ratio Personal Lines Agency 2.4 % 97.6 4.0 % 96.0 6.4 % 93.6 4.9 % 95.1 Direct 4.7 95.3 3.5 96.5 5.9 94.1 3.4 96.6 Total Personal Lines 3.6 96.4 3.8 96.2 6.1 93.9 4.2 95.8 Commercial Lines 6.0 94.0 0.7 99.3 8.2 91.8 5.5 94.5 Property 1 (27.0 ) 127.0 4.6 95.4 (8.0 ) 108.0 (4.3 ) 104.3 Other indemnity 2 NM NM NM NM NM NM NM NM Total underwriting operations 2.6 97.4 3.4 96.6 5.8 94.2 4.0 96.0 1 Included in both the three and nine months ended September 30, 2017 are 6.7 points of amortization expense predominately associated with the acquisition of a controlling interest in ARX and 6.8 points and 7.3 points, respectively, for the three and nine months ended September 30, 2016. The nine months ended September 30, 2016, also include 0.7 points of expense related to the loss on the exchange transaction that occurred in 2016. 2 Underwriting margins and combined ratios are not meaningful (NM) for our other indemnity businesses due to the low level of premiums earned by, and the variability of loss costs in, such businesses.

Dividends

Dividends	9 Months Ended
	Sep. 30, 2017
Text Block [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 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 2016 target percentage. 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 bonus program currently in place for our employees (our “Gainsharing program”). On a year-to-date basis, as of September 30, 2017 , the Gainshare factor was 1.67 . Since the final factor will be determined based on our results for the full year, the final factor may vary from the current factor. 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 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 two years: (millions, except per share amounts) Amount Dividend Type Declared Paid Per Share Total 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.

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	9 Months Ended
	Sep. 30, 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, were as follows: Components of Changes in (millions) Pretax total Total tax After tax total Total net Net Foreign (Income)loss attributable to NCI Balance at June 30, 2017 $ 1,774.1 $ (623.4 ) $ 1,150.7 $ 1,164.6 $ (15.1 ) $ (0.9 ) $ 2.1 Other comprehensive income (loss) before reclassifications: Investment securities 122.7 (43.0 ) 79.7 79.7 0 0 0 Net non-credit related OTTI losses, adjusted for valuation changes 0 0 0 0 0 0 0 Forecasted transactions 0 0 0 0 0 0 0 Foreign currency translation adjustment 0.9 (0.3 ) 0.6 0 0 0.6 0 Loss attributable to noncontrolling interest (NCI) (1.1 ) 0.4 (0.7 ) 0 0 0 (0.7 ) Total other comprehensive income (loss) before reclassifications 122.5 (42.9 ) 79.6 79.7 0 0.6 (0.7 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (9.3 ) 3.3 (6.0 ) (6.0 ) 0 0 0 Net realized gains (losses) on securities 15.7 (5.5 ) 10.2 10.2 0 0 0 Interest expense (0.2 ) 0.1 (0.1 ) 0 (0.1 ) 0 0 Total reclassification adjustment for amounts realized in net income 6.2 (2.1 ) 4.1 4.2 (0.1 ) 0 0 Total other comprehensive income (loss) 116.3 (40.8 ) 75.5 75.5 0.1 0.6 (0.7 ) Balance at September 30, 2017 $ 1,890.4 $ (664.2 ) $ 1,226.2 $ 1,240.1 $ (15.0 ) $ (0.3 ) $ 1.4 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 (Income)loss attributable to NCI Balance 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 554.4 (194.3 ) 360.1 360.1 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 1.2 (0.4 ) 0.8 0 0 0.8 0 Loss attributable to noncontrolling interest (NCI) (4.5 ) 1.6 (2.9 ) 0 0 0 (2.9 ) Total other comprehensive income (loss) before reclassifications 543.1 (190.3 ) 352.8 360.1 (5.2 ) 0.8 (2.9 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (12.9 ) 4.6 (8.3 ) (8.3 ) 0 0 0 Net realized gains (losses) on securities 104.5 (36.6 ) 67.9 67.9 0 0 0 Interest expense 0.6 (0.2 ) 0.4 0 0.4 0 0 Total reclassification adjustment for amounts realized in net income 92.2 (32.2 ) 60.0 59.6 0.4 0 0 Total other comprehensive income (loss) 450.9 (158.1 ) 292.8 300.5 (5.6 ) 0.8 (2.9 ) Balance at September 30, 2017 $ 1,890.4 $ (664.2 ) $ 1,226.2 $ 1,240.1 $ (15.0 ) $ (0.3 ) $ 1.4 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 Net unrealized gains on forecasted transactions Foreign currency translation adjustment (Income)loss attributable to NCI Balance at June 30, 2016 $ 1,477.0 $ (519.2 ) $ 957.8 $ 970.1 $ (8.8 ) $ (1.1 ) $ (2.4 ) Other comprehensive income (loss) before reclassifications: Investment securities 146.8 (51.4 ) 95.4 95.4 0 0 0 Net non-credit related OTTI losses, adjusted for valuation changes 0 0 0 0 0 0 0 Forecasted transactions 0 0 0 0 0 0 0 Foreign currency translation adjustment 0.4 (0.1 ) 0.3 0 0 0.3 0 Loss attributable to noncontrolling interest (NCI) 1.8 (0.6 ) 1.2 0 0 0 1.2 Total other comprehensive income (loss) before reclassifications 149.0 (52.1 ) 96.9 95.4 0 0.3 1.2 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (26.8 ) 9.4 (17.4 ) (17.4 ) 0 0 0 Net realized gains (losses) on securities 39.6 (13.9 ) 25.7 25.7 0 0 0 Interest expense 0.5 (0.2 ) 0.3 0 0.3 0 0 Total reclassification adjustment for amounts realized in net income 13.3 (4.7 ) 8.6 8.3 0.3 0 0 Total other comprehensive income (loss) 135.7 (47.4 ) 88.3 87.1 (0.3 ) 0.3 1.2 Balance at September 30, 2016 $ 1,612.7 $ (566.6 ) $ 1,046.1 $ 1,057.2 $ (9.1 ) $ (0.8 ) $ (1.2 ) 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 Net unrealized gains on forecasted transactions Foreign currency translation adjustment (Income)loss attributable to NCI Balance 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 470.4 (165.2 ) 305.2 305.2 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 1.0 (0.3 ) 0.7 0 0 0.7 0 Loss attributable to noncontrolling interest (NCI) (3.7 ) 1.4 (2.3 ) 0 0 0 (2.3 ) Total other comprehensive income (loss) before reclassifications 467.6 (164.0 ) 303.6 305.2 0 0.7 (2.3 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (27.0 ) 9.5 (17.5 ) (17.5 ) 0 0 0 Net realized gains (losses) on securities 114.9 (40.4 ) 74.5 74.5 0 0 0 Interest expense 1.5 (0.6 ) 0.9 0 0.9 0 0 Total reclassification adjustment for amounts realized in net income 89.4 (31.5 ) 57.9 57.0 0.9 0 0 Total other comprehensive income (loss) 378.2 (132.5 ) 245.7 248.2 (0.9 ) 0.7 (2.3 ) Balance at September 30, 2016 $ 1,612.7 $ (566.6 ) $ 1,046.1 $ 1,057.2 $ (9.1 ) $ (0.8 ) $ (1.2 ) In an effort to manage interest rate risk, we entered into forecasted transactions on each of Progressive’s outstanding debt issuances. Upon issuing the debt, the gains (losses) recognized on the effective cash flow hedges are recorded as unrealized gains (losses) in accumulated other comprehensive income and amortized into interest expense over the term of the related debt issuance. We expect to reclassify $1.0 million (pretax) into interest expense during the next 12 months, related to net unrealized losses on forecasted transactions. For the three and nine months ended September 30, 2016, the net impairment losses recognized in earnings were revised to reflect a decrease in a reclassification adjustment rather than an increase as previously disclosed. The offset was to other comprehensive income before reclassifications-investment securities.The impacts to classification within the table are immaterial. Overall, this revision had no impact on the total other comprehensive income (loss) reported for these periods.

Litigation

Litigation	9 Months Ended
	Sep. 30, 2017
Commitments and Contingencies Disclosure [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 subsidiaries are named as defendants in a number of class action or individual lawsuits that challenge certain of the operations of the insurance subsidiaries. These cases include those alleging damages as a result of our subsidiaries’ practices in evaluating or paying medical or injury claims or benefits, including, but not limited to, personal injury protection, medical payments, and bodily injury benefits; the utilization, content, or appearance of policy documents; labor rates paid to auto body repair shops; wage and hour issues; and cases challenging other aspects of our subsidiaries’ claims or marketing practices or other business operations. Other insurance companies face many of these same issues. The nature and volume of litigation to which The Progressive Corporation is subject is similar to that which was disclosed in Note 12 – Litigation in our 2016 Annual Report to Shareholders. We plan to contest the pending lawsuits vigorously, but may pursue settlement negotiations in some cases, if appropriate. The outcomes of pending cases are uncertain at this time. We establish accruals for these lawsuits when it is probable that a loss has been or will be incurred and we can reasonably estimate potential loss exposure, which may include a range of loss. As to lawsuits for which the loss is considered neither probable or estimable, or 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. With respect to our pending lawsuits that are not related to claims under insurance policies, the accruals that we have established, if any, were not material at September 30, 2017 or 2016, and there were no material settlements during the first nine months of 2017 or 2016. For most of these lawsuits, we do not consider any losses to be both probable and estimable, and we are unable to estimate a meaningful range of loss, if any, at this time, due to the factors discussed in Note 12 – Litigation in our 2016 Annual Report to Shareholders. In the event that any one or more of these lawsuits results in a substantial judgment against or settlement by us, or if our accruals (if any) prove to be inadequate by a significant amount, the resulting liability could have a material adverse effect on our consolidated financial condition, cash flows, and/or results of operations. For a further discussion on our pending litigation and related reserving policies, see Note 12 – Litigation in our 2016 Annual Report to Shareholders.

Redeemable Noncontrolling Inter

Redeemable Noncontrolling Interest	9 Months Ended
	Sep. 30, 2017
Temporary Equity Disclosure [Abstract]
Redeemable Noncontrolling Interest	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. See Note 15 – Redeemable Noncontrolling Interest in our 2016 Annual Report to Shareholders for a discussion of the purchase prices for shares to be purchased by Progressive pursuant to these put or call rights. 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. In addition to these minority shares, at September 30, 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 $32.2 million is not recorded as part of redeemable NCI. The changes in the components of redeemable NCI during the nine months ended September 30, 2017 and 2016 , and the year ended December 31, 2016 , were: (millions) September 30, 2017 September 30, 2016 December 31, 2016 Balance, Beginning of period $ 483.7 $ 464.9 $ 464.9 Net income attributable to NCI 1.9 11.3 26.2 Other comprehensive income (loss) attributable to NCI 2.9 2.3 (3.2 ) Exercise of employee stock options 3.4 0 0 Change in redemption value of NCI 6.3 (6.0 ) (4.2 ) Balance, End of period $ 498.2 $ 472.5 $ 483.7

Goodwill and Intangible Assets

Goodwill and Intangible Assets	9 Months Ended
	Sep. 30, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	Goodwill and Intangible Assets Goodwill The carrying amount of goodwill increased $3.3 million as a result of the acquisition of a small excess and surplus lines insurance company during the second quarter 2017, which will provide us flexibility in our Commercial Lines business going forward. Goodwill recorded at September 30, 2017 , was $452.7 million . No accumulated goodwill impairment losses exist. Intangible Assets The following table is a summary of the net carrying amount of other intangible assets as of September 30, 2017 and 2016 , and December 31, 2016 : (millions) September 30, 2017 September 30, 2016 December 31, 2016 Intangible assets subject to amortization $ 372.2 $ 435.9 $ 420.4 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 384.6 $ 448.3 $ 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 all periods presented. Intangible assets subject to amortization consisted of the following: (millions) September 30, 2017 September 30, 2016 December 31, 2016 Category Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Policies in force $ 256.2 $ 91.5 $ 164.7 $ 256.2 $ 55.0 $ 201.2 $ 256.2 $ 64.1 $ 192.1 Agency relationships 159.2 28.4 130.8 159.2 17.1 142.1 159.2 19.9 139.3 Software rights 79.1 26.8 52.3 79.1 16.1 63.0 79.1 18.8 60.3 Trade name 34.8 10.4 24.4 34.8 5.2 29.6 34.8 6.1 28.7 Total $ 529.3 $ 157.1 $ 372.2 $ 529.3 $ 93.4 $ 435.9 $ 529.3 $ 108.9 $ 420.4 Amortization expense was $17.2 million and $48.2 million for the three and nine months ended September 30, 2017 , respectively, compared to $15.5 million and $46.6 million during the same period last year. During the third quarter 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 September 30, 2017, the remaining average life of all of our intangible assets is 5.2 years .

