The Progressive Corporation (PGR) 10-Q 31 Oct 18 2018 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information	9 Months Ended
	Sep. 30, 2018shares
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, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q3
Amendment Flag	false
Entity Common Stock, Shares Outstanding	583,144,753
Trading Symbol	PGR
Entity Small Business	false
Entity Emerging Growth Company	false

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) shares in Millions, $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenues
Net premiums earned		$ 7,930.5	$ 6,544	$ 22,738.7	$ 18,884
Period over prior year period percent change in net premiums earned		21.00%		20.00%
Investment income		$ 218.1	142.9	$ 576.5	410.9
Period over prior year period percent change in investment income		53.00%		40.00%
Net realized gains (losses) on securities:
Gross Realized Gain (Loss), excluding Holding Period and OTTI		$ 108.4		$ 205.8
Net Holding Period Realized Gain (Loss)		95.8		(5.9)
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net		(22.1)	(43)	(33.2)	(57.8)
Total net realized gains (losses) on securities		182.1		166.7
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments			19.2		116.8
Unrealized Gain (Loss) on Hybrid Instrument, Net		$ 1.3	(0.9)	$ (4.2)	0.3
Available-for-sale Securities, Gross Realized Gain (Loss)			(24.7)		59.3
Period Over Prior Year Period, Percent Increase (Decrease), Net Realized Gains (Losses) On Security Sales		465.00%		76.00%
Period Over Prior Year Period, Percent Increase (Decrease), Net Impairment Losses Recognized In Earnings		(49.00%)		(43.00%)
Period Over Prior Year Period, Percent Increase (Decrease), Total Net Realized Gains (Losses) On Securities				181.00%
Fees and other revenues		$ 122.6	96.3	$ 342.4	270.3
Period over prior year period percent change in fees and other revenues		27.00%		27.00%
Service revenues		$ 42.5	33.3	$ 119.6	94.5
Period over prior year period percent change in service revenues		28.00%		27.00%
Other gains (losses)		$ 0	0	$ 0	0.2
Period Over Prior Year Period, Percent Increase (Decrease), Other Gains (Losses)				(100.00%)
Total revenues		$ 8,495.8	6,791.8	$ 23,943.9	19,719.2
Period over prior year period percent change in total revenues		25.00%		21.00%
Expenses
Losses and loss adjustment expenses		$ 5,523.1	5,050.5		13,928.8
Period over prior year period percent change in losses and loss adjustment expenses		9.00%		13.00%
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims				$ 15,769.2	13,928.8
Policy acquisition costs		$ 662.7	540.1	$ 1,889.7	1,557.2
Period over prior year period percent change in policy acquisition costs		23.00%		21.00%
Other underwriting expenses		$ 1,095.9	877.7	$ 3,123	2,568.3
Period over prior year period percent change in other underwriting expenses		25.00%		22.00%
Investment expenses		$ 5.8	5.8	$ 18	18
Period over prior year period percent change in investment expenses		0.00%		0.00%
Service expenses		$ 35.8	28.9	$ 102.1	81.8
Period over prior year period percent change in service expenses		24.00%		25.00%
Interest expense		$ 42	37.4	$ 120.5	117.6
Period over prior year period percent change in interest expense		12.00%		2.00%
Total expenses		$ 7,365.3	6,540.4	$ 21,022.5	18,271.7
Period over prior year period percent change in total expenses		13.00%		15.00%
Net Income
Income before income taxes		$ 1,130.5	251.4	$ 2,921.4	1,447.5
Period over prior year period percent change in income before income taxes		350.00%		102.00%
Provision for income taxes		$ 200.3	36.6	$ 560.2	429.7
Period over prior year period percent change in provision for income taxes		447.00%		30.00%
Net income		$ 930.2	214.8	$ 2,361.2	1,017.8
Period over prior year period percent change in net income		333.00%		132.00%
Net (income) loss attributable to noncontrolling interest (NCI)		$ (1.8)	9.2	$ (10.6)	(1.9)
Period over prior year period percent change in net (income) loss attributable to noncontrolling interest		(120.00%)		458.00%
Net income attributable to Progressive		$ 928.4	224	$ 2,350.6	1,015.9
Period over prior year period percent change in net income attributable to Progressive		314.00%		131.00%
Other Comprehensive Income (Loss)
Total net unrealized gains (losses) on securities		$ (39)	75.5	$ (243.6)	300.5
Period over prior year period percent change in total net unrealized gains (losses) on securities		(152.00%)		(181.00%)
Net unrealized losses on forecasted transactions		$ 0.2	0.1	$ 0.6	(5.6)
Period Over Prior Year Period, Percent Increase (Decrease), Unrealized Gain (Loss) On Forecasted Transactions		100.00%		(111.00%)
Foreign currency translation adjustment		$ 0	0.6	$ 0	0.8
Period over prior year period percent change in foreign currency translation adjustment		(100.00%)		(100.00%)
Other comprehensive income (loss)		$ (38.8)	76.2	$ (243)	295.7
Period over prior year period percent change in other comprehensive income		(151.00%)		(182.00%)
Other comprehensive (income) loss attributable to NCI		$ 0.4	(0.7)	$ 5	(2.9)