New Accounting Standards

New Accounting Standards	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
New Accounting Standards	New Accounting Standards Issued In March 2017, the Financial Accounting Standards Boards (FASB) issued an accounting standards update (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, 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. Adopted 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 first nine months of 2017, we recognized $23.1 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 (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2017
Investments Schedule [Abstract]
Schedule of Available-for-sale Securities Reconciliation [Table Text Block]	The following tables present the composition of our investment portfolio by major security type, consistent with our 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. ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value September 30, 2017 Fixed maturities: U.S. government obligations $ 4,612.2 $ 3.0 $ (20.2 ) $ 0 $ 4,595.0 17.0 % State and local government obligations 2,332.2 35.0 (3.1 ) 0.1 2,364.2 8.7 Foreign government obligations 24.2 0 0 0 24.2 0.1 Corporate debt securities 5,195.7 32.7 (4.2 ) 1.4 5,225.6 19.4 Residential mortgage-backed securities 911.3 12.2 (3.2 ) 0 920.3 3.4 Agency residential pass-through obligations 35.7 0 (0.4 ) 0 35.3 0.1 Commercial mortgage-backed securities 2,763.7 17.0 (12.8 ) 0 2,767.9 10.3 Other asset-backed securities 2,485.6 6.4 (2.0 ) 0.2 2,490.2 9.2 Redeemable preferred stocks 222.5 16.4 (2.0 ) 0.4 237.3 0.9 Total fixed maturities 18,583.1 122.7 (47.9 ) 2.1 18,660.0 69.1 Equity securities: Nonredeemable preferred stocks 700.6 120.4 (7.3 ) 0 813.7 3.0 Common equities 1,485.5 1,729.5 (5.5 ) 0 3,209.5 11.9 Short-term investments 4,311.5 0 0 0 4,311.5 16.0 Total portfolio 1,2 $ 25,080.7 $ 1,972.6 $ (60.7 ) $ 2.1 $ 26,994.7 100.0 % ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value September 30, 2016 Fixed maturities: U.S. government obligations $ 726.0 $ 8.9 $ (0.3 ) $ 0 $ 734.6 3.1 % State and local government obligations 2,508.9 58.7 (1.6 ) 0 2,566.0 10.9 Foreign government obligations 25.4 0 0 0 25.4 0.1 Corporate debt securities 4,327.5 76.9 (1.8 ) 0.7 4,403.3 18.7 Residential mortgage-backed securities 1,582.6 25.3 (15.5 ) 1.8 1,594.2 6.8 Agency residential pass-through obligations 43.6 0.5 0 0 44.1 0.2 Commercial mortgage-backed securities 2,249.4 35.1 (6.1 ) 0 2,278.4 9.7 Other asset-backed securities 2,037.5 7.9 (0.3 ) 0.3 2,045.4 8.7 Redeemable preferred stocks 207.1 19.6 (1.2 ) 0 225.5 1.0 Total fixed maturities 13,708.0 232.9 (26.8 ) 2.8 13,916.9 59.2 Equity securities: Nonredeemable preferred stocks 726.6 146.2 (13.9 ) 1.2 860.1 3.6 Common equities 1,576.1 1,296.3 (4.4 ) 0 2,868.0 12.2 Short-term investments 5,876.2 0 0 0 5,876.2 25.0 Total portfolio 1,2 $ 21,886.9 $ 1,675.4 $ (45.1 ) $ 4.0 $ 23,521.2 100.0 % ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of 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 1,448.5 23.7 (15.0 ) 1.5 1,458.7 6.2 Agency residential pass-through obligations 41.2 0 (0.6 ) 0 40.6 0.2 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 open derivative positions; at September 30, 2017 and 2016 , and December 31, 2016 , we had $238.3 million , $185.3 million , and $27.8 million , respectively, included in “other liabilities.” 2 The total fair value of the portfolio at September 30, 2017 and 2016 , and December 31, 2016 , included $1.1 billion , $1.0 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 Instruments Table [Table Text Block]	Included in our fixed-maturity and equity securities are hybrid securities, which are reported at fair value: September 30, December 31, (millions) 2017 2016 Fixed maturities: State and local government obligations $ 2.4 $ 0 $ 0 Corporate debt securities 114.9 31.2 40.1 Residential mortgage-backed securities 0 175.3 170.5 Other asset-backed securities 7.1 9.5 8.9 Redeemable preferred stocks 35.5 0 0 Total fixed maturities 159.9 216.0 219.5 Equity securities: Nonredeemable preferred stocks 0 13.6 0 Total hybrid securities $ 159.9 $ 229.6 $ 219.5
Investments Classified by Contractual Maturity Date [Table Text Block]	The composition of fixed maturities by maturity at September 30, 2017 , was: (millions) Cost Fair Value Less than one year $ 4,088.1 $ 4,105.7 One to five years 11,476.9 11,508.8 Five to ten years 2,873.9 2,901.3 Ten years or greater 144.2 144.2 Total $ 18,583.1 $ 18,660.0
Schedule of Unrealized Loss on Investments [Table Text Block]	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 Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses September 30, 2017 Fixed maturities: U.S. government obligations 54 $ 3,804.0 $ (20.2 ) 31 $ 2,302.2 $ (7.1 ) 23 $ 1,501.8 $ (13.1 ) State and local government obligations 169 610.4 (3.1 ) 40 141.0 (0.7 ) 129 469.4 (2.4 ) Corporate debt securities 113 1,523.8 (4.2 ) 59 636.0 (1.1 ) 54 887.8 (3.1 ) Residential mortgage-backed securities 126 308.8 (3.2 ) 14 37.4 (0.1 ) 112 271.4 (3.1 ) Agency residential pass-through obligations 65 32.7 (0.4 ) 12 1.8 0 53 30.9 (0.4 ) Commercial mortgage-backed securities 105 1,588.9 (12.8 ) 61 1,073.5 (4.6 ) 44 515.4 (8.2 ) Other asset-backed securities 148 1,393.2 (2.0 ) 103 920.8 (1.0 ) 45 472.4 (1.0 ) Redeemable preferred stocks 2 21.4 (2.0 ) 1 10.9 0 1 10.5 (2.0 ) Total fixed maturities 782 9,283.2 (47.9 ) 321 5,123.6 (14.6 ) 461 4,159.6 (33.3 ) Equity securities: Nonredeemable preferred stocks 3 72.4 (7.3 ) 0 0 0 3 72.4 (7.3 ) Common equities 67 52.0 (5.5 ) 62 49.3 (4.9 ) 5 2.7 (0.6 ) Total equity securities 70 124.4 (12.8 ) 62 49.3 (4.9 ) 8 75.1 (7.9 ) Total portfolio 852 $ 9,407.6 $ (60.7 ) 383 $ 5,172.9 $ (19.5 ) 469 $ 4,234.7 $ (41.2 ) Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses September 30, 2016 Fixed maturities: U.S. government obligations 4 $ 104.8 $ (0.3 ) 4 $ 104.8 $ (0.3 ) 0 $ 0 $ 0 State and local government obligations 151 512.0 (1.6 ) 117 409.6 (1.0 ) 34 102.4 (0.6 ) Corporate debt securities 52 598.3 (1.8 ) 42 541.5 (0.8 ) 10 56.8 (1.0 ) Residential mortgage-backed securities 138 915.8 (15.5 ) 26 108.8 (0.4 ) 112 807.0 (15.1 ) Agency residential pass-through obligations 15 4.2 0 7 1.9 0 8 2.3 0 Commercial mortgage-backed securities 56 610.6 (6.1 ) 17 194.7 (1.7 ) 39 415.9 (4.4 ) Other asset-backed securities 43 488.7 (0.3 ) 16 219.5 (0.1 ) 27 269.2 (0.2 ) Redeemable preferred stocks 2 31.8 (1.2 ) 0 0 0 2 31.8 (1.2 ) Total fixed maturities 461 3,266.2 (26.8 ) 229 1,580.8 (4.3 ) 232 1,685.4 (22.5 ) Equity securities: Nonredeemable preferred stocks 7 187.5 (13.9 ) 1 9.9 (0.2 ) 6 177.6 (13.7 ) Common equities 86 53.4 (4.4 ) 83 49.5 (4.2 ) 3 3.9 (0.2 ) Total equity securities 93 240.9 (18.3 ) 84 59.4 (4.4 ) 9 181.5 (13.9 ) Total portfolio 554 $ 3,507.1 $ (45.1 ) 313 $ 1,640.2 $ (8.7 ) 241 $ 1,866.9 $ (36.4 ) Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair 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 178 917.7 (15.0 ) 69 175.8 (1.1 ) 109 741.9 (13.9 ) Agency residential pass-through obligations 55 36.0 (0.6 ) 48 33.9 (0.6 ) 7 2.1 0 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 )
Non Credit Portion Of Other Than Temporary Impairment Recorded In Accumulated Other Comprehensive Income Table [Table Text Block]	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): September 30, December 31, (millions) 2017 2016 Fixed maturities: Residential mortgage-backed securities $ (19.7 ) $ (43.3 ) $ (43.3 ) Commercial mortgage-backed securities (0.4 ) (0.6 ) (0.6 ) Total fixed maturities $ (20.1 ) $ (43.9 ) $ (43.9 )
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Table Text Block]	The following tables provide rollforwards of the amounts related to credit losses recognized in earnings for the periods ended September 30, 2017 and 2016 , 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: Three Months Ended September 30, 2017 Mortgage-Backed (millions) Residential Commercial Total Balance at June 30, 2017 $ 0.2 $ 0.1 $ 0.3 Credit losses for which an OTTI was not previously recognized 0 0.4 0.4 Reductions for securities sold/matured 0 0 0 Change in recoveries of future cash flows expected to be collected 1 0 0 0 Balance at September 30, 2017 $ 0.2 $ 0.5 $ 0.7 Nine Months Ended September 30, 2017 Mortgage-Backed (millions) Residential Commercial Total Balance 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 0 0 Balance at September 30, 2017 $ 0.2 $ 0.5 $ 0.7 Three Months Ended September 30, 2016 Mortgage-Backed (millions) Residential Commercial Total Balance at June 30, 2016 $ 11.8 $ 0.4 $ 12.2 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 (0.3 ) 0 (0.3 ) Balance at September 30, 2016 $ 11.5 $ 0.4 $ 11.9 Nine Months Ended September 30, 2016 Mortgage-Backed (millions) Residential Commercial Total Balance 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 (0.9 ) 0 (0.9 ) Balance at September 30, 2016 $ 11.5 $ 0.4 $ 11.9 1 Reflects the current period change in the expected recovery of prior impairments that will be accreted into income over the remaining life of the security.
Realized Gain (Loss) on Investments [Table Text Block]	The components of net realized gains (losses) for the three and nine months ended September 30, were: Three Months Nine Months (millions) 2017 2016 2017 2016 Gross realized gains on security sales Fixed maturities: U.S. government obligations $ 0.9 $ 6.4 $ 5.8 $ 24.1 State and local government obligations 4.0 0.6 7.1 16.0 Corporate and other debt securities 5.1 15.6 16.5 38.1 Residential mortgage-backed securities 2.8 0.2 23.8 1.9 Agency residential pass-through obligations 0 0 0 0.1 Commercial mortgage-backed securities 0 5.5 2.4 12.0 Other asset-backed securities 0 0 0.3 0 Redeemable preferred stocks 7.7 0 8.0 0 Total fixed maturities 20.5 28.3 63.9 92.2 Equity securities: Nonredeemable preferred stocks 3.0 4.9 54.6 11.9 Common equities 5.7 14.4 23.0 43.3 Short-term investments 0 0.1 0 0.1 Subtotal gross realized gains on security sales 29.2 47.7 141.5 147.5 Gross realized losses on security sales Fixed maturities: U.S. government obligations (1.0 ) (0.8 ) (4.6 ) (1.2 ) State and local government obligations 0 0 (0.1 ) (1.6 ) Corporate and other debt securities (1.8 ) (0.2 ) (4.6 ) (1.9 ) Residential mortgage-backed securities (0.1 ) 0 (0.4 ) 0 Agency residential pass-through obligations 0 0 0 (0.2 ) Commercial mortgage-backed securities (0.5 ) 0 (3.6 ) (4.1 ) Redeemable preferred stocks (6.4 ) (6.5 ) (6.4 ) (6.5 ) Total fixed maturities (9.8 ) (7.5 ) (19.7 ) (15.5 ) Equity securities: Nonredeemable preferred stocks (0.1 ) (0.4 ) (5.9 ) (3.1 ) Common equities (0.2 ) (0.3 ) (0.3 ) (5.3 ) Subtotal gross realized losses on security sales (10.1 ) (8.2 ) (25.9 ) (23.9 ) Net realized gains (losses) on security sales Fixed maturities: U.S. government obligations (0.1 ) 5.6 1.2 22.9 State and local government obligations 4.0 0.6 7.0 14.4 Corporate and other debt securities 3.3 15.4 11.9 36.2 Residential mortgage-backed securities 2.7 0.2 23.4 1.9 Agency residential pass-through obligations 0 0 0 (0.1 ) Commercial mortgage-backed securities (0.5 ) 5.5 (1.2 ) 7.9 Other asset-backed securities 0 0 0.3 0 Redeemable preferred stocks 1.3 (6.5 ) 1.6 (6.5 ) Total fixed maturities 10.7 20.8 44.2 76.7 Equity securities: Nonredeemable preferred stocks 2.9 4.5 48.7 8.8 Common equities 5.5 14.1 22.7 38.0 Short-term investments 0 0.1 0 0.1 Subtotal net realized gains (losses) on security sales 19.1 39.5 115.6 123.6 Other-than-temporary impairment losses Fixed maturities: Commercial mortgage-backed securities (0.4 ) 0 (0.4 ) 0 Redeemable preferred stocks 0 (25.4 ) 0 (25.4 ) Total fixed maturities (0.4 ) (25.4 ) (0.4 ) (25.4 ) Equity securities: Common equities (8.9 ) (1.4 ) (12.5 ) (1.6 ) Subtotal investment other-than-temporary impairment losses (9.3 ) (26.8 ) (12.9 ) (27.0 ) Other asset impairment (33.7 ) (34.8 ) (44.9 ) (34.8 ) Subtotal other-than-temporary impairment losses (43.0 ) (61.6 ) (57.8 ) (61.8 ) Other gains (losses) Hybrid securities (0.9 ) 0.6 0.3 2.9 Derivative instruments 0 0.7 0 (35.8 ) Litigation settlements 0.1 0.1 1.2 0.1 Subtotal other gains (losses) (0.8 ) 1.4 1.5 (32.8 ) Total net realized gains (losses) on securities $ (24.7 ) $ (20.7 ) $ 59.3 $ 29.0
Components of Net Investment Income	The components of net investment income for the three and nine months ended September 30, were: Three Months Nine Months (millions) 2017 2016 2017 2016 Fixed maturities: U.S. government obligations $ 18.4 $ 3.3 $ 48.1 $ 12.4 State and local government obligations 12.9 12.6 39.3 39.3 Foreign government obligations 0.1 0.1 0.3 0.3 Corporate debt securities 32.2 27.3 93.1 81.9 Residential mortgage-backed securities 7.8 11.1 27.3 34.9 Agency residential pass-through obligations 0.2 0.3 0.6 0.9 Commercial mortgage-backed securities 20.0 22.4 57.1 62.6 Other asset-backed securities 12.1 6.8 33.8 18.7 Redeemable preferred stocks 2.8 3.8 9.1 11.5 Total fixed maturities 106.5 87.7 308.7 262.5 Equity securities: Nonredeemable preferred stocks 10.9 12.4 33.0 36.8 Common equities 14.9 13.9 42.8 42.0 Short-term investments 10.6 5.3 26.4 11.4 Investment income 142.9 119.3 410.9 352.7 Investment expenses (5.8 ) (4.8 ) (18.0 ) (14.9 ) Net investment income $ 137.1 $ 114.5 $ 392.9 $ 337.8
Schedule of Derivative Instruments [Table Text Block]	The following table shows the status of our derivative instruments at September 30, 2017 and 2016 , and December 31, 2016 , and for the three and nine months ended September 30, 2017 and 2016 : (millions) Balance Sheet 2 Comprehensive Income Statement Assets (Liabilities) Pretax Net Realized Gains (Losses) Notional Value 1 Three Months Ended Nine Months Ended September 30, Dec. 31, September 30, Dec. 31, September 30, September 30, Derivatives designated as: 2017 2016 2016 Purpose Classification 2017 2016 2016 2017 2016 2017 2016 Hedging instrument Closed: Ineffective cash flow hedge $ 31 $ 370 $ 370 Manage interest rate risk NA $ 0 $ 0 $ 0 $ 0 $ (1.4 ) $ 0 $ (1.3 ) Non-hedging instruments Liabilities: Interest rate swaps 0 315 0 Manage portfolio duration Other liabilities 0 (13.0 ) 0 0 1.7 0 (17.8 ) Closed: Interest rate swaps 0 435 750 Manage portfolio duration NA 0 0 0 0 0.3 0 (17.0 ) U.S. Treasury Note futures 0 135 135 Manage NA 0 0 0 0 0.1 0 0.3 Total NA NA NA $ 0 $ (13.0 ) $ 0 $ 0 $ 0.7 $ 0 $ (35.8 ) NA= Not applicable 1 The amounts represent the value held at quarter and year end for open positions and the maximum amount held during the period for closed positions. 2 To the extent we hold both derivative assets and liabilities with the same counterparty that are subject to an enforceable master netting arrangement, we reported them on a gross basis on our balance sheets, consistent with our historical presentation.

Fair Value (Tables)

Fair Value (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2017 Fixed maturities: U.S. government obligations $ 4,595.0 $ 0 $ 0 $ 4,595.0 $ 4,612.2 State and local government obligations 0 2,364.2 0 2,364.2 2,332.2 Foreign government obligations 24.2 0 0 24.2 24.2 Corporate debt securities 0 5,225.6 0 5,225.6 5,195.7 Subtotal 4,619.2 7,589.8 0 12,209.0 12,164.3 Asset-backed securities: Residential mortgage-backed 0 920.3 0 920.3 911.3 Agency residential pass-through obligations 0 35.3 0 35.3 35.7 Commercial mortgage-backed 0 2,767.9 0 2,767.9 2,763.7 Other asset-backed 0 2,490.2 0 2,490.2 2,485.6 Subtotal asset-backed securities 0 6,213.7 0 6,213.7 6,196.3 Redeemable preferred stocks: Financials 0 63.8 0 63.8 61.4 Utilities 0 31.9 0 31.9 30.5 Industrials 0 141.6 0 141.6 130.6 Subtotal redeemable preferred stocks 0 237.3 0 237.3 222.5 Total fixed maturities 4,619.2 14,040.8 0 18,660.0 18,583.1 Equity securities: Nonredeemable preferred stocks: Financials 82.2 726.5 0 808.7 695.6 Industrials 0 0 5.0 5.0 5.0 Subtotal nonredeemable preferred stocks 82.2 726.5 5.0 813.7 700.6 Common equities: Common stocks 3,209.2 0 0 3,209.2 1,485.2 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 3,209.2 0 0.3 3,209.5 1,485.5 Total fixed maturities and equity securities 7,910.6 14,767.3 5.3 22,683.2 20,769.2 Short-term investments 3,175.4 1,136.1 0 4,311.5 4,311.5 Total portfolio $ 11,086.0 $ 15,903.4 $ 5.3 $ 26,994.7 $ 25,080.7 Debt $ 0 $ 3,574.6 $ 43.3 $ 3,617.9 $ 3,312.2 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost September 30, 2016 Fixed maturities: U.S. government obligations $ 734.6 $ 0 $ 0 $ 734.6 $ 726.0 State and local government obligations 0 2,566.0 0 2,566.0 2,508.9 Foreign government obligations 25.4 0 0 25.4 25.4 Corporate debt securities 0 4,403.3 0 4,403.3 4,327.5 Subtotal 760.0 6,969.3 0 7,729.3 7,587.8 Asset-backed securities: Residential mortgage-backed 0 1,594.2 0 1,594.2 1,582.6 Agency residential pass-through obligations 0 44.1 0 44.1 43.6 Commercial mortgage-backed 0 2,277.8 0.6 2,278.4 2,249.4 Other asset-backed 0 2,045.4 0 2,045.4 2,037.5 Subtotal asset-backed securities 0 5,961.5 0.6 5,962.1 5,913.1 Redeemable preferred stocks: Financials 0 95.7 0 95.7 77.1 Utilities 0 30.4 0 30.4 30.6 Industrials 0 99.4 0 99.4 99.4 Subtotal redeemable preferred stocks 0 225.5 0 225.5 207.1 Total fixed maturities 760.0 13,156.3 0.6 13,916.9 13,708.0 Equity securities: Nonredeemable preferred stocks: Financials 154.6 705.5 0 860.1 726.6 Industrials 0 0 0 0 0 Subtotal nonredeemable preferred stocks 154.6 705.5 0 860.1 726.6 Common equities: Common stocks 2,867.6 0 0 2,867.6 1,575.7 Other risk investments 0 0 0.4 0.4 0.4 Subtotal common equities 2,867.6 0 0.4 2,868.0 1,576.1 Total fixed maturities and equity securities 3,782.2 13,861.8 1.0 17,645.0 16,010.7 Short-term investments 5,051.5 824.7 0 5,876.2 5,876.2 Total portfolio $ 8,833.7 $ 14,686.5 $ 1.0 $ 23,521.2 $ 21,886.9 Debt $ 0 $ 3,371.6 $ 133.6 $ 3,505.2 $ 3,153.9 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 1,458.7 0 1,458.7 1,448.5 Agency residential pass-through obligations 0 40.6 0 40.6 41.2 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 three and nine months ended September 30, 2017 and 2016 : Level 3 Fair Value Three Months Ended September 30, 2017 (millions) Fair Value at June 30, 2017 Calls/ Maturities/ Paydowns Purchases Sales Net Realized (Gain) Loss on Sales Change in Valuation Net Transfers In (Out) Fair Value at September 30, 2017 Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 $ 0 Total fixed maturities 0 0 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 0.3 0 0 0 0 0 0 0.3 Total Level 3 securities $ 0.3 $ 0 $ 5.0 $ 0 $ 0 $ 0 $ 0 $ 5.3 Level 3 Fair Value Nine months ended September 30, 2017 (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 September 30, 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 Three Months Ended September 30, 2016 (millions) Fair Value at June 30, 2016 Calls/ Purchases Sales Net Realized (Gain) Loss on Sales Change in Net Fair Value at September 30, 2016 Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 9.2 $ (8.7 ) $ 0 $ 0 $ 0 $ 0.1 $ 0 $ 0.6 Total fixed maturities 9.2 (8.7 ) 0 0 0 0.1 0 0.6 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 $ 9.5 $ (8.7 ) $ 0 $ 0 $ 0 $ 0.2 $ 0 $ 1.0 Level 3 Fair Value Nine months ended September 30, 2016 (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 September 30, 2016 Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 9.9 $ (9.3 ) $ 0 $ 0 $ 0 $ 0 $ 0 $ 0.6 Total fixed maturities 9.9 (9.3 ) 0 0 0 0 0 0.6 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 $ (9.3 ) $ 0 $ 0 $ 0 $ 0.1 $ 0 $ 1.0
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 September 30, 2017 and 2016 , and December 31, 2016 : Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at September 30, 2017 Valuation Technique Unobservable Input Unobservable 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 Purchase price 3.9 Subtotal Level 3 securities 5.0 Pricing exemption securities 2 0.3 Total Level 3 securities $ 5.3 NA= Not applicable, since we did not hold any commercial mortgage-backed Level 3 securities at September 30, 2017 . 1 The security was internally-priced since it is privately held and it was valued at September 30, 2017 using the purchase price. 2 The fair values for these securities were determined with unobservable inputs not reasonably available to us. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at September 30, 2016 Valuation Technique Unobservable Input Unobservable Fixed maturities: Asset-backed securities: Commercial mortgage-backed $ 0.6 External vendor Prepayment rate 1 0 % Total fixed maturities 0.6 Equity securities: Nonredeemable preferred stocks: Industrials 0 NA NA NA Subtotal Level 3 securities 0.6 Pricing exemption securities 2 0.4 Total Level 3 securities $ 1.0 NA= Not applicable, since we did not hold any nonredeemable preferred stock Level 3 securities at September 30, 2016 . 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 determined with unobservable inputs 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 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 Pricing exemption securities 2 0.4 Total Level 3 securities $ 0.7 NA= Not applicable, since we did not hold any nonredeemable preferred stock Level 3 securities at December 31, 2016 . 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 determined with unobservable inputs not reasonably available to us.

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2017
Debt Table	Debt at each of the balance sheet periods consisted of: September 30, 2017 September 30, 2016 December 31, 2016 (millions) Carrying Value Fair Value Carrying Value Fair Value Carrying Value Fair Value 3.75% Senior Notes due 2021 $ 498.7 $ 525.7 $ 498.4 $ 544.2 $ 498.4 $ 528.8 2.45% Senior Notes due 2027 496.0 475.3 495.6 497.5 495.8 464.6 6 5/8% Senior Notes due 2029 296.0 388.3 295.8 402.6 295.9 380.1 6.25% Senior Notes due 2032 395.3 518.7 395.1 533.3 395.2 499.0 4.35% Senior Notes due 2044 346.5 379.8 346.4 403.0 346.4 362.3 3.70% Senior Notes due 2045 395.2 392.6 395.1 418.7 395.1 372.5 4.125% Senior Notes due 2047 841.2 894.2 0 0 0 0 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 0 0 593.9 572.3 594.1 581.2 Other debt instruments 43.3 43.3 133.6 133.6 127.3 127.3 Total $ 3,312.2 $ 3,617.9 $ 3,153.9 $ 3,505.2 $ 3,148.2 $ 3,315.8
Other debt instruments
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: September 30, 2017 September 30, 2016 December 31, 2016 ($ in millions) Type of debt instrument Number of Instruments Carrying Value Number of Instruments Carrying Number of Instruments Carrying Stated Maturity Date(s) Term loans 2 $ 43.3 2 $ 68.4 2 $ 62.1 December 2018 and 2019 Junior subordinated notes 1 0 0 2 41.2 2 41.2 June 2036 and 2037 Senior notes 1 0 0 4 24.0 4 24.0 Various 2 Total $ 43.3 $ 133.6 $ 127.3 1 The notes were redeemed during the third quarter 2017 (discussed below). 2 The senior notes original maturity dates were May 2033, April 2034, December 2034, and June 2035.

Loss and Loss Adjustment Expe24

Loss and Loss Adjustment Expense Reserves (Tables)	9 Months Ended
	Sep. 30, 2017
Loss and Loss Adjustment Expenses Reserves [Abstract]
Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block]	Activity in the loss and loss adjustment expense reserves during the nine month periods is summarized as follows: September 30, (millions) 2017 2016 Balance, Beginning of period $ 11,368.0 $ 10,039.0 Less reinsurance recoverables on unpaid losses 1,801.0 1,442.7 Net balance, Beginning of period 9,567.0 8,596.3 Net loss and loss adjustment expense reserves disposed 1 0 (2.5 ) Total beginning reserves 9,567.0 8,593.8 Incurred related to: Current year 13,886.3 12,577.5 Prior years 42.5 (22.9 ) Total incurred 13,928.8 12,554.6 Paid related to: Current year 8,379.4 7,768.2 Prior years 4,387.8 3,986.2 Total paid 12,767.2 11,754.4 Net balance, End of period 10,728.6 9,394.0 Plus reinsurance recoverables on unpaid losses 2,624.7 1,834.2 Balance, End of period $ 13,353.3 $ 11,228.2 1 During 2016, $2.5 million net reserves were disposed by ARX in an exchange transaction.

Supplemental Cash Flow Inform25

Supplemental Cash Flow Information (Tables)	9 Months Ended
	Sep. 30, 2017
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information Table	We paid the following in the respective periods: Nine Months Ended September 30, (millions) 2017 2016 Income taxes $ 538.7 $ 380.8 Interest 107.4 98.0

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2017
Segment Reporting [Abstract]
Reconciliation of Revenue and operating Income from Segments to Consolidated	Following are the operating results for the respective periods: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 (millions) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Personal Lines Agency $ 2,840.0 $ 69.5 $ 2,474.3 $ 98.6 $ 8,224.0 $ 524.7 $ 7,245.5 $ 354.5 Direct 2,734.8 128.7 2,391.2 84.4 7,908.5 466.7 6,946.7 237.5 Total Personal Lines 1 5,574.8 198.2 4,865.5 183.0 16,132.5 991.4 14,192.2 592.0 Commercial Lines 714.0 42.8 630.2 4.4 2,031.2 166.4 1,772.4 96.7 Property 2 255.2 (69.0 ) 227.7 10.4 720.3 (57.5 ) 638.0 (27.7 ) Other indemnity 0 0 0 (0.8 ) 0 (0.3 ) 0 (1.6 ) Total underwriting operations 6,544.0 172.0 5,723.4 197.0 18,884.0 1,100.0 16,602.6 659.4 Fees and other revenues 3 96.3 NA 86.8 NA 270.3 NA 248.2 NA Service businesses 33.3 4.4 26.2 3.0 94.5 12.7 77.7 9.2 Investments 4 118.2 112.4 98.6 93.8 470.2 452.2 381.7 366.8 Gains on extinguishment of debt 0 0 0 0 0.2 0.2 1.6 1.6 Interest expense NA (37.4 ) NA (35.3 ) NA (117.6 ) NA (103.8 ) Consolidated total $ 6,791.8 $ 251.4 $ 5,935.0 $ 258.5 $ 19,719.2 $ 1,447.5 $ 17,311.8 $ 933.2 NA = Not applicable 1 Personal auto insurance accounted for 93% of the total Personal Lines segment net premiums earned in the three and nine months ended September 30, 2017 , and 92% for the same periods in 2016 ; insurance for our special lines products (e.g., motorcycles, watercraft, and RVs) accounted for the balance of the Personal Lines net premiums earned. 2 For the three and nine months ended September 30, 2017 , pretax profit (loss) includes $17.2 million and $48.2 million , respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX and $15.5 million and $46.6 million for the same periods in 2016. 3 Pretax profit (loss) for fees and other revenues are attributable to operating segments. 4 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 and combined ratios for our underwriting operations for the respective periods: Three Months Ended September 30, Nine Months Ended September 30, 2017 2016 2017 2016 Under-writing Combined Ratio Under-writing Combined Ratio Under-writing Margin Combined Ratio Under-writing Margin Combined Ratio Personal Lines Agency 2.4 % 97.6 4.0 % 96.0 6.4 % 93.6 4.9 % 95.1 Direct 4.7 95.3 3.5 96.5 5.9 94.1 3.4 96.6 Total Personal Lines 3.6 96.4 3.8 96.2 6.1 93.9 4.2 95.8 Commercial Lines 6.0 94.0 0.7 99.3 8.2 91.8 5.5 94.5 Property 1 (27.0 ) 127.0 4.6 95.4 (8.0 ) 108.0 (4.3 ) 104.3 Other indemnity 2 NM NM NM NM NM NM NM NM Total underwriting operations 2.6 97.4 3.4 96.6 5.8 94.2 4.0 96.0 1 Included in both the three and nine months ended September 30, 2017 are 6.7 points of amortization expense predominately associated with the acquisition of a controlling interest in ARX and 6.8 points and 7.3 points, respectively, for the three and nine months ended September 30, 2016. The nine months ended September 30, 2016, also include 0.7 points of expense related to the loss on the exchange transaction that occurred in 2016. 2 Underwriting margins and combined ratios are not meaningful (NM) for our other indemnity businesses due to the low level of premiums earned by, and the variability of loss costs in, such businesses.