Period over prior year period change in Comprehensive (income) loss attributable to NCI		(157.00%)		(272.00%)
Comprehensive income attributable to Progressive		$ 890	299.5	$ 2,112.6	1,308.7
Period over prior year period percent change in comprehensive income		197.00%		61.00%
Computation of Per Share Earnings Available to Progressive Common Shareholders
Net income attributable to Progressive		$ 928.4	224	$ 2,350.6	1,015.9
Period over prior year period percent change in net income attributable to Progressive		314.00%		131.00%
Less: Preferred share dividends		$ 6.7	0	$ 14.6	0
Net Income (Loss) Available to Common Stockholders, Basic		$ 921.7	$ 224	$ 2,336	$ 1,015.9
Period Over Prior Year Period, Percent Increase (Decrease), Net Income (Loss) Available to Common Stockholders		311.00%		130.00%
Average common shares outstanding - Basic		582.7	581.3	582.2	580.7
Period over prior year period percent change in average shares outstanding - basic		0.00%		0.00%
Net effect of dilutive stock-based compensation		3.9	4.3	4.5	5
Period over prior year period percent change in net effect of dilutive stock-based compensation		(9.00%)		(10.00%)
Total average equivalent common shares - Diluted		586.6	585.6	586.7	585.7
Period over prior year period percent change in total equivalent shares - diluted		0.00%		0.00%
Basic: Earnings per common share		$ 1.58	$ 0.39	$ 4.01	$ 1.75
Period over prior year period percent change in basic: earnings per share		310.00%		129.00%
Diluted: Earnings per common share		$ 1.57	0.38	$ 3.98	1.73
Period over prior year period percent change in diluted: earnings per share		311.00%		130.00%
Dividends declared per share	[1]	$ 0	$ 0	$ 0	$ 0
[1]	Progressive maintains an annual variable common share dividend program. See Note 9 – Dividends for further discussion.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Assets
Fixed maturities (amortized cost: $25,963.4, $18,583.1, and $20,209.9)		$ 25,642.7
Short-term investments (amortized cost: $2,809.7, $4,311.5, and $2,869.4)		2,809.7
Total available-for-sale securities		28,452.4
Nonredeemable preferred stocks (cost: $758.9, $700.6, and $698.6)		840.9
Common equities (cost: $1,135.0, $1,485.5, and $1,499.0)		3,057.3
Total equity securities		3,898.2
Total investments		32,350.6
Fixed maturities (amortized cost: $25,963.4, $18,583.1, and $20,209.9)			$ 20,201.7	$ 18,660
Short-term investments (amortized cost: $2,809.7, $4,311.5, and $2,869.4)			2,869.4	4,311.5
Total available-for-sale securities			23,071.1	22,971.5
Nonredeemable preferred stocks (cost: $758.9, $700.6, and $698.6)			803.8	813.7
Common equities (cost: $1,135.0, $1,485.5, and $1,499.0)			3,399.8	3,209.5
Total equity securities			4,203.6	4,023.2
Total investments			27,274.7	26,994.7
Cash and cash equivalents		121.9	265	224.9
Restricted cash		0.8	10.3	31.4
Cash and cash equivalents		122.7	275.3	256.3
Accrued investment income		159.5	119.7	113
Premiums receivable, net of allowance for doubtful accounts of $230.1, $189.3, and $210.9		6,776.6	5,422.5	5,519.9
Reinsurance recoverables, including $114.1, $76.4, and $103.3 on paid losses and loss adjustment expenses		2,490.5	2,273.4	2,701.1
Prepaid reinsurance premiums		379.6	203.3	211.7
Deferred acquisition costs		962.7	780.5	782.6
Property and equipment, net of accumulated depreciation of $1,004.6, $908.4, and $940.6		1,139.3	1,119.6	1,129.4
Goodwill		452.7	452.7	452.7
Intangible assets, net of accumulated amortization of $229.7, $157.7, and $175.7		312.6	366.6	384.6
Other assets		396.9	412.9	386.6
Total assets		45,543.7	38,701.2	38,932.6
Liabilities
Unearned premiums		11,009.2	8,903.5	9,005.3
Loss and loss adjustment expense reserves		14,620.8	13,086.9	13,353.3
Net deferred income taxes		53.5	135	201.5
Accounts payable, accrued expenses, and other liabilities		3,924.1	3,481	3,272.7
Debt	[1]	3,859.9	3,306.3	3,312.2
Total liabilities		33,467.5	28,912.7	29,145
Redeemable noncontrolling interest (NCI)	[2]	217.4	503.7	498.2
Shareholders’ Equity
Serial Preferred Shares, Series B, no par value (cumulative, liquidation preference $1,000 per share) (authorized, issued, and outstanding 0.5, 0, and 0)		493.9	0	0
Common shares, $1.00 par value (authorized 900.0; issued 797.5, including treasury shares of 214.4, 215.9, and 215.8)		583.1	581.7	581.6
Paid-in capital		1,443.4	1,389.2	1,365.1
Retained earnings		9,602.1	6,031.7	6,116.5
Accumulated other comprehensive income (loss):
Net unrealized gains (losses) on securities		(249.9)	1,295	1,240.1
Net unrealized losses on forecasted transactions		(17.4)	(14.8)	(15)
Foreign currency translation adjustment		0	0	(0.3)
Accumulated other comprehensive (income) loss attributable to NCI		3.6	2	1.4
Total accumulated other comprehensive income (loss) attributable to Progressive		(263.7)	1,282.2	1,226.2
Total shareholders’ equity		11,858.8	9,284.8	9,289.4
Total liabilities, redeemable NCI, and shareholders’ equity		$ 45,543.7	$ 38,701.2	$ 38,932.6
[1]	Consists of long-term debt. See Note 4 – Debt for further discussion.