Dividends Dividends (Tables)

Dividends Dividends (Tables)	9 Months Ended
	Sep. 30, 2017
Dividends [Abstract]
Dividends	Following is a summary of our shareholder dividends that were declared in the last two years: (millions, except per share amounts) Amount Dividend Type Declared Paid Per Share Total 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.

Other Comprehensive Income (L28

Other Comprehensive Income (Loss) (Tables)	9 Months Ended
	Sep. 30, 2017
Equity [Abstract]
Components of Other Comprehensive Income (Loss)	The components of other comprehensive income (loss), including reclassification adjustments by income statement line item, were as follows: Components of Changes in (millions) Pretax total Total tax After tax total Total net Net Foreign (Income)loss attributable to NCI Balance at June 30, 2017 $ 1,774.1 $ (623.4 ) $ 1,150.7 $ 1,164.6 $ (15.1 ) $ (0.9 ) $ 2.1 Other comprehensive income (loss) before reclassifications: Investment securities 122.7 (43.0 ) 79.7 79.7 0 0 0 Net non-credit related OTTI losses, adjusted for valuation changes 0 0 0 0 0 0 0 Forecasted transactions 0 0 0 0 0 0 0 Foreign currency translation adjustment 0.9 (0.3 ) 0.6 0 0 0.6 0 Loss attributable to noncontrolling interest (NCI) (1.1 ) 0.4 (0.7 ) 0 0 0 (0.7 ) Total other comprehensive income (loss) before reclassifications 122.5 (42.9 ) 79.6 79.7 0 0.6 (0.7 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (9.3 ) 3.3 (6.0 ) (6.0 ) 0 0 0 Net realized gains (losses) on securities 15.7 (5.5 ) 10.2 10.2 0 0 0 Interest expense (0.2 ) 0.1 (0.1 ) 0 (0.1 ) 0 0 Total reclassification adjustment for amounts realized in net income 6.2 (2.1 ) 4.1 4.2 (0.1 ) 0 0 Total other comprehensive income (loss) 116.3 (40.8 ) 75.5 75.5 0.1 0.6 (0.7 ) Balance at September 30, 2017 $ 1,890.4 $ (664.2 ) $ 1,226.2 $ 1,240.1 $ (15.0 ) $ (0.3 ) $ 1.4 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 (Income)loss attributable to NCI Balance 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 554.4 (194.3 ) 360.1 360.1 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 1.2 (0.4 ) 0.8 0 0 0.8 0 Loss attributable to noncontrolling interest (NCI) (4.5 ) 1.6 (2.9 ) 0 0 0 (2.9 ) Total other comprehensive income (loss) before reclassifications 543.1 (190.3 ) 352.8 360.1 (5.2 ) 0.8 (2.9 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (12.9 ) 4.6 (8.3 ) (8.3 ) 0 0 0 Net realized gains (losses) on securities 104.5 (36.6 ) 67.9 67.9 0 0 0 Interest expense 0.6 (0.2 ) 0.4 0 0.4 0 0 Total reclassification adjustment for amounts realized in net income 92.2 (32.2 ) 60.0 59.6 0.4 0 0 Total other comprehensive income (loss) 450.9 (158.1 ) 292.8 300.5 (5.6 ) 0.8 (2.9 ) Balance at September 30, 2017 $ 1,890.4 $ (664.2 ) $ 1,226.2 $ 1,240.1 $ (15.0 ) $ (0.3 ) $ 1.4 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 Net unrealized gains on forecasted transactions Foreign currency translation adjustment (Income)loss attributable to NCI Balance at June 30, 2016 $ 1,477.0 $ (519.2 ) $ 957.8 $ 970.1 $ (8.8 ) $ (1.1 ) $ (2.4 ) Other comprehensive income (loss) before reclassifications: Investment securities 146.8 (51.4 ) 95.4 95.4 0 0 0 Net non-credit related OTTI losses, adjusted for valuation changes 0 0 0 0 0 0 0 Forecasted transactions 0 0 0 0 0 0 0 Foreign currency translation adjustment 0.4 (0.1 ) 0.3 0 0 0.3 0 Loss attributable to noncontrolling interest (NCI) 1.8 (0.6 ) 1.2 0 0 0 1.2 Total other comprehensive income (loss) before reclassifications 149.0 (52.1 ) 96.9 95.4 0 0.3 1.2 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (26.8 ) 9.4 (17.4 ) (17.4 ) 0 0 0 Net realized gains (losses) on securities 39.6 (13.9 ) 25.7 25.7 0 0 0 Interest expense 0.5 (0.2 ) 0.3 0 0.3 0 0 Total reclassification adjustment for amounts realized in net income 13.3 (4.7 ) 8.6 8.3 0.3 0 0 Total other comprehensive income (loss) 135.7 (47.4 ) 88.3 87.1 (0.3 ) 0.3 1.2 Balance at September 30, 2016 $ 1,612.7 $ (566.6 ) $ 1,046.1 $ 1,057.2 $ (9.1 ) $ (0.8 ) $ (1.2 ) 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 Net unrealized gains on forecasted transactions Foreign currency translation adjustment (Income)loss attributable to NCI Balance 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 470.4 (165.2 ) 305.2 305.2 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 1.0 (0.3 ) 0.7 0 0 0.7 0 Loss attributable to noncontrolling interest (NCI) (3.7 ) 1.4 (2.3 ) 0 0 0 (2.3 ) Total other comprehensive income (loss) before reclassifications 467.6 (164.0 ) 303.6 305.2 0 0.7 (2.3 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (27.0 ) 9.5 (17.5 ) (17.5 ) 0 0 0 Net realized gains (losses) on securities 114.9 (40.4 ) 74.5 74.5 0 0 0 Interest expense 1.5 (0.6 ) 0.9 0 0.9 0 0 Total reclassification adjustment for amounts realized in net income 89.4 (31.5 ) 57.9 57.0 0.9 0 0 Total other comprehensive income (loss) 378.2 (132.5 ) 245.7 248.2 (0.9 ) 0.7 (2.3 ) Balance at September 30, 2016 $ 1,612.7 $ (566.6 ) $ 1,046.1 $ 1,057.2 $ (9.1 ) $ (0.8 ) $ (1.2 )

Redeemable Noncontrolling Int29

Redeemable Noncontrolling Interest (Tables)	9 Months Ended
	Sep. 30, 2017
Temporary Equity Disclosure [Abstract]
Redeemable Noncontrolling Interest	The changes in the components of redeemable NCI during the nine months ended September 30, 2017 and 2016 , and the year ended December 31, 2016 , were: (millions) September 30, 2017 September 30, 2016 December 31, 2016 Balance, Beginning of period $ 483.7 $ 464.9 $ 464.9 Net income attributable to NCI 1.9 11.3 26.2 Other comprehensive income (loss) attributable to NCI 2.9 2.3 (3.2 ) Exercise of employee stock options 3.4 0 0 Change in redemption value of NCI 6.3 (6.0 ) (4.2 ) Balance, End of period $ 498.2 $ 472.5 $ 483.7

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 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 September 30, 2017 and 2016 , and December 31, 2016 : (millions) September 30, 2017 September 30, 2016 December 31, 2016 Intangible assets subject to amortization $ 372.2 $ 435.9 $ 420.4 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 384.6 $ 448.3 $ 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 all periods presented.
Intangible Assets Subject to Amortization	Intangible assets subject to amortization consisted of the following: (millions) September 30, 2017 September 30, 2016 December 31, 2016 Category Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Policies in force $ 256.2 $ 91.5 $ 164.7 $ 256.2 $ 55.0 $ 201.2 $ 256.2 $ 64.1 $ 192.1 Agency relationships 159.2 28.4 130.8 159.2 17.1 142.1 159.2 19.9 139.3 Software rights 79.1 26.8 52.3 79.1 16.1 63.0 79.1 18.8 60.3 Trade name 34.8 10.4 24.4 34.8 5.2 29.6 34.8 6.1 28.7 Total $ 529.3 $ 157.1 $ 372.2 $ 529.3 $ 93.4 $ 435.9 $ 529.3 $ 108.9 $ 420.4

Basis of Presentation Basis of

Basis of Presentation Basis of Presentation-Additional Information (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Business Acquisition [Line Items]
Assets Held-for-sale, Property, Plant and Equipment	$ 5.3	$ 8.7	$ 8.7
ARX Holding Corp. [Member]
Business Acquisition [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage	69.00%	69.20%	69.20%

Investments Investment Portfoli

Investments Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 25,080.7	$ 22,031.7	$ 21,886.9
Gross Unrealized Gains	1,972.6	1,588.9	1,675.4
Gross Unrealized Losses	(60.7)	(139.8)	(45.1)
Net Realized Gains (Losses)	2.1	1.8	4
Fair Value	$ 26,994.7	$ 23,482.6	$ 23,521.2
% of Total Fair Value	100.00%	100.00%	100.00%
Other Payables to Broker-Dealers and Clearing Organizations	$ 238.3	$ 27.8	$ 185.3
Securities Held In Consolidated Non Insurance Subsidiary	1,100	1,300	1,000
Fixed maturities
Schedule of Available-for-sale Securities [Line Items]
Cost	18,583.1	16,287.1	13,708
Gross Unrealized Gains	122.7	76.5	232.9
Gross Unrealized Losses	(47.9)	(121.6)	(26.8)
Net Realized Gains (Losses)	2.1	1.8	2.8
Fair Value	$ 18,660	$ 16,243.8	$ 13,916.9
% of Total Fair Value	69.10%	69.20%	59.20%
Fixed maturities | U.S. government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 4,612.2	$ 2,899.2	$ 726
Gross Unrealized Gains	3	0	8.9
Gross Unrealized Losses	(20.2)	(29.1)	(0.3)
Net Realized Gains (Losses)	0	0	0
Fair Value	$ 4,595	$ 2,870.1	$ 734.6
% of Total Fair Value	17.00%	12.20%	3.10%
Fixed maturities | State and local government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 2,332.2	$ 2,509.5	$ 2,508.9
Gross Unrealized Gains	35	13.8	58.7
Gross Unrealized Losses	(3.1)	(20.7)	(1.6)
Net Realized Gains (Losses)	0.1	0	0
Fair Value	$ 2,364.2	$ 2,502.6	$ 2,566
% of Total Fair Value	8.70%	10.70%	10.90%
Fixed maturities | Foreign government obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 24.2	$ 24.5	$ 25.4
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Realized Gains (Losses)	0	0	0
Fair Value	$ 24.2	$ 24.5	$ 25.4
% of Total Fair Value	0.10%	0.10%	0.10%
Fixed maturities | Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 5,195.7	$ 4,557.8	$ 4,327.5
Gross Unrealized Gains	32.7	17.3	76.9
Gross Unrealized Losses	(4.2)	(24.3)	(1.8)
Net Realized Gains (Losses)	1.4	0.1	0.7
Fair Value	$ 5,225.6	$ 4,550.9	$ 4,403.3
% of Total Fair Value	19.40%	19.40%	18.70%
Fixed maturities | Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 911.3	$ 1,448.5	$ 1,582.6
Gross Unrealized Gains	12.2	23.7	25.3
Gross Unrealized Losses	(3.2)	(15)	(15.5)
Net Realized Gains (Losses)	0	1.5	1.8
Fair Value	$ 920.3	$ 1,458.7	$ 1,594.2
% of Total Fair Value	3.40%	6.20%	6.80%
Fixed maturities | Agency residential pass-through obligations
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 35.7	$ 41.2	$ 43.6
Gross Unrealized Gains	0	0	0.5
Gross Unrealized Losses	(0.4)	(0.6)	0
Net Realized Gains (Losses)	0	0	0
Fair Value	$ 35.3	$ 40.6	$ 44.1
% of Total Fair Value	0.10%	0.20%	0.20%
Fixed maturities | Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 2,763.7	$ 2,266.9	$ 2,249.4
Gross Unrealized Gains	17	12	35.1
Gross Unrealized Losses	(12.8)	(25.5)	(6.1)
Net Realized Gains (Losses)	0	0	0
Fair Value	$ 2,767.9	$ 2,253.4	$ 2,278.4
% of Total Fair Value	10.30%	9.60%	9.70%
Fixed maturities | Other asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 2,485.6	$ 2,350.7	$ 2,037.5
Gross Unrealized Gains	6.4	4.6	7.9
Gross Unrealized Losses	(2)	(4.4)	(0.3)
Net Realized Gains (Losses)	0.2	0.2	0.3
Fair Value	$ 2,490.2	$ 2,351.1	$ 2,045.4
% of Total Fair Value	9.20%	10.00%	8.70%
Fixed maturities | Redeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 222.5	$ 188.8	$ 207.1
Gross Unrealized Gains	16.4	5.1	19.6
Gross Unrealized Losses	(2)	(2)	(1.2)
Net Realized Gains (Losses)	0.4	0	0
Fair Value	$ 237.3	$ 191.9	$ 225.5
% of Total Fair Value	0.90%	0.80%	1.00%
Equity securities | Nonredeemable preferred stocks
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 700.6	$ 734.2	$ 726.6
Gross Unrealized Gains	120.4	135.4	146.2
Gross Unrealized Losses	(7.3)	(16.1)	(13.9)
Net Realized Gains (Losses)	0	0	1.2
Fair Value	$ 813.7	$ 853.5	$ 860.1
% of Total Fair Value	3.00%	3.60%	3.60%
Equity securities | Common equities
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 1,485.5	$ 1,437.5	$ 1,576.1
Gross Unrealized Gains	1,729.5	1,377	1,296.3
Gross Unrealized Losses	(5.5)	(2.1)	(4.4)
Net Realized Gains (Losses)	0	0	0
Fair Value	$ 3,209.5	$ 2,812.4	$ 2,868
% of Total Fair Value	11.90%	12.00%	12.20%
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Cost	$ 4,311.5	$ 3,572.9	$ 5,876.2
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Net Realized Gains (Losses)	0	0	0
Fair Value	$ 4,311.5	$ 3,572.9	$ 5,876.2
% of Total Fair Value	16.00%	15.20%	25.00%

Investments Hybrid Securities (

Investments Hybrid Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Hybrid Securities [Line Items]
Hybrid securities	$ 159.9	$ 219.5	$ 229.6
Fixed maturities
Hybrid Securities [Line Items]
Hybrid securities	159.9	219.5	216
Fixed maturities | State and local government obligations
Hybrid Securities [Line Items]
Hybrid securities	2.4	0	0
Fixed maturities | Corporate debt securities
Hybrid Securities [Line Items]
Hybrid securities	114.9	40.1	31.2
Fixed maturities | Residential mortgage-backed securities
Hybrid Securities [Line Items]
Hybrid securities	0	170.5	175.3
Fixed maturities | Other asset-backed securities
Hybrid Securities [Line Items]
Hybrid securities	7.1	8.9	9.5
Fixed maturities | Redeemable preferred stocks
Hybrid Securities [Line Items]
Hybrid securities	35.5	0	0
Equity securities | Nonredeemable preferred stocks
Hybrid Securities [Line Items]
Hybrid securities	$ 0	$ 0	$ 13.6