[2]	See Note 12 – Redeemable Noncontrolling Interest for further discussion.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Fixed maturities, amortized cost	$ 25,963.4
Available-for-Sale Fixed Maturities, Amortized Cost		$ 20,209.9	$ 18,583.1
Nonredeemable preferred stocks, cost	758.9
Available-for-Sale Nonredeemable Preferred Stocks, Cost		698.6	700.6
Common equities, cost	1,135
Available-for-Sale Common Equities, Cost		1,499	1,485.5
Short-term investments, amortized cost	2,809.7
Available-for-Sale Short Term Investments, Amortized Cost		2,869.4	4,311.5
Premiums receivable, allowance for doubtful accounts	230.1	210.9	189.3
Reinsurance recoverables, paid losses and loss adjustment expenses	114.1	103.3	76.4
Intangible Assets, Accumulated Amortization	229.7	175.7	157.7
Property and equipment, accumulated depreciation	$ 1,004.6	$ 940.6	$ 908.4
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)	214,400,000	215,800,000	215,900,000
Serial Preferred Stock [Member]
Preferred Stock, Shares Authorized (shares)	20,000,000	20,000,000	20,000,000
Series B Preferred Stock [Member]
Preferred Stock, No Par Value	$ 0		$ 0
Preferred Stock, Liquidation Preference Per Share (USD per share)		$ 0	0
Series B Preferred Stock [Member] | Serial Preferred Stock [Member]
Preferred Stock, No Par Value	$ 0	$ 0	$ 0
Preferred Stock, Shares Authorized (shares)	500,000	0	0
Preferred Stock, Shares Issued (shares)	500,000	0	0
Preferred Stock, Shares Outstanding (shares)	500,000	0	0
Preferred Stock, Liquidation Preference Per Share (USD per share)	$ 1,000

Consolidated Statements of Chan

Consolidated Statements of Changes in Shareholders' Equity Statement - USD ($) $ in Millions		Total	Preferred Shares, No Par Value	Common Shares, $1.00 Par Value	Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss) Attributable to Progressive	Series B Preferred Stock [Member]	Voting Preference Shares [Member]
Preferred Stock, Dividends Per Share, Declared								$ 0
Balance, Beginning of period at Dec. 31, 2016			$ 0	$ 579.9	$ 1,303.4	$ 5,140.4	$ 933.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of Serial Preferred Shares, Series B			0
Treasury shares purchased				(1.5)	(3.4)	(57.2)
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)
Net income attributable to Progressive		1,015.9				1,015.9
Cash dividends declared on Serial Preferred Shares, Series B ($27.024 per share and $0)		0
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	0	581.6	1,365.1	6,116.5	1,226.2
Balance, Beginning of period at Dec. 31, 2016			0	579.9	1,303.4	5,140.4	933.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment to carrying amount of redeemable noncontrolling interest		8.4
Attributable to noncontrolling interest		(2.3)
Balance, End of period at Dec. 31, 2017		$ 9,284.8	0	581.7	1,389.2	6,031.7	1,282.2
Preferred Stock, No Par Value								0
Preferred Stock, Shares Authorized (shares)									5,000,000
Preferred Stock, Shares Issued (shares)									0
Common Shares, par value (USD per share)		$ 1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-01 [Member]	[1]					0	0
Reclassification of disproportionate tax effects | Accounting Standards Update 2018-02 [Member]	[1]					0	0
Common Shares, par value (USD per share)		$ 1
Cumulative Effect of New Accounting Principle in Period of Adoption		$ 1,300.2
Reclassification of disproportionate tax effects		7.7
Preferred Stock, Dividends Per Share, Declared								27.024
Issuance of Serial Preferred Shares, Series B			493.9
Treasury shares purchased				(1.4)	(3.3)	(74)
Net restricted equity awards issued/vested				2.8	(2.8)
Amortization of equity-based compensation					53.3
Reinvested dividends on restricted stock units					(0.8)	0.8
Adjustment to carrying amount of redeemable noncontrolling interest		(3.1)			7.8
Net income attributable to Progressive		2,350.6				2,350.6
Cash dividends declared on Serial Preferred Shares, Series B ($27.024 per share and $0)		(13.5)
Attributable to noncontrolling interest		5					1.6
Other comprehensive income							(243)
Other, net						2
Balance, End of period at Sep. 30, 2018		11,858.8	493.9	583.1	1,443.4	9,602.1	(263.7)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of disproportionate tax effects		3.4
Preferred Stock, Dividends Per Share, Declared								27.024
Net income attributable to Progressive		928.4
Cash dividends declared on Serial Preferred Shares, Series B ($27.024 per share and $0)		(13.5)
Attributable to noncontrolling interest		0.4
Balance, End of period at Sep. 30, 2018		$ 11,858.8	$ 493.9	$ 583.1	$ 1,443.4	9,602.1	(263.7)
Preferred Stock, No Par Value								$ 0
Preferred Stock, Shares Authorized (shares)									5,000,000
Preferred Stock, Shares Issued (shares)									0
Common Shares, par value (USD per share)		$ 1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-01 [Member]	[1]					1,300.2	(1,300.2)
Reclassification of disproportionate tax effects | Accounting Standards Update 2018-02 [Member]	[1]					$ 4.3	$ (4.3)
[1]	See Note 14 – New Accounting Standards for further discussion.

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Statement of Cash Flows [Abstract]
Net income	$ 2,361.2	$ 1,017.8
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	137.8	126.7
Amortization of intangible assets	54	48.2
Net amortization of fixed-income securities	29.7	67.5
Amortization of equity-based compensation	54	76.6
Net realized (gains) losses on securities	(166.7)	(59.3)
Net (gains) losses on disposition of property and equipment	7.4	5.3
Other (gains) losses	0	(0.2)
Changes in:
Premiums receivable	(1,354.1)	(1,010.3)
Reinsurance recoverables	(217.1)	(816.1)
Prepaid reinsurance premiums	(176.3)	(41.2)
Deferred acquisition costs	(182.2)	(131.4)
Income taxes	25	(111.2)
Unearned premiums	2,105.7	1,536.7
Loss and loss adjustment expense reserves	1,533.9	1,985.1
Accounts payable, accrued expenses, and other liabilities	1,032.8	622.1
Other, net	(43.6)	(93.2)
Net cash provided by operating activities	5,201.5	3,223.1
Purchases:
Fixed maturities	(14,430.6)	(9,623.6)
Equity securities	(234.9)	(155.2)
Sales:
Fixed maturities	4,659.6	3,431.7
Equity securities	773.5	150.6
Maturities, paydowns, calls, and other:
Fixed maturities	3,911.5	3,872.5
Equity securities	15	50
Net sales (purchases) of short-term investments	91.9	(721.8)
Net unsettled security transactions	11	210.5
Purchases of property and equipment	(164.8)	(109.7)
Sales of property and equipment	8.1	13.8
Payments to Acquire Additional Interest in Subsidiaries	(296.9)	0
Payments to Acquire Businesses, Net of Cash Acquired	0	(18.1)
Net cash used in investing activities	(5,656.6)	(2,899.3)
Cash Flows From Financing Activities
Net proceeds from issuance of Serial Preferred Shares, Series B	493.9	0
Net proceeds from debt issuances	589.5	841.1
Payments of debt	(37.1)	(42.8)
Redemption/reacquisition of subordinated debt	0	(635.6)
Dividends paid to common shareholders	(654.9)	(395.4)
Dividends paid to preferred shareholders	(13.5)	0
Proceeds from exercise of equity options	3.3	0.5
Acquisition of treasury shares for restricted stock tax liabilities	(78.3)	(57.2)
Acquisition of treasury shares acquired in open market	(0.4)	(4.9)
Net cash provided by (used in) financing activities	302.5	(294.3)
Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	0	0.4
Increase (decrease) in cash, cash equivalents, and restricted cash	(152.6)	29.9
Cash, cash equivalents, and restricted cash – January 1	275.3	226.4
Cash, cash equivalents, and restricted cash – September 30	$ 122.7	$ 256.3

Basis of Presentation

Basis of Presentation	9 Months Ended
	Sep. 30, 2018
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 86.8% of the outstanding capital stock of ARX at September 30, 2018 and 69.0% at September 30, 2017 and December 31, 2017 . The increase in Progressive’s ownership of ARX at September 30, 2018, is primarily due to the "put" by minority ARX shareholders of 204,527 of their ARX shares during the second quarter 2018, including exercised stock options, to Progressive pursuant to the stockholders’ agreement. 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, 2018 , 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, 2017 ( “ 2017 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 $20.5 million at September 30, 2018 , and $5.3 million at both September 30, 2017 and December 31, 2017.