Investments Composition of Fixe

Investments Composition of Fixed Maturities by Maturity (Detail) $ in Millions	Sep. 30, 2017USD ($)
Available for sale, cost
Less than one year	$ 4,088.1
One to five years	11,476.9
Five to ten years	2,873.9
Ten years or greater	144.2
Total	18,583.1
Available for sale, Fair value
Less than one year	4,105.7
One to five years	11,508.8
Five to ten years	2,901.3
Ten years or greater	144.2
Total	$ 18,660

Investments Gross Unrealized lo

Investments Gross Unrealized losses by Major Security (Detail) $ in Millions	Sep. 30, 2017USD ($)security	Dec. 31, 2016USD ($)security	Sep. 30, 2016USD ($)security
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	852	1,369	554
Total Fair Value	$ 9,407.6	$ 11,175.9	$ 3,507.1
Gross Unrealized Losses	$ (60.7)	$ (139.8)	$ (45.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	383	1,157	313
Less than 12 Months Fair Value	$ 5,172.9	$ 9,627.3	$ 1,640.2
Less than 12 Months Unrealized Losses	$ (19.5)	$ (106.7)	$ (8.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	469	212	241
12 Months or Greater Fair Value	$ 4,234.7	$ 1,548.6	$ 1,866.9
12 Months or Greater Unrealized Losses	$ (41.2)	$ (33.1)	$ (36.4)
Fixed maturities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	782	1,281	461
Total Fair Value	$ 9,283.2	$ 10,824.2	$ 3,266.2
Gross Unrealized Losses	$ (47.9)	$ (121.6)	$ (26.8)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	321	1,080	229
Less than 12 Months Fair Value	$ 5,123.6	$ 9,432.4	$ 1,580.8
Less than 12 Months Unrealized Losses	$ (14.6)	$ (101.2)	$ (4.3)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	461	201	232
12 Months or Greater Fair Value	$ 4,159.6	$ 1,391.8	$ 1,685.4
12 Months or Greater Unrealized Losses	$ (33.3)	$ (20.4)	$ (22.5)
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	54	30	4
Total Fair Value	$ 3,804	$ 2,774	$ 104.8
Gross Unrealized Losses	$ (20.2)	$ (29.1)	$ (0.3)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	31	30	4
Less than 12 Months Fair Value	$ 2,302.2	$ 2,774	$ 104.8
Less than 12 Months Unrealized Losses	$ (7.1)	$ (29.1)	$ (0.3)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	23	0	0
12 Months or Greater Fair Value	$ 1,501.8	$ 0	$ 0
12 Months or Greater Unrealized Losses	$ (13.1)	$ 0	$ 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	169	618	151
Total Fair Value	$ 610.4	$ 1,497.9	$ 512
Gross Unrealized Losses	$ (3.1)	$ (20.7)	$ (1.6)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	40	584	117
Less than 12 Months Fair Value	$ 141	$ 1,404.3	$ 409.6
Less than 12 Months Unrealized Losses	$ (0.7)	$ (19.6)	$ (1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	129	34	34
12 Months or Greater Fair Value	$ 469.4	$ 93.6	$ 102.4
12 Months or Greater Unrealized Losses	$ (2.4)	$ (1.1)	$ (0.6)
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	113	184	52
Total Fair Value	$ 1,523.8	$ 2,615.1	$ 598.3
Gross Unrealized Losses	$ (4.2)	$ (24.3)	$ (1.8)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	59	175	42
Less than 12 Months Fair Value	$ 636	$ 2,559.9	$ 541.5
Less than 12 Months Unrealized Losses	$ (1.1)	$ (24)	$ (0.8)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	54	9	10
12 Months or Greater Fair Value	$ 887.8	$ 55.2	$ 56.8
12 Months or Greater Unrealized Losses	$ (3.1)	$ (0.3)	$ (1)
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	126	178	138
Total Fair Value	$ 308.8	$ 917.7	$ 915.8
Gross Unrealized Losses	$ (3.2)	$ (15)	$ (15.5)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	14	69	26
Less than 12 Months Fair Value	$ 37.4	$ 175.8	$ 108.8
Less than 12 Months Unrealized Losses	$ (0.1)	$ (1.1)	$ (0.4)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	112	109	112
12 Months or Greater Fair Value	$ 271.4	$ 741.9	$ 807
12 Months or Greater Unrealized Losses	$ (3.1)	$ (13.9)	$ (15.1)
Fixed maturities | Agency residential pass-through obligations
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	65	55	15
Total Fair Value	$ 32.7	$ 36	$ 4.2
Gross Unrealized Losses	$ (0.4)	$ (0.6)	$ 0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	12	48	7
Less than 12 Months Fair Value	$ 1.8	$ 33.9	$ 1.9
Less than 12 Months Unrealized Losses	$ 0	$ (0.6)	$ 0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	53	7	8
12 Months or Greater Fair Value	$ 30.9	$ 2.1	$ 2.3
12 Months or Greater Unrealized Losses	$ (0.4)	$ 0	$ 0
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	56
Total Fair Value	$ 1,588.9	$ 1,347.3	$ 610.6
Gross Unrealized Losses	$ (12.8)	$ (25.5)	$ (6.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	61	85	17
Less than 12 Months Fair Value	$ 1,073.5	$ 1,061.2	$ 194.7
Less than 12 Months Unrealized Losses	$ (4.6)	$ (22.9)	$ (1.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	44	26	39
12 Months or Greater Fair Value	$ 515.4	$ 286.1	$ 415.9
12 Months or Greater Unrealized Losses	$ (8.2)	$ (2.6)	$ (4.4)
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	148	103	43
Total Fair Value	$ 1,393.2	$ 1,605.2	$ 488.7
Gross Unrealized Losses	$ (2)	$ (4.4)	$ (0.3)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	103	89	16
Less than 12 Months Fair Value	$ 920.8	$ 1,423.3	$ 219.5
Less than 12 Months Unrealized Losses	$ (1)	$ (3.9)	$ (0.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	45	14	27
12 Months or Greater Fair Value	$ 472.4	$ 181.9	$ 269.2
12 Months or Greater Unrealized Losses	$ (1)	$ (0.5)	$ (0.2)
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	2
Total Fair Value	$ 21.4	$ 31	$ 31.8
Gross Unrealized Losses	$ (2)	$ (2)	$ (1.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	1	0	0
Less than 12 Months Fair Value	$ 10.9	$ 0	$ 0
Less than 12 Months Unrealized Losses	$ 0	$ 0	$ 0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	1	2	2
12 Months or Greater Fair Value	$ 10.5	$ 31	$ 31.8
12 Months or Greater Unrealized Losses	$ (2)	$ (2)	$ (1.2)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	70	88	93
Total Fair Value	$ 124.4	$ 351.7	$ 240.9
Gross Unrealized Losses	$ (12.8)	$ (18.2)	$ (18.3)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	62	77	84
Less than 12 Months Fair Value	$ 49.3	$ 194.9	$ 59.4
Less than 12 Months Unrealized Losses	$ (4.9)	$ (5.5)	$ (4.4)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	8	11	9
12 Months or Greater Fair Value	$ 75.1	$ 156.8	$ 181.5
12 Months or Greater Unrealized Losses	$ (7.9)	$ (12.7)	$ (13.9)
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	3	13	7
Total Fair Value	$ 72.4	$ 329.6	$ 187.5
Gross Unrealized Losses	$ (7.3)	$ (16.1)	$ (13.9)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	0	8	1
Less than 12 Months Fair Value	$ 0	$ 175.2	$ 9.9
Less than 12 Months Unrealized Losses	$ 0	$ (3.8)	$ (0.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	3	5	6
12 Months or Greater Fair Value	$ 72.4	$ 154.4	$ 177.6
12 Months or Greater Unrealized Losses	$ (7.3)	$ (12.3)	$ (13.7)
Equity securities | Common equities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	67	75	86
Total Fair Value	$ 52	$ 22.1	$ 53.4
Gross Unrealized Losses	$ (5.5)	$ (2.1)	$ (4.4)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	62	69	83
Less than 12 Months Fair Value	$ 49.3	$ 19.7	$ 49.5
Less than 12 Months Unrealized Losses	$ (4.9)	$ (1.7)	$ (4.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	5	6	3
12 Months or Greater Fair Value	$ 2.7	$ 2.4	$ 3.9
12 Months or Greater Unrealized Losses	$ (0.6)	$ (0.4)	$ (0.2)

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	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Other Than Temporary Impairment Non Credit Losses Recognized In Accumulated Other Comprehensive Income [Line Items]
Non-credit portion of other-than-temporary impairments recognized in accumulated other comprehensive income	$ (20.1)	$ (43.9)	$ (43.9)
Residential mortgage-backed securities
Other Than Temporary Impairment Non Credit Losses Recognized In Accumulated Other Comprehensive Income [Line Items]
Non-credit portion of other-than-temporary impairments recognized in accumulated other comprehensive income	(19.7)	(43.3)	(43.3)
Commercial mortgage-backed securities
Other Than Temporary Impairment Non Credit Losses Recognized In Accumulated Other Comprehensive Income [Line Items]
Non-credit portion of other-than-temporary impairments recognized in accumulated other comprehensive income	$ (0.4)	$ (0.6)	$ (0.6)

Investments OTTI Credit Losses

Investments OTTI Credit Losses Recognized in Earnings (Detail) - Fixed maturities - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	$ 0.3	$ 12.2	$ 11.5	$ 12.8
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	0.4	0	0.4	0
Reductions for securities sold/matured	0	0	(11.2)	0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0	(0.3)	0	(0.9)
Ending balance	0.7	11.9	0.7	11.9
Residential mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	0.2	11.8	11.1	12.4
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	0	0	0	0
Reductions for securities sold/matured	0	0	(10.9)	0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0	(0.3)	0	(0.9)
Ending balance	0.2	11.5	0.2	11.5
Commercial mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	0.1	0.4	0.4	0.4
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	0.4	0	0.4	0
Reductions for securities sold/matured	0	0	(0.3)	0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0	0	0	0
Ending balance	$ 0.5	$ 0.4	$ 0.5	$ 0.4

Investments Components of Net R

Investments Components of Net Realized Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	$ 29.2	$ 47.7	$ 141.5	$ 147.5
Gross realized losses on securities sales	(10.1)	(8.2)	(25.9)	(23.9)
Net realized gains (losses) on securities sales	19.1	39.5	115.6	123.6
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales, Available for Sale Securities	(9.3)	(26.8)	(12.9)	(27)
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales	(43)	(61.6)	(57.8)	(61.8)
Other gains (losses)	(0.8)	1.4	1.5	(32.8)
Total net realized gains (losses) on securities	(24.7)	(20.7)	59.3	29
Fixed maturities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	20.5	28.3	63.9	92.2
Gross realized losses on securities sales	(9.8)	(7.5)	(19.7)	(15.5)
Net realized gains (losses) on securities sales	10.7	20.8	44.2	76.7
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales, Available for Sale Securities	(0.4)	(25.4)	(0.4)	(25.4)
Fixed maturities | U.S. government obligations
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.9	6.4	5.8	24.1
Gross realized losses on securities sales	(1)	(0.8)	(4.6)	(1.2)
Net realized gains (losses) on securities sales	(0.1)	5.6	1.2	22.9
Fixed maturities | State and local government obligations
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	4	0.6	7.1	16
Gross realized losses on securities sales	0	0	(0.1)	(1.6)
Net realized gains (losses) on securities sales	4	0.6	7	14.4
Fixed maturities | Corporate and other debt securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	5.1	15.6	16.5	38.1
Gross realized losses on securities sales	(1.8)	(0.2)	(4.6)	(1.9)
Net realized gains (losses) on securities sales	3.3	15.4	11.9	36.2
Fixed maturities | Residential mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	2.8	0.2	23.8	1.9
Gross realized losses on securities sales	(0.1)	0	(0.4)	0
Net realized gains (losses) on securities sales	2.7	0.2	23.4	1.9
Fixed maturities | Agency residential pass-through obligations
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0	0	0	0.1
Gross realized losses on securities sales	0	0	0	(0.2)
Net realized gains (losses) on securities sales	0	0	0	(0.1)
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0	5.5	2.4	12
Gross realized losses on securities sales	(0.5)	0	(3.6)	(4.1)
Net realized gains (losses) on securities sales	(0.5)	5.5	(1.2)	7.9
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales, Available for Sale Securities	(0.4)	0	(0.4)	0
Fixed maturities | Other asset-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0	0	0.3	0
Net realized gains (losses) on securities sales	0	0	0.3	0
Fixed maturities | Redeemable preferred stocks
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	7.7	0	8	0
Gross realized losses on securities sales	(6.4)	(6.5)	(6.4)	(6.5)
Net realized gains (losses) on securities sales	1.3	(6.5)	1.6	(6.5)
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales, Available for Sale Securities	0	(25.4)	0	(25.4)
Equity securities | Nonredeemable preferred stocks
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	3	4.9	54.6	11.9
Gross realized losses on securities sales	(0.1)	(0.4)	(5.9)	(3.1)
Net realized gains (losses) on securities sales	2.9	4.5	48.7	8.8
Equity securities | Common equities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	5.7	14.4	23	43.3
Gross realized losses on securities sales	(0.2)	(0.3)	(0.3)	(5.3)
Net realized gains (losses) on securities sales	5.5	14.1	22.7	38
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales, Available for Sale Securities	(8.9)	(1.4)	(12.5)	(1.6)
Other Assets
Gain (Loss) on Investments [Line Items]
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales	(33.7)	(34.8)	(44.9)	(34.8)
Short-term investments
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0	0.1	0	0.1
Net realized gains (losses) on securities sales	0	0.1	0	0.1
Hybrid and Derivative Instruments and Litigation Settlements | Litigation Settlements
Gain (Loss) on Investments [Line Items]
Other gains (losses)	0.1	0.1	1.2	0.1
Hybrid and Derivative Instruments and Litigation Settlements | Hybrid Preferred Stock
Gain (Loss) on Investments [Line Items]
Other gains (losses)	(0.9)	0.6	0.3	2.9
Hybrid and Derivative Instruments and Litigation Settlements | Derivative Instruments
Gain (Loss) on Investments [Line Items]
Other gains (losses)	$ 0	$ 0.7	$ 0	$ (35.8)