Investments

Investments	9 Months Ended
	Sep. 30, 2018
Investments Schedule [Abstract]
Investments	Investments — 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. Our securities are reported in our Consolidated Balance Sheets at fair value. The changes in fair value for our fixed-maturity securities (other than hybrid securities) are reported as a component of accumulated other comprehensive income, net of deferred income taxes, in our Consolidated Balance Sheets. The net holding period gains (losses) reported below represent the inception-to-date changes in fair value. The changes in the net holding period gains (losses) between periods for the hybrid securities and, beginning in 2018 , equity securities are recorded as a component of net realized gains (losses) on securities in our Consolidated Statements of Comprehensive Income. Prior to 2018, the change in fair value of our equity securities was part of accumulated other comprehensive income (see Note 14 – New Accounting Standards for further discussion). ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value September 30, 2018 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 9,942.0 $ 0 $ (189.5 ) $ 0 $ 9,752.5 30.2 % State and local government obligations 1,610.1 1.9 (23.2 ) 0 1,588.8 4.9 Foreign government obligations 0 0 0 0 0 0 Corporate debt securities 7,229.1 4.9 (85.2 ) (2.0 ) 7,146.8 22.1 Residential mortgage-backed securities 767.8 7.6 (5.6 ) 0 769.8 2.4 Commercial mortgage-backed securities 2,986.5 3.3 (30.5 ) 0 2,959.3 9.1 Other asset-backed securities 3,205.0 1.4 (13.6 ) 0.1 3,192.9 9.9 Redeemable preferred stocks 222.9 13.2 (1.4 ) (2.1 ) 232.6 0.7 Total fixed maturities 25,963.4 32.3 (349.0 ) (4.0 ) 25,642.7 79.3 Short-term investments 2,809.7 0 0 0 2,809.7 8.7 Total available-for-sale securities 28,773.1 32.3 (349.0 ) (4.0 ) 28,452.4 88.0 Equity securities: Nonredeemable preferred stocks 758.9 0 0 82.0 840.9 2.6 Common equities 1,135.0 0 0 1,922.3 3,057.3 9.4 Total equity securities 1,893.9 0 0 2,004.3 3,898.2 12.0 Total portfolio 1,2 $ 30,667.0 $ 32.3 $ (349.0 ) $ 2,000.3 $ 32,350.6 100.0 % ($ 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 947.0 12.2 (3.6 ) 0 955.6 3.5 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 December 31, 2017 Fixed maturities: U.S. government obligations $ 6,688.8 $ 1.1 $ (44.0 ) $ 0 $ 6,645.9 24.4 % State and local government obligations 2,285.6 20.7 (9.3 ) 0.1 2,297.1 8.4 Foreign government obligations 0 0 0 0 0 0 Corporate debt securities 4,997.2 14.8 (14.4 ) 0.1 4,997.7 18.3 Residential mortgage-backed securities 828.8 11.3 (3.4 ) 0 836.7 3.1 Commercial mortgage-backed securities 2,760.1 11.8 (13.3 ) 0 2,758.6 10.1 Other asset-backed securities 2,454.5 4.5 (4.5 ) 0.2 2,454.7 9.0 Redeemable preferred stocks 194.9 17.8 (1.5 ) (0.2 ) 211.0 0.8 Total fixed maturities 20,209.9 82.0 (90.4 ) 0.2 20,201.7 74.1 Equity securities: Nonredeemable preferred stocks 698.6 114.0 (8.8 ) 0 803.8 2.9 Common equities 1,499.0 1,901.0 (0.2 ) 0 3,399.8 12.5 Short-term investments 2,869.4 0 0 0 2,869.4 10.5 Total portfolio 1,2 $ 25,276.9 $ 2,097.0 $ (99.4 ) $ 0.2 $ 27,274.7 100.0 % 1 Our portfolio reflects the effect of unsettled security transactions; at September 30, 2018 and 2017 , we had $5.2 million and $238.3 million , respectively, included in “other liabilities,” compared to $5.8 million included in “other assets” at December 31, 2017 . 2 The total fair value of the portfolio at September 30, 2018 and 2017 , and December 31, 2017 , included $1.8 billion , $1.1 billion , and $1.6 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 or are redeemable within one year. We did not have any open repurchase or reverse repurchase transactions in our short-term investment portfolio at September 30, 2018 and 2017 , or December 31, 2017 . 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 securities are hybrid securities, which are reported at fair value: September 30, (millions) 2018 2017 December 31, 2017 Fixed maturities: State and local government obligations $ 3.6 $ 2.4 $ 6.1 Corporate debt securities 159.7 114.9 99.8 Other asset-backed securities 5.0 7.1 6.7 Redeemable preferred stocks 72.0 35.5 30.3 Total hybrid securities $ 240.3 $ 159.9 $ 142.9 Certain securities in our portfolio are accounted for as hybrid securities because they contain embedded derivatives that are not deemed to be clearly and closely related to the host investments. Since the embedded derivatives (e.g., change-in-control put option, debt-to-equity conversion, or any other feature unrelated to the credit quality or risk of default of the issuer that could impact the amount or timing of our expected future cash flows) do not have observable intrinsic values, we have elected to record the changes in fair value of these securities through income as realized gains or losses. Fixed Maturities The composition of fixed maturities by maturity at September 30, 2018 , was: (millions) Cost Fair Value Less than one year $ 4,192.3 $ 4,196.4 One to five years 16,545.0 16,315.6 Five to ten years 5,123.1 5,028.3 Ten years or greater 103.0 102.4 Total $ 25,963.4 $ 25,642.7 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, 2018 , we had $349.0 million of gross unrealized losses in our fixed-maturity securities. A review of our fixed-maturity securities indicated that the issuers were current with respect to their interest obligations and that there was no evidence of deterioration of the current cash flow projections that would indicate we would not receive the remaining principal at maturity. The following tables show the composition of gross unrealized losses by major security type and by the length of time that individual securities have been in a continuous unrealized loss position: Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses September 30, 2018 Fixed maturities: U.S. government obligations 91 $ 9,752.5 $ (189.5 ) 39 $ 5,025.0 $ (67.2 ) 52 $ 4,727.5 $ (122.3 ) State and local government obligations 459 1,394.3 (23.2 ) 227 680.5 (6.4 ) 232 713.8 (16.8 ) Corporate debt securities 369 5,959.2 (85.2 ) 221 3,963.9 (45.3 ) 148 1,995.3 (39.9 ) Residential mortgage-backed securities 234 521.7 (5.6 ) 44 325.0 (0.6 ) 190 196.7 (5.0 ) Commercial mortgage-backed securities 151 2,392.7 (30.5 ) 80 1,164.2 (8.3 ) 71 1,228.5 (22.2 ) Other asset-backed securities 219 2,828.6 (13.6 ) 111 1,821.8 (4.0 ) 108 1,006.8 (9.6 ) Redeemable preferred stocks 3 30.8 (1.4 ) 1 4.7 (0.1 ) 2 26.1 (1.3 ) Total fixed maturities 1,526 $ 22,879.8 $ (349.0 ) 723 $ 12,985.1 $ (131.9 ) 803 $ 9,894.7 $ (217.1 ) 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 191 341.5 (3.6 ) 26 39.2 (0.1 ) 165 302.3 (3.5 ) 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 December 31, 2017 Fixed maturities: U.S. government obligations 58 $ 5,817.0 $ (44.0 ) 41 $ 4,869.3 $ (34.6 ) 17 $ 947.7 $ (9.4 ) State and local government obligations 358 1,200.3 (9.3 ) 230 737.6 (4.4 ) 128 462.7 (4.9 ) Corporate debt securities 222 2,979.4 (14.4 ) 171 2,072.9 (9.1 ) 51 906.5 (5.3 ) Residential mortgage-backed securities 201 300.9 (3.4 ) 30 75.1 (0.2 ) 171 225.8 (3.2 ) Commercial mortgage-backed securities 105 1,682.3 (13.3 ) 63 1,221.2 (5.9 ) 42 461.1 (7.4 ) Other asset-backed securities 197 1,837.3 (4.5 ) 134 1,377.8 (3.3 ) 63 459.5 (1.2 ) Redeemable preferred stocks 2 21.8 (1.5 ) 1 10.8 (0.1 ) 1 11.0 (1.4 ) Total fixed maturities 1,143 13,839.0 (90.4 ) 670 10,364.7 (57.6 ) 473 3,474.3 (32.8 ) Equity securities: Nonredeemable preferred stocks 4 127.8 (8.8 ) 1 56.5 (0.5 ) 3 71.3 (8.3 ) Common equities 19 13.4 (0.2 ) 18 13.4 (0.2 ) 1 0 0 Total equity securities 23 141.2 (9.0 ) 19 69.9 (0.7 ) 4 71.3 (8.3 ) Total portfolio 1,166 $ 13,980.2 $ (99.4 ) 689 $ 10,434.6 $ (58.3 ) 477 $ 3,545.6 $ (41.1 ) Since both September 30, 2017 and December 31, 2017 , the number of securities in our fixed-maturity portfolio with unrealized losses increased as a result of rising interest rates. We had no material decreases in valuation as a result of credit rating downgrades and all of the securities in the table above are current with respect to required principal and interest payments. 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) 2018 2017 Fixed maturities: Residential mortgage-backed securities $ (19.7 ) $ (19.7 ) $ (19.7 ) Commercial mortgage-backed securities (0.1 ) (0.4 ) (0.3 ) Total fixed maturities $ (19.8 ) $ (20.1 ) $ (20.0 ) The following tables provide rollforwards of the amounts related to credit losses recognized in earnings for the periods ended September 30, 2018 and 2017 , 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, 2018 Mortgage-Backed (millions) Residential Commercial Total Balance at June 30, 2018 $ 0.3 $ 0.5 $ 0.8 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 0 0 Balance at September 30, 2018 $ 0.3 $ 0.5 $ 0.8 Nine Months Ended September 30, 2018 Mortgage-Backed (millions) Residential Commercial Total Balance at December 31, 2017 $ 0 $ 0.5 $ 0.5 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, 2018 $ 0.3 $ 0.5 $ 0.8 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 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, 2018 . 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) 2018 2017 2018 2017 Gross realized gains on security sales Available-for-sale securities: U.S. government obligations $ 0.1 $ 0.9 $ 1.8 $ 5.8 State and local government obligations 0.2 4.0 9.4 7.1 Corporate and other debt securities 1.7 5.1 2.1 16.5 Residential mortgage-backed securities 0 2.8 0 23.8 Commercial mortgage-backed securities 0 0 2.0 2.4 Other asset-backed securities 0 0 0.1 0.3 Redeemable preferred stocks 0.2 7.7 4.5 8.0 Total available-for-sale securities 2.2 20.5 19.9 63.9 Equity securities: Nonredeemable preferred stocks 0.1 3.0 3.7 54.6 Common equities 126.8 5.7 265.2 23.0 Total equity securities 126.9 8.7 268.9 77.6 Subtotal gross realized gains on security sales 129.1 29.2 288.8 141.5 Gross realized losses on security sales Available-for-sale securities: U.S. government obligations (7.6 ) (1.0 ) (46.4 ) (4.6 ) State and local government obligations (0.6 ) 0 (2.5 ) (0.1 ) Corporate and other debt securities (3.2 ) (1.8 ) (7.3 ) (4.6 ) Residential mortgage-backed securities 0 (0.1 ) 0 (0.4 ) Commercial mortgage-backed securities 0 (0.5 ) (6.3 ) (3.6 ) Other asset-backed securities (0.1 ) 0 (1.1 ) 0 Redeemable preferred stocks 0 (6.4 ) 0 (6.4 ) Total available-for-sale securities (11.5 ) (9.8 ) (63.6 ) (19.7 ) Equity securities: Nonredeemable preferred stocks 