Investments Components of Net I

Investments Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net Investment Income [Line Items]
Investment income	$ 142.9	$ 119.3	$ 410.9	$ 352.7
Investment expenses	(5.8)	(4.8)	(18)	(14.9)
Net Investment Income	137.1	114.5	392.9	337.8
Fixed maturities
Net Investment Income [Line Items]
Investment income	106.5	87.7	308.7	262.5
Fixed maturities | U.S. government obligations
Net Investment Income [Line Items]
Investment income	18.4	3.3	48.1	12.4
Fixed maturities | State and local government obligations
Net Investment Income [Line Items]
Investment income	12.9	12.6	39.3	39.3
Fixed maturities | Foreign government obligations
Net Investment Income [Line Items]
Investment income	0.1	0.1	0.3	0.3
Fixed maturities | Corporate debt securities
Net Investment Income [Line Items]
Investment income	32.2	27.3	93.1	81.9
Fixed maturities | Residential mortgage-backed securities
Net Investment Income [Line Items]
Investment income	7.8	11.1	27.3	34.9
Fixed maturities | Agency residential pass-through obligations
Net Investment Income [Line Items]
Investment income	0.2	0.3	0.6	0.9
Fixed maturities | Commercial mortgage-backed securities
Net Investment Income [Line Items]
Investment income	20	22.4	57.1	62.6
Fixed maturities | Other asset-backed securities
Net Investment Income [Line Items]
Investment income	12.1	6.8	33.8	18.7
Fixed maturities | Redeemable preferred stocks
Net Investment Income [Line Items]
Investment income	2.8	3.8	9.1	11.5
Equity securities | Nonredeemable preferred stocks
Net Investment Income [Line Items]
Investment income	10.9	12.4	33	36.8
Equity securities | Common equities
Net Investment Income [Line Items]
Investment income	14.9	13.9	42.8	42
Short-term investments
Net Investment Income [Line Items]
Investment income	$ 10.6	$ 5.3	$ 26.4	$ 11.4

Investments Derivative Instrume

Investments Derivative Instruments (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Aug. 25, 2016
Derivative [Line Items]
Balance Sheet - Fair Value	$ 0	$ (13,000,000)	$ 0	$ (13,000,000)	$ 0
Income Statement - Net Realized Gain (Loss) on Securities	0	700,000	0	(35,800,000)
Designated as Hedging Instruments | Ineffective Cash Flow Hedge [Member]
Derivative [Line Items]
Derivative, Notional Amount	31,000,000	370,000,000	$ 31,000,000	$ 370,000,000	$ 370,000,000	$ 350,000,000
Purpose			Manage  interest  rate risk	Manage interest rate risk	Manage interest rate risk
Balance Sheet - Fair Value	0	0	$ 0	$ 0	$ 0
Income Statement - Net Realized Gain (Loss) on Securities	0	(1,400,000)	0	(1,300,000)
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Derivative [Line Items]
Balance Sheet - Fair Value	0		0		0
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member] | Other Liabilities [Member]
Derivative [Line Items]
Derivative, Notional Amount	0	315,000,000	0	$ 315,000,000	0
Purpose				Manage  portfolio duration
Balance Sheet - Fair Value	0	(13,000,000)	0	$ (13,000,000)	0
Income Statement - Net Realized Gain (Loss) on Securities	0	1,700,000	0	(17,800,000)
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member] | Closed Positions [Member]
Derivative [Line Items]
Derivative, Notional Amount	0	435,000,000	0	$ 435,000,000	$ 750,000,000
Purpose				Manage  portfolio duration	Manage portfolio duration
Balance Sheet - Fair Value	0	0	0	$ 0	$ 0
Income Statement - Net Realized Gain (Loss) on Securities	0	300,000	0	(17,000,000)
Not Designated as Hedging Instrument [Member] | Future [Member]
Derivative [Line Items]
Balance Sheet - Fair Value	0	0	0	0	0
Not Designated as Hedging Instrument [Member] | Future [Member] | Closed Positions [Member]
Derivative [Line Items]
Derivative, Notional Amount	0	135,000,000	0	$ 135,000,000	$ 135,000,000
Purpose				Manage  portfolio duration	Manage portfolio duration
Balance Sheet - Fair Value	0	0	0	$ 0	$ 0
Income Statement - Net Realized Gain (Loss) on Securities	$ 0	$ 100,000	$ 0	$ 300,000

Investments - Additional Inform

Investments - Additional Information (Detail)	Apr. 03, 2017USD ($)	Sep. 30, 2017USD ($)security	Jun. 30, 2017USD ($)	Sep. 30, 2016USD ($)security	Sep. 30, 2017USD ($)security	Sep. 30, 2016USD ($)security	Dec. 31, 2016USD ($)security	Aug. 25, 2016USD ($)
Schedule of Investments [Line Items]
Proceeds from (Payments for) in Securities Sold under Agreements to Repurchase					$ 0	$ 0
Securities Sold under Agreements to Repurchase		$ 0		$ 0	0	0	$ 0
Open reverse repurchase commitments		0		68,000,000	0	68,000,000	0
Proceeds from Securities Purchased under Agreements to Resell					0
Largest Outstanding Balance of Reverse Repurchase Agreements						$ 265,000,000
RRP Largest Outstanding Balance Number of Days Open						1 day
Reverse Repurchase Agreements, Total Days of Activity						28 days
Reverse Repurchase Agreement Average Daily Balance						$ 122,600,000
Gross unrealized losses		$ 60,700,000		$ 45,100,000	$ 60,700,000	$ 45,100,000	$ 139,800,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security		852		554	852	554	1,369
Trading securities		$ 0		$ 0	$ 0	$ 0	$ 0
Net realized gains (losses) on securities		18,300,000		40,900,000	117,100,000	90,800,000
Gain (Loss) on Derivative Instruments, Net, Pretax		0		(700,000)	0	35,800,000
Balance Sheet - Fair Value		$ 0		(13,000,000)	$ 0	(13,000,000)	0
4.125% Senior Notes due 2047
Schedule of Investments [Line Items]
Debt, face amount	$ 850,000,000		$ 850,000,000
Debt, stated interest rate	4.125%	4.125%	4.125%		4.125%
Unrealized gain (loss) on interest rate cash flow hedges	$ (8,000,000)		$ (8,000,000)
2.45% Senior Notes due 2027
Schedule of Investments [Line Items]
Debt, face amount				$ 500,000,000		$ 500,000,000
Debt, stated interest rate		2.45%		2.45%	2.45%	2.45%
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Schedule of Investments [Line Items]
Debt, stated interest rate		6.70%	6.70%		6.70%
Debt Instrument, Repurchased Face Amount		$ 31,000,000		$ 20,000,000	$ 31,000,000	$ 20,000,000
Fixed Income Securities
Schedule of Investments [Line Items]
Gross unrealized losses		55,200,000			55,200,000
Equity securities
Schedule of Investments [Line Items]
Gross unrealized losses		$ 12,800,000		$ 18,300,000	$ 12,800,000	$ 18,300,000	$ 18,200,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security		70		93	70	93	88
Equity securities | Common equities
Schedule of Investments [Line Items]
Gross unrealized losses		$ 5,500,000		$ 4,400,000	$ 5,500,000	$ 4,400,000	$ 2,100,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security		67		86	67	86	75
Equity securities | Nonredeemable preferred stocks
Schedule of Investments [Line Items]
Gross unrealized losses		$ 7,300,000		$ 13,900,000	$ 7,300,000	$ 13,900,000	$ 16,100,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security		3		7	3	7	13
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Percentage Decline		9.00%			9.00%
Fixed maturities
Schedule of Investments [Line Items]
Gross unrealized losses		$ 47,900,000		$ 26,800,000	$ 47,900,000	$ 26,800,000	$ 121,600,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security		782		461	782	461	1,281
Fixed maturities | Corporate debt securities
Schedule of Investments [Line Items]
Percentage which the issuer receives the security back at once the change of control is triggered		1.00%			1.00%
Realized investment losses					$ 9,600,000
Gross unrealized losses		$ 4,200,000		$ 1,800,000	$ 4,200,000	$ 1,800,000	$ 24,300,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security		113		52	113	52	184
Categories of Investments, Marketable Securities, Trading Securities
Schedule of Investments [Line Items]
Net realized gains (losses) on securities		$ 0		$ 0	$ 0	$ 0
Russell One Thousand | Equity securities | Common equities
Schedule of Investments [Line Items]
Percentage of common stock portfolio		95.00%			95.00%
Managed Equity Strategy | Equity securities | Common equities
Schedule of Investments [Line Items]
Percentage of common stock portfolio		5.00%			5.00%
Designated as Hedging Instruments | Ineffective Cash Flow Hedge [Member]
Schedule of Investments [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax		$ 0		1,400,000	$ 0	1,300,000
Derivative, Notional Amount		31,000,000		370,000,000	31,000,000	370,000,000	$ 370,000,000	$ 350,000,000
Balance Sheet - Fair Value		0		0	0	0	0
Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Schedule of Investments [Line Items]
Balance Sheet - Fair Value		0			0		0
Derivative, Collateral, Right to Reclaim Cash				15,100,000		15,100,000
Not Designated as Hedging Instrument [Member] | Future [Member]
Schedule of Investments [Line Items]
Balance Sheet - Fair Value		0		0	0	0	0
Closed Positions [Member] | Not Designated as Hedging Instrument [Member] | Interest Rate Swap [Member]
Schedule of Investments [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax		0		(300,000)	0	17,000,000
Derivative, Notional Amount		0		435,000,000	0	435,000,000	750,000,000
Balance Sheet - Fair Value		0		0	0	0	0
Closed Positions [Member] | Not Designated as Hedging Instrument [Member] | Future [Member]
Schedule of Investments [Line Items]
Gain (Loss) on Derivative Instruments, Net, Pretax		0		(100,000)	0	(300,000)
Derivative, Notional Amount		0		135,000,000	0	135,000,000	135,000,000
Balance Sheet - Fair Value		$ 0		$ 0	$ 0	$ 0	$ 0

Fair Value Composition of Inves

Fair Value Composition of Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	$ 26,994.7	$ 23,482.6	$ 23,521.2
Long-Term Debt, Fair Value	3,617.9	3,315.8	3,505.2
Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	18,660	16,243.8	13,916.9
Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	12,209	9,948.1	7,729.3
Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	6,213.7	6,103.8	5,962.1
Fixed maturities | U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	4,595	2,870.1	734.6
Fixed maturities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	4,595	2,870.1	734.6
Fixed maturities | State and local government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,364.2	2,502.6	2,566
Fixed maturities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,364.2	2,502.6	2,566
Fixed maturities | Foreign government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	24.2	24.5	25.4
Fixed maturities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	24.2	24.5	25.4
Fixed maturities | Corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	5,225.6	4,550.9	4,403.3
Fixed maturities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	5,225.6	4,550.9	4,403.3
Fixed maturities | Residential mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	920.3	1,458.7	1,594.2
Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	920.3	1,458.7	1,594.2
Fixed maturities | Agency residential pass-through obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	35.3	40.6	44.1
Fixed maturities | Agency residential pass-through obligations | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	35.3	40.6	44.1
Fixed maturities | Commercial mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,767.9	2,253.4	2,278.4
Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,767.9	2,253.4	2,278.4
Fixed maturities | Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,490.2	2,351.1	2,045.4
Fixed maturities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	2,490.2	2,351.1	2,045.4
Fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	237.3	191.9	225.5
Fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	63.8	59.5	95.7
Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	31.9	30.9	30.4
Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	141.6	101.5	99.4
Equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	813.7	853.5	860.1
Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	808.7	853.5	860.1
Equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	5	0	0
Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	3,209.5	2,812.4	2,868
Equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	3,209.2	2,812	2,867.6
Equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.3	0.4	0.4
Total Fixed Maturities and Equity Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	22,683.2	19,909.7	17,645
Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	4,311.5	3,572.9	5,876.2
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	25,080.7	22,031.7	21,886.9
Long-term Debt	3,312.2	3,148.2	3,153.9
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	18,583.1	16,287.1	13,708
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	12,164.3	9,991	7,587.8
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	6,196.3	6,107.3	5,913.1
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]
Available-for-sale Securities	4,612.2	2,899.2	726
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]
Available-for-sale Securities	2,332.2	2,509.5	2,508.9
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]
Available-for-sale Securities	24.2	24.5	25.4
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]
Available-for-sale Securities	5,195.7	4,557.8	4,327.5
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]
Available-for-sale Securities	911.3	1,448.5	1,582.6
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Agency residential pass-through obligations | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	35.7	41.2	43.6
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]
Available-for-sale Securities	2,763.7	2,266.9	2,249.4
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]
Available-for-sale Securities	2,485.6	2,350.7	2,037.5
Carrying (Reported) Amount, Fair Value Disclosure | Fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	222.5	188.8	207.1
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]
Available-for-sale Securities	61.4	59.8	77.1
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]
Available-for-sale Securities	30.5	30.5	30.6
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]
Available-for-sale Securities	130.6	98.5	99.4
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	700.6	734.2	726.6
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]
Available-for-sale Securities	695.6	734.2	726.6
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]
Available-for-sale Securities	5	0	0
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,485.5	1,437.5	1,576.1
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]
Available-for-sale Securities	1,485.2	1,437.1	1,575.7
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]
Available-for-sale Securities	0.3	0.4	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]
Available-for-sale Securities	20,769.2	18,458.8	16,010.7
Carrying (Reported) Amount, Fair Value Disclosure | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	4,311.5	3,572.9	5,876.2
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	11,086	8,854	8,833.7
Long-Term Debt, Fair Value	0	0	0
Fair Value, Inputs, Level 1 | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	4,619.2	2,894.6	760
Fair Value, Inputs, Level 1 | Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	4,619.2	2,894.6	760
Fair Value, Inputs, Level 1 | Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
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]
Available-for-sale Securities	4,595	2,870.1	734.6
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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	24.2	24.5	25.4
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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 1 | Fixed maturities | Agency residential pass-through obligations | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	82.2	138.1	154.6
Fair Value, Inputs, Level 1 | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	82.2	138.1	154.6
Fair Value, Inputs, Level 1 | Equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 1 | Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	3,209.2	2,812	2,867.6
Fair Value, Inputs, Level 1 | Equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	3,209.2	2,812	2,867.6
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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	7,910.6	5,844.7	3,782.2
Fair Value, Inputs, Level 1 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	3,175.4	3,009.3	5,051.5
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	15,903.4	14,627.9	14,686.5
Long-Term Debt, Fair Value	3,574.6	3,188.5	3,371.6
Fair Value, Inputs, Level 2 | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	14,040.8	13,348.9	13,156.3
Fair Value, Inputs, Level 2 | Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	7,589.8	7,053.5	6,969.3
Fair Value, Inputs, Level 2 | Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	6,213.7	6,103.5	5,961.5
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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	2,364.2	2,502.6	2,566
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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	5,225.6	4,550.9	4,403.3
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]
Available-for-sale Securities	920.3	1,458.7	1,594.2
Fair Value, Inputs, Level 2 | Fixed maturities | Agency residential pass-through obligations | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	35.3	40.6	44.1
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]
Available-for-sale Securities	2,767.9	2,253.1	2,277.8
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]
Available-for-sale Securities	2,490.2	2,351.1	2,045.4
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	237.3	191.9	225.5
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	63.8	59.5	95.7
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	31.9	30.9	30.4
Fair Value, Inputs, Level 2 | Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	141.6	101.5	99.4
Fair Value, Inputs, Level 2 | Equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	726.5	715.4	705.5
Fair Value, Inputs, Level 2 | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	726.5	715.4	705.5
Fair Value, Inputs, Level 2 | Equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 2 | Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	14,767.3	14,064.3	13,861.8
Fair Value, Inputs, Level 2 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	1,136.1	563.6	824.7
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	5.3	0.7	1
Long-Term Debt, Fair Value	43.3	127.3	133.6
Fair Value, Inputs, Level 3 | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0.3	0.6
Fair Value, Inputs, Level 3 | Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	0.3	0.6
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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	0	0
Fair Value, Inputs, Level 3 | Fixed maturities | Agency residential pass-through obligations | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	0.3	0.6
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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	5	0	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	5	0	0
Fair Value, Inputs, Level 3 | Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	0.3	0.4	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]
Available-for-sale Securities	0	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]
Available-for-sale Securities	0.3	0.4	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]
Available-for-sale Securities	5.3	0.7	1
Fair Value, Inputs, Level 3 | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale Securities	$ 0	$ 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	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	$ 0.3	$ 9.5	$ 0.7	$ 10.2
Calls/ Maturities/ Paydowns	0	(8.7)	(0.4)	(9.3)
Purchases	5	0	5	0
Sales	0	0	0	0
Net Realized (Gain) Loss on Sales	0	0	0	0
Change in Valuation	0	0.2	0	0.1
Net Transfers in (out)	0	0	0	0
Ending Fair value	5.3	1	5.3	1
Fixed maturities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0	9.2	0.3	9.9
Calls/ Maturities/ Paydowns	0	(8.7)	(0.3)	(9.3)
Purchases	0	0	0	0
Sales	0	0	0	0
Net Realized (Gain) Loss on Sales	0	0	0	0
Change in Valuation	0	0.1	0	0
Net Transfers in (out)	0	0	0	0
Ending Fair value	0	0.6	0	0.6
Equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0	0	0	0
Calls/ Maturities/ Paydowns	0	0	0	0
Purchases	5	0	5	0
Sales	0	0	0	0
Net Realized (Gain) Loss on Sales	0	0	0	0
Change in Valuation	0	0	0	0
Net Transfers in (out)	0	0	0	0
Ending Fair value	5	0	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	0.3	0.3	0.4	0.3
Calls/ Maturities/ Paydowns	0	0	(0.1)	0
Purchases	0	0	0	0
Sales	0	0	0	0
Net Realized (Gain) Loss on Sales	0	0	0	0
Change in Valuation	0	0.1	0	0.1
Net Transfers in (out)	0	0	0	0
Ending Fair value	0.3	0.4	0.3	0.4
Asset-backed Securities | Fixed maturities | Commercial mortgage-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Fair Value	0	9.2	0.3	9.9
Calls/ Maturities/ Paydowns	0	(8.7)	(0.3)	(9.3)
Purchases	0	0	0	0
Sales	0	0	0	0
Net Realized (Gain) Loss on Sales	0	0	0	0
Change in Valuation	0	0.1	0	0
Net Transfers in (out)	0	0	0	0
Ending Fair value	$ 0	$ 0.6	$ 0	$ 0.6