0 (0.1 ) (2.3 ) (5.9 ) Common equities (9.2 ) (0.2 ) (17.1 ) (0.3 ) Total equity securities (9.2 ) (0.3 ) (19.4 ) (6.2 ) Subtotal gross realized losses on security sales (20.7 ) (10.1 ) (83.0 ) (25.9 ) Net realized gains (losses) on security sales Available-for-sale securities: U.S. government obligations (7.5 ) (0.1 ) (44.6 ) 1.2 State and local government obligations (0.4 ) 4.0 6.9 7.0 Corporate and other debt securities (1.5 ) 3.3 (5.2 ) 11.9 Residential mortgage-backed securities 0 2.7 0 23.4 Commercial mortgage-backed securities 0 (0.5 ) (4.3 ) (1.2 ) Other asset-backed securities (0.1 ) 0 (1.0 ) 0.3 Redeemable preferred stocks 0.2 1.3 4.5 1.6 Total available-for-sale securities (9.3 ) 10.7 (43.7 ) 44.2 Equity securities: Nonredeemable preferred stocks 0.1 2.9 1.4 48.7 Common equities 117.6 5.5 248.1 22.7 Total equity securities 117.7 8.4 249.5 71.4 Litigation settlements and other gains (losses) 0 0.1 0 1.2 Subtotal net realized gains (losses) on security sales 108.4 19.2 205.8 116.8 Net holding period gains (losses) Hybrid securities 1.3 (0.9 ) (4.2 ) 0.3 Equity securities 94.5 0 (1.7 ) 0 Subtotal net holding period gains (losses) 95.8 (0.9 ) (5.9 ) 0.3 Other-than-temporary impairment losses Fixed maturities: Commercial mortgage-backed securities 0 (0.4 ) 0 (0.4 ) Total fixed maturities 0 (0.4 ) 0 (0.4 ) Equity securities: Common equities 0 (8.9 ) 0 (12.5 ) Subtotal investment other-than-temporary impairment losses 0 (9.3 ) 0 (12.9 ) Other asset impairment (22.1 ) (33.7 ) (33.2 ) (44.9 ) Subtotal other-than-temporary impairment losses (22.1 ) (43.0 ) (33.2 ) (57.8 ) Total net realized gains (losses) on securities $ 182.1 $ (24.7 ) $ 166.7 $ 59.3 Gross realized gains were predominantly the result of sales in our indexed common stock portfolio in order to reduce the overall portfolio risk, while gross realized losses were predominantly in our available-for-sale securities and were largely the result of an increase in interest rates. Also included are holding period change in valuation gains and losses on equity securities and hybrid securities, recoveries from litigation settlements related to investments, and 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. The following table reflects our holding period realized gains (losses) on equity securities recognized for the three and nine months ended September 30, 2018 for equity securities held at quarter end: Three Months Nine Months (millions) 2018 2018 Total net gains (losses) recognized during the period on equity securities $ 212.2 $ 247.8 Less: Net gains (losses) recognized on equity securities sold during the period 117.7 249.5 Net holding period gains (losses) recognized during the period on equity securities held at period end $ 94.5 $ (1.7 ) Note: Comparative disclosure for the prior year period is not meaningful. Net Investment Income The components of net investment income for the three and nine months ended September 30, were: Three Months Nine Months (millions) 2018 2017 2018 2017 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 47.7 $ 18.4 $ 133.4 $ 48.1 State and local government obligations 9.0 12.9 28.1 39.3 Foreign government obligations 0 0.1 0 0.3 Corporate debt securities 62.1 32.2 150.1 93.1 Residential mortgage-backed securities 6.9 8.0 20.4 27.9 Commercial mortgage-backed securities 24.1 20.0 65.5 57.1 Other asset-backed securities 21.2 12.1 50.9 33.8 Redeemable preferred stocks 3.3 2.8 8.8 9.1 Total fixed maturities 174.3 106.5 457.2 308.7 Short-term investments 15.7 10.6 40.7 26.4 Total available-for-sale securities 190.0 117.1 497.9 335.1 Equity securities: Nonredeemable preferred stocks 11.2 10.9 32.5 33.0 Common equities 16.9 14.9 46.1 42.8 Total equity securities 28.1 25.8 78.6 75.8 Investment income 218.1 142.9 576.5 410.9 Investment expenses (5.8 ) (5.8 ) (18.0 ) (18.0 ) Net investment income $ 212.3 $ 137.1 $ 558.5 $ 392.9 The amount of investment income (interest and dividends) we recognize varies based on the average assets held during the year and the book yields of the securities in our portfolio. The increase in net investment income on a year-over-year basis for the three and nine months ended September 30, 2018, was due to a combination of an increase in average assets and an increase in portfolio yields. The increase in average assets was due to strong underwriting growth and profitability, as well as the $600 million debt and $500 million preferred stock issuances in the first quarter of 2018. The increase in portfolio yields was a result of our decisions to hold a short-duration portfolio, which allowed us to reinvest significant maturities and paydowns of principal at higher yields, and to increase the portfolio duration from 2.2 years at the end of the third quarter 2017 to 2.6 years at the end of the third quarter 2018 as interest rates generally rose. Trading Securities At September 30, 2018 and 2017 , and December 31, 2017 , 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, 2018 and 2017 . Derivative Instruments At September 30, 2018 and 2017 , and December 31, 2017 , we had no open derivative positions. 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 unrealized loss of $8.0 million in April 2017. See Note 4 – Debt for further discussion.