Fair Value Summary of Quantitat

Fair Value Summary of Quantitative Information about Level 3 Fair Value Measurements (Detail)	9 Months Ended		12 Months Ended
	Sep. 30, 2017USD ($)$ / shares	Sep. 30, 2016USD ($)securityInvestment	Dec. 31, 2016USD ($)securityInvestment
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	$ 26,994,700,000	$ 23,521,200,000	$ 23,482,600,000
Fixed maturities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	18,660,000,000	13,916,900,000	16,243,800,000
Fixed maturities | Commercial mortgage-backed securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	2,767,900,000	2,278,400,000	2,253,400,000
Equity securities | Nonredeemable preferred stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	813,700,000	860,100,000	853,500,000
Fair Value, Inputs, Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	5,300,000	1,000,000	700,000
Fair Value, Inputs, Level 3 | Third Party Pricing Exemption Securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	300,000	400,000	400,000
Fair Value, Inputs, Level 3 | Subtotal Level 3 Securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	5,000,000	$ 600,000	$ 300,000
Fair Value, Inputs, Level 3 | Internal Pricing
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Number of securities | Investment		0	0
Fair Value, Inputs, Level 3 | Fixed maturities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	0	$ 600,000	$ 300,000
Fair Value, Inputs, Level 3 | Fixed maturities | External Pricing
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	0	$ 600,000	$ 300,000
Fair Value, Inputs, Level 3 | Fixed maturities | Commercial mortgage-backed securities | External Pricing
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Number of securities | security		1	1
Available-for-sale Securities	0	$ 600,000	$ 300,000
Valuation Technique		External vendor	External vendor
Unobservable Input, prepayment rate		0.00%	0.00%
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	5,000,000	$ 0	$ 0
Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks | Internal Pricing
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Available-for-sale Securities	$ 5,000,000	$ 0	$ 0
Valuation Technique	Internal price
Fair Value Inputs, Original Issue Price | $ / shares	$ 3.9

Fair Value Fair Value - Additio

Fair Value Fair Value - Additional Information (Detail)	Sep. 30, 2017USD ($)Investment	Dec. 31, 2016USD ($)Investment	Sep. 30, 2016USD ($)Investment
Fair Value Measurements [Line Items]
Fair Value	$ 26,994,700,000	$ 23,482,600,000	$ 23,521,200,000
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	$ 0	$ 0	$ 0
Quoted prices percentage of fair value assets	100.00%	100.00%	100.00%
Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Fair Value	$ 11,086,000,000	$ 8,854,000,000	$ 8,833,700,000
Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Fair Value	15,903,400,000	14,627,900,000	14,686,500,000
Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Fair Value	5,300,000	700,000	1,000,000
Fixed maturities
Fair Value Measurements [Line Items]
Fair Value	$ 18,660,000,000	$ 16,243,800,000	$ 13,916,900,000
Quoted prices percentage of fair value assets	69.10%	69.20%	59.20%
Fixed maturities | Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Fair Value	$ 4,619,200,000	$ 2,894,600,000	$ 760,000,000
Fixed maturities | Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Fair Value	14,040,800,000	13,348,900,000	13,156,300,000
Fixed maturities | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Fair Value	$ 0	$ 300,000	$ 600,000
Internal Pricing | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Number of securities | Investment		0	0
Dealer Quoted Pricing | Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Quoted prices percentage of fair value assets	2.00%	1.00%	1.00%
Vendor Quoted Pricing | Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Quoted prices percentage of fair value assets	59.00%	52.00%	24.00%
Vendor Quoted Pricing | Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Quoted prices percentage of fair value assets	98.00%	99.00%	99.00%
Nonredeemable preferred stocks | Equity securities
Fair Value Measurements [Line Items]
Fair Value	$ 813,700,000	$ 853,500,000	$ 860,100,000
Quoted prices percentage of fair value assets	3.00%	3.60%	3.60%
Nonredeemable preferred stocks | Equity securities | Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Fair Value	$ 82,200,000	$ 138,100,000	$ 154,600,000
Nonredeemable preferred stocks | Equity securities | Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Fair Value	726,500,000	715,400,000	705,500,000
Nonredeemable preferred stocks | Equity securities | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Fair Value	5,000,000	0	0
Nonredeemable preferred stocks | Internal Pricing | Equity securities | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Fair Value	5,000,000	$ 0	$ 0
Internal Pricing | Equity securities | Nonredeemable preferred stocks | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Fair Value	$ 5,000,000
Number of securities | Investment	1

Debt (Detail)

Debt (Detail)		Sep. 30, 2017USD ($)instrument	Dec. 31, 2016USD ($)instrument	Sep. 30, 2016USD ($)instrument
Debt Instrument [Line Items]
Fair Value		$ 3,617,900,000	$ 3,315,800,000	$ 3,505,200,000
Debt, Long-term and Short-term, Combined Amount	[1]	3,312,200,000	3,148,200,000	3,153,900,000
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Long-term Debt		498,700,000	498,400,000	498,400,000
Fair Value		525,700,000	528,800,000	544,200,000
2.45% Senior Notes due 2027
Debt Instrument [Line Items]
Long-term Debt		496,000,000	495,800,000	495,600,000
Fair Value		475,300,000	464,600,000	497,500,000
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Long-term Debt		296,000,000	295,900,000	295,800,000
Fair Value		388,300,000	380,100,000	402,600,000
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Long-term Debt		395,300,000	395,200,000	395,100,000
Fair Value		518,700,000	499,000,000	533,300,000
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Long-term Debt		346,500,000	346,400,000	346,400,000
Fair Value		379,800,000	362,300,000	403,000,000
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Long-term Debt		395,200,000	395,100,000	395,100,000
Fair Value		392,600,000	372,500,000	418,700,000
4.125% Senior Notes due 2047
Debt Instrument [Line Items]
Long-term Debt		841,200,000	0	0
Fair Value		894,200,000	0	0
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Long-term Debt		0	594,100,000	593,900,000
Fair Value		0	581,200,000	572,300,000
Other debt instruments
Debt Instrument [Line Items]
Debt, Long-term and Short-term, Combined Amount		$ 43,300,000	$ 127,300,000	$ 133,600,000
Notes Payable, Other Payables | Other debt instruments
Debt Instrument [Line Items]
Number of debt instruments | instrument		2	2	2
Debt, Long-term and Short-term, Combined Amount		$ 43,300,000	$ 62,100,000	$ 68,400,000
Trust Preferred Debt Securities | Other debt instruments
Debt Instrument [Line Items]
Number of debt instruments | instrument		0	2	2
Long-term Debt		$ 0	$ 41,200,000	$ 41,200,000
Senior Notes | Other debt instruments
Debt Instrument [Line Items]
Number of debt instruments | instrument		0	4	4
Long-term Debt		$ 0	$ 24,000,000	$ 24,000,000
[1]	Consists of both short-term and long-term debt. See Note 4 – Debt for further discussion.

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($)	Apr. 03, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Debt Instrument [Line Items]
Repayments of Debt						$ 42,800,000	$ 19,200,000
Repayment of debt						635,600,000	18,200,000
Gains (losses) on extinguishment of debt		$ 0			$ 0	200,000	1,600,000
Discretionary Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 250,000,000
Debt term		30 days
Borrowings outstanding						$ 0		$ 0
Terminated Discretionary Line of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity			100,000,000
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Debt, stated interest rate		3.75%				3.75%
2.45% Senior Notes due 2027
Debt Instrument [Line Items]
Debt, face amount					$ 500,000,000		$ 500,000,000
Debt, stated interest rate		2.45%			2.45%	2.45%	2.45%
Proceeds, net of underwriter's discounts and commissions					$ 495,600,000
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Debt, stated interest rate		6.625%				6.625%
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Debt, stated interest rate		6.25%				6.25%
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Debt, stated interest rate		4.35%				4.35%
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Debt, stated interest rate		3.70%				3.70%
4.125% Senior Notes due 2047
Debt Instrument [Line Items]
Debt, face amount	$ 850,000,000		$ 850,000,000
Debt, stated interest rate	4.125%	4.125%	4.125%			4.125%
Proceeds, net of underwriter's discounts and commissions			$ 841,100,000
Unrealized gain (loss) on interest rate cash flow hedges	$ (8,000,000)		$ (8,000,000)
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Debt, stated interest rate		6.70%	6.70%			6.70%
Repayment of debt			$ 563,700,000	$ 30,900,000				19,800,000
Gains (losses) on extinguishment of debt				$ 200,000				$ 1,600,000
Federal Funds Open Rate | Discretionary Line of Credit
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate						0.50%
Trust Preferred Debt Securities [Member] | Other debt instruments
Debt Instrument [Line Items]
Repayments of Debt		$ 65,200,000
Loans Payable [Member] | Fixed Rate, Five Year Loan [Member]
Debt Instrument [Line Items]
Debt term		5 years

Income Taxes Additional Informa

Income Taxes Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Valuation allowance	$ 0	$ 0	$ 0	$ 0	$ 0
Effective Income Tax Rate Reconciliation, Percent	14.60%	20.50%	29.70%	29.40%

Loss and Loss Adjustment Expe49

Loss and Loss Adjustment Expense Reserves (Details) - USD ($)	6 Months Ended	9 Months Ended
	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves		$ 13,353,300,000	$ 11,228,200,000	$ 11,368,000,000	$ 10,039,000,000
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments and Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item		2,624,700,000	1,834,200,000	1,801,000,000	1,442,700,000
Liability for Unpaid Claims and Claims Adjustment Expense, Net		10,728,600,000	9,394,000,000	9,567,000,000	8,596,300,000
Current Year Claims and Claims Adjustment Expense		13,886,300,000	12,577,500,000
Prior Year Claims and Claims Adjustment Expense		(42,500,000)	22,900,000
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims		13,928,800,000	12,554,600,000
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year		8,379,400,000	7,768,200,000
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years		4,387,800,000	3,986,200,000
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid		12,767,200,000	11,754,400,000
Liability for Unpaid Claims and Claims Adjustment Expense, Business Acquisitions	$ 2,500,000	0	2,500,000
Liability for Unpaid Claims and Claims Adjustment Expense, Net, Adjusted Beginning Balance				$ 9,567,000,000	$ 8,593,800,000
Accident year 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(51,000,000)
Short-duration Insurance Contracts, Accident Years 2014 and Prior [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		8,000,000
Accident year 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			30,000,000
Accident year 2014
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			(26,000,000)
Personal Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			9,000,000
Personal Lines | Personal auto businesses
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(76,000,000)
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		$ 24,000,000	46,000,000
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense			$ (31,000,000)

Supplemental Cash Flow Inform50

Supplemental Cash Flow Information (Detail) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Supplemental Cash Flow Elements [Abstract]
Income Taxes Paid	$ 538.7	$ 380.8
Interest Paid	$ 107.4	$ 98

Segment Information Operating R

Segment Information Operating Results (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Revenues	$ 6,791.8	$ 5,935	$ 19,719.2	$ 17,311.8
Pretax Profit (Loss)	251.4	258.5	1,447.5	933.2
Amortization of intangible assets	17.2	15.5	48.2	46.6
Personal Lines | Agency | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	2,840	2,474.3	8,224	7,245.5
Pretax Profit (Loss)	69.5	98.6	524.7	354.5
Personal Lines | Direct | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	2,734.8	2,391.2	7,908.5	6,946.7
Pretax Profit (Loss)	128.7	84.4	466.7	237.5
Operating Segments | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	6,544	5,723.4	18,884	16,602.6
Pretax Profit (Loss)	172	197	1,100	659.4
Operating Segments | Personal Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	5,574.8	4,865.5	16,132.5	14,192.2
Pretax Profit (Loss)	$ 198.2	$ 183	$ 991.4	$ 592
Personal auto insurance percentage of the total personal lines segment net premiums earned	93.00%	92.00%	93.00%	92.00%
Operating Segments | Commercial Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	$ 714	$ 630.2	$ 2,031.2	$ 1,772.4
Pretax Profit (Loss)	42.8	4.4	166.4	96.7
Operating Segments | Property | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	255.2	227.7	720.3	638
Pretax Profit (Loss)	(69)	10.4	(57.5)	(27.7)
Amortization of intangible assets	17.2	15.5	48.2	46.6
Operating Segments | Other Indemnity | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Pretax Profit (Loss)	0	(0.8)	(0.3)	(1.6)
Segment Reconciling Items | Fees and other revenues
Segment Reporting Information [Line Items]
Revenues	96.3	86.8	270.3	248.2
Segment Reconciling Items | Service businesses
Segment Reporting Information [Line Items]
Revenues	33.3	26.2	94.5	77.7
Pretax Profit (Loss)	4.4	3	12.7	9.2
Segment Reconciling Items | Investment income
Segment Reporting Information [Line Items]
Revenues	118.2	98.6	470.2	381.7
Pretax Profit (Loss)	112.4	93.8	452.2	366.8
Segment Reconciling Items | Gains on extinguishment of debt
Segment Reporting Information [Line Items]
Revenues	0	0	0.2	1.6
Pretax Profit (Loss)	0	0	0.2	1.6
Segment Reconciling Items | Interest expense
Segment Reporting Information [Line Items]
Pretax Profit (Loss)	$ (37.4)	$ (35.3)	$ (117.6)	$ (103.8)

Segment Information Underwritin

Segment Information Underwriting Margins and Combined Ratios for our Underwriting Operations (Detail) - Operating Segments - Underwriting operations	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Underwriting Margin	2.60%	3.40%	5.80%	4.00%
Combined Ratio	97.40%	96.60%	94.20%	96.00%
Personal Lines
Segment Reporting Information [Line Items]
Underwriting Margin	3.60%	3.80%	6.10%	4.20%
Combined Ratio	96.40%	96.20%	93.90%	95.80%
Personal Lines | Agency
Segment Reporting Information [Line Items]
Underwriting Margin	2.40%	4.00%	6.40%	4.90%
Combined Ratio	97.60%	96.00%	93.60%	95.10%
Personal Lines | Direct
Segment Reporting Information [Line Items]
Underwriting Margin	4.70%	3.50%	5.90%	3.40%
Combined Ratio	95.30%	96.50%	94.10%	96.60%
Commercial Lines
Segment Reporting Information [Line Items]
Underwriting Margin	6.00%	0.70%	8.20%	5.50%
Combined Ratio	94.00%	99.30%	91.80%	94.50%
Property
Segment Reporting Information [Line Items]
Underwriting Margin	(27.00%)	4.60%	(8.00%)	(4.30%)
Combined Ratio	127.00%	95.40%	108.00%	104.30%
Combined ratio amortization points	0.067	0.068	0.067	0.073
Combined ratio exchange loss				0.007

Dividends Dividends (Details)

Dividends Dividends (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
	Feb. 28, 2017	Feb. 29, 2016	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Payments of Ordinary Dividends, Common Stock	$ 395.4	$ 519
Dividends payable			$ 0	$ 395.4	$ 0	$ 519.2
Annual variable dividend [Member]
Common Stock, Dividends, Per Share, Cash Paid	$ 0.6808	$ 0.8882

Dividends - Additional Informat

Dividends - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2017USD ($)	Dec. 31, 2016
Dividends [Abstract]
Target percentage used to determine annual variable dividend	33.33%	33.33%
Gainshare factor used to determine annual variable dividend range, minimum	0
Minimum Payments of Ordinary Dividends, Common Stock	$ 0
Gainshare factor used to determine annual variable dividend range, maximum	2
Gainshare factor used to determine annual variable dividend	1.67

Other Comprehensive Income (L55

Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
AOCI before Tax, Attributable to Parent, beginning balance	$ 1,774.1	$ 1,477	$ 1,439.5	$ 1,234.5	$ 1,234.5
Investment securities, pre tax	122.7	146.8	554.4	470.4
Net non-credit related OTTI losses, adjusted for valuation changes, pre tax	0	0	0	(0.1)
Forecasted transactions, pre tax	0	0	(8)	0
Foreign currency translation adjustment, pre tax	0.9	0.4	1.2	1
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Noncontrolling Interest	(1.1)	1.8	(4.5)	(3.7)
Total other comprehensive income (loss) before reclassifications, pre tax	122.5	149	543.1	467.6
Total reclassification adjustment for amounts realized in net income, pre tax	6.2	13.3	92.2	89.4
Total other comprehensive income (loss), pre tax	116.3	135.7	450.9	378.2
AOCI before Tax, Attributable to Parent, ending balance	1,890.4	1,612.7	1,890.4	1,612.7	1,439.5
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
AOCI Tax, Attributable to Parent, beginning balance	(623.4)	(519.2)	(506.1)	(434.1)	(434.1)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax	(43)	(51.4)	(194.3)	(165.2)
Net non-credit related OTTI losses, adjusted for valuation changes, Total tax (provision) benefit	0	0	0	(0.1)
Forecasted transactions, Total tax (provision) benefit	0	0	2.8	0
Foreign currency translation adjustment, Total tax (provision) benefit	(0.3)	(0.1)	(0.4)	(0.3)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Noncontrolling Interest	0.4	(0.6)	1.6	1.4
Other Comprehensive Income (Loss), Available-for-sale Securities, before Reclassification Adjustments, Tax	(42.9)	(52.1)	(190.3)	(164)
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	(2.1)	(4.7)	(32.2)	(31.5)
Total other comprehensive income (loss), Total tax (provision) benefit	(40.8)	(47.4)	(158.1)	(132.5)
AOCI Tax, Attributable to Parent, ending balance	(664.2)	(566.6)	(664.2)	(566.6)	(506.1)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	1,150.7	957.8	933.4	800.4	800.4
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	79.7	95.4	360.1	305.2
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, Net of Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	0	0	0	0
Forecasted transactions, after tax	0	0	(5.2)	0
Foreign currency translation adjustment, after tax	0.6	0.3	0.8	0.7
Other comprehensive (income) loss attributable to NCI	(0.7)	1.2	(2.9)	(2.3)	3.2
Total other comprehensive income (loss) before reclassifications, after tax	79.6	96.9	352.8	303.6
Total reclassification adjustment for amounts realized in net income, after tax	4.1	8.6	60	57.9
Other comprehensive income (loss)	75.5	88.3	292.8	245.7
After tax total accumulated other comprehensive income, ending balance	1,226.2	1,046.1	1,226.2	1,046.1	933.4
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, before Tax	9.3	26.8	12.9	27
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Tax	3.3	9.4	4.6	9.5
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	6	17.4	8.3	17.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	15.7	39.6	104.5	114.9
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	(5.5)	(13.9)	(36.6)	(40.4)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	10.2	25.7	67.9	74.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	(0.2)	0.5	0.6	1.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	0.1	(0.2)	(0.2)	(0.6)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.1)	0.3	0.4	0.9
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	1,164.6	970.1	939.6	809	809
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	79.7	95.4	360.1	305.2
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, Net of Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	0	0	0	0
Forecasted transactions, after tax	0	0	0	0
Foreign currency translation adjustment, after tax	0	0	0	0
Other comprehensive (income) loss attributable to NCI	0	0	0	0
Total other comprehensive income (loss) before reclassifications, after tax	79.7	95.4	360.1	305.2
Total reclassification adjustment for amounts realized in net income, after tax	4.2	8.3	59.6	57
Other comprehensive income (loss)	75.5	87.1	300.5	248.2
After tax total accumulated other comprehensive income, ending balance	1,240.1	1,057.2	1,240.1	1,057.2	939.6
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	6	17.4	8.3	17.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	10.2	25.7	67.9	74.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0	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	(15.1)	(8.8)	(9.4)	(8.2)	(8.2)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	0	0	0	0
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, Net of Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	0	0	0	0
Forecasted transactions, after tax	0	0	(5.2)	0
Foreign currency translation adjustment, after tax	0	0	0	0
Other comprehensive (income) loss attributable to NCI	0	0	0	0
Total other comprehensive income (loss) before reclassifications, after tax	0	0	(5.2)	0
Total reclassification adjustment for amounts realized in net income, after tax	(0.1)	0.3	0.4	0.9
Other comprehensive income (loss)	0.1	(0.3)	(5.6)	(0.9)
After tax total accumulated other comprehensive income, ending balance	(15)	(9.1)	(15)	(9.1)	(9.4)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0	0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.1)	0.3	0.4	0.9
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(0.9)	(1.1)	(1.1)	(1.5)	(1.5)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	0	0	0	0
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, Net of Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	0	0	0	0
Forecasted transactions, after tax	0	0	0	0
Foreign currency translation adjustment, after tax	0.6	0.3	0.8	0.7
Other comprehensive (income) loss attributable to NCI	0	0	0	0
Total other comprehensive income (loss) before reclassifications, after tax	0.6	0.3	0.8	0.7
Total reclassification adjustment for amounts realized in net income, after tax	0	0	0	0
Other comprehensive income (loss)	0.6	0.3	0.8	0.7
After tax total accumulated other comprehensive income, ending balance	(0.3)	(0.8)	(0.3)	(0.8)	(1.1)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0	0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0	0	0	0
Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	2.1	(2.4)	4.3	1.1	1.1
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	0	0	0	0
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, Net of Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	0	0	0	0
Forecasted transactions, after tax	0	0	0	0
Foreign currency translation adjustment, after tax	0	0	0	0
Other comprehensive (income) loss attributable to NCI	(0.7)	1.2	(2.9)	(2.3)
Total other comprehensive income (loss) before reclassifications, after tax	(0.7)	1.2	(2.9)	(2.3)
Total reclassification adjustment for amounts realized in net income, after tax	0	0	0	0
Other comprehensive income (loss)	(0.7)	1.2	(2.9)	(2.3)
After tax total accumulated other comprehensive income, ending balance	1.4	(1.2)	1.4	(1.2)	$ 4.3
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0	0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	$ 0	$ 0	$ 0	$ 0

Other Comprehensive Income (L56

Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement, Additional Information (Detail) $ in Millions	Sep. 30, 2017USD ($)
Statement of Comprehensive Income [Abstract]
Net unrealized gains on forecasted transactions, expected to reclassify into income within the next 12 months	$ 1

Redeemable Noncontrolling Int57

Redeemable Noncontrolling Interest (Details) $ in Millions		3 Months Ended		9 Months Ended			12 Months Ended
		Sep. 30, 2017USD ($)shares	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)shares		Sep. 30, 2016USD ($)	Dec. 31, 2016USD ($)	Apr. 01, 2021installment	Apr. 01, 2018installment	Apr. 01, 2015installment
Redeemable Noncontrolling Interest [Line Items]
Number of Installments Related to Stockholders Agreement | installment										2
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance, Beginning of period				$ 483.7	[1]	$ 464.9	$ 464.9
Net income attributable to NCI		$ (9.2)	$ 6.8	1.9		11.3	26.2
Other comprehensive income (loss) attributable to NCI		0.7	(1.2)	2.9		2.3	(3.2)
Exercise of employee stock options				3.4		0	0
Change in redemption value of NCI				6.3		(6)	(4.2)
Balance, End of period	[1]	$ 498.2	$ 472.5	$ 498.2		$ 472.5	$ 483.7
ARX Holding Corp. [Member]
Redeemable Noncontrolling Interest [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage		69.00%	69.20%	69.00%		69.20%	69.20%
Shares held in Employee Stock Option Plan, Suspense Shares | shares		22,550		22,550
Employee Stock Ownership Plan (ESOP), Deferred Shares, Fair Value		$ 32.2		$ 32.2
Scenario, Forecast [Member]
Redeemable Noncontrolling Interest [Line Items]
Number of Installments Related to Stockholders Agreement | installment								1	1
Scenario, Forecast [Member] | ARX Holding Corp. [Member]
Redeemable Noncontrolling Interest [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage								100.00%	80.00%
[1]	See Note 12 – Redeemable Noncontrolling Interest for further discussion.

Goodwill and Intangible Asset58

Goodwill and Intangible Assets Goodwill (Details) - USD ($)	3 Months Ended			6 Months Ended	9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2016	Jun. 30, 2017	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Goodwill [Line Items]
Goodwill, Acquired During Period		$ 3,300,000
Goodwill	$ 452,700,000		$ 449,400,000		$ 452,700,000	$ 449,400,000	$ 449,400,000
Goodwill, Impaired, Accumulated Impairment Loss	0				0
Amortization of intangible assets	$ 17,200,000		$ 15,500,000		$ 48,200,000	$ 46,600,000
Weighted Average [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Remaining Amortization Period					5 years 2 months 12 days
Trade name [Member]
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life	2 years			10 years

Goodwill and Intangible Asset59

Goodwill and Intangible Assets - Net Carrying Amount of Other Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Schedule of Other Intangible Assets [Line Items]
Indefinite lived intangible assets	$ 12.4	$ 12.4	$ 12.4
Total	384.6	432.8	448.3
Accumulated amortization	157.7	109.5	94
Licensing Agreements [Member]
Schedule of Other Intangible Assets [Line Items]
Accumulated amortization	$ 0.6	$ 0.6	$ 0.6

Goodwill and Intangible Asset60

Goodwill and Intangible Assets - Finite-lived Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	$ 157.7	$ 109.5	$ 94
Policies in force [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	256.2	256.2	256.2
Accumulated Amortization	91.5	64.1	55
Net Carrying Amount	164.7	192.1	201.2
Agency relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	159.2	159.2	159.2
Accumulated Amortization	28.4	19.9	17.1
Net Carrying Amount	130.8	139.3	142.1
Software rights [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	79.1	79.1	79.1
Accumulated Amortization	26.8	18.8	16.1
Net Carrying Amount	52.3	60.3	63
Trade name [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	34.8	34.8	34.8
Accumulated Amortization	10.4	6.1	5.2
Net Carrying Amount	24.4	28.7	29.6
Finite-Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	529.3	529.3	529.3
Accumulated Amortization	157.1	108.9	93.4
Net Carrying Amount	$ 372.2	$ 420.4	$ 435.9

New Accounting Standards Effect

New Accounting Standards Effect of Standard Change (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Item Effected [Line Items]
Provision (benefit) for income taxes	$ 36.6	$ 53	$ 429.7	$ 274.1
Adjustments for New Accounting Pronouncement
Item Effected [Line Items]
Provision (benefit) for income taxes			$ (23.1)