Fair Value

Fair Value	9 Months Ended
	Sep. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value	Fair Value — We have categorized our financial instruments, based on the degree of subjectivity inherent in the method by which they are valued, into a fair value hierarchy of three levels, as follows: • Level 1 : Inputs are unadjusted quoted prices in active markets for identical instruments at the measurement date (e.g., U.S. government obligations, which are continually priced on a daily basis, active exchange-traded equity securities, and certain short-term securities). • Level 2 : Inputs (other than quoted prices included within Level 1) that are observable for the instrument either directly or indirectly (e.g., certain corporate and municipal bonds and certain preferred stocks). This includes: (i) quoted prices for similar instruments in active markets, (ii) quoted prices for identical or similar instruments in markets that are not active, (iii) inputs other than quoted prices that are observable for the instruments, and (iv) inputs that are derived principally from or corroborated by observable market data by correlation or other means. • Level 3 : Inputs that are unobservable. Unobservable inputs reflect our subjective evaluation about the assumptions market participants would use in pricing the financial instrument (e.g., certain structured securities and privately held investments). Determining the fair value of the investment portfolio is the responsibility of management. As part of the responsibility, we evaluate whether a market is distressed or inactive in determining the fair value for our portfolio. We review certain market level inputs to evaluate whether sufficient activity, volume, and new issuances exist to create an active market. Based on this evaluation, we concluded that there was sufficient activity related to the sectors and securities for which we obtained valuations. The composition of the investment portfolio by major security type and our outstanding debt was: Fair Value (millions) Level 1 Level 2 Level 3 Total Cost September 30, 2018 Fixed maturities: U.S. government obligations $ 9,752.5 $ 0 $ 0 $ 9,752.5 $ 9,942.0 State and local government obligations 0 1,588.8 0 1,588.8 1,610.1 Foreign government obligations 0 0 0 0 0 Corporate debt securities 0 7,146.8 0 7,146.8 7,229.1 Subtotal 9,752.5 8,735.6 0 18,488.1 18,781.2 Asset-backed securities: Residential mortgage-backed 0 769.8 0 769.8 767.8 Commercial mortgage-backed 0 2,959.3 0 2,959.3 2,986.5 Other asset-backed 0 3,192.9 0 3,192.9 3,205.0 Subtotal asset-backed securities 0 6,922.0 0 6,922.0 6,959.3 Redeemable preferred stocks: Financials 0 67.8 0 67.8 65.2 Utilities 0 4.7 0 4.7 4.8 Industrials 10.1 150.0 0 160.1 152.9 Subtotal redeemable preferred stocks 10.1 222.5 0 232.6 222.9 Total fixed maturities 9,762.6 15,880.1 0 25,642.7 25,963.4 Short-term investments 2,573.4 236.3 0 2,809.7 2,809.7 Total available-for-sale securities 12,336.0 16,116.4 0 28,452.4 28,773.1 Equity securities: Nonredeemable preferred stocks: Financials 76.3 724.0 0 800.3 718.9 Utilities 0 35.6 0 35.6 35.0 Industrials 0 0 5.0 5.0 5.0 Subtotal nonredeemable preferred stocks 76.3 759.6 5.0 840.9 758.9 Common equities: Common stocks 3,057.0 0 0 3,057.0 1,134.7 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 3,057.0 0 0.3 3,057.3 1,135.0 Total equity securities 3,133.3 759.6 5.3 3,898.2 1,893.9 Total portfolio $ 15,469.3 $ 16,876.0 $ 5.3 $ 32,350.6 $ 30,667.0 Debt $ 0 $ 3,926.9 $ 0 $ 3,926.9 $ 3,859.9 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 0 955.6 0 955.6 947.0 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 Utilities 0 0 0 0 0 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 December 31, 2017 Fixed maturities: U.S. government obligations $ 6,645.9 $ 0 $ 0 $ 6,645.9 $ 6,688.8 State and local government obligations 0 2,297.1 0 2,297.1 2,285.6 Foreign government obligations 0 0 0 0 0 Corporate debt securities 0 4,997.7 0 4,997.7 4,997.2 Subtotal 6,645.9 7,294.8 0 13,940.7 13,971.6 Asset-backed securities: Residential mortgage-backed 0 836.7 0 836.7 828.8 Commercial mortgage-backed 0 2,758.6 0 2,758.6 2,760.1 Other asset-backed 0 2,454.7 0 2,454.7 2,454.5 Subtotal asset-backed securities 0 6,050.0 0 6,050.0 6,043.4 Redeemable preferred stocks: Financials 0 64.1 0 64.1 61.3 Utilities 0 11.4 0 11.4 10.1 Industrials 0 135.5 0 135.5 123.5 Subtotal redeemable preferred stocks 0 211.0 0 211.0 194.9 Total fixed maturities 6,645.9 13,555.8 0 20,201.7 20,209.9 Equity securities: Nonredeemable preferred stocks: Financials 80.6 718.2 0 798.8 693.6 Utilities 0 0 0 0 0 Industrials 0 0 5.0 5.0 5.0 Subtotal nonredeemable preferred stocks 80.6 718.2 5.0 803.8 698.6 Common equities: Common stocks 3,399.5 0 0 3,399.5 1,498.7 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 3,399.5 0 0.3 3,399.8 1,499.0 Total fixed maturities and equity securities 10,126.0 14,274.0 5.3 24,405.3 22,407.5 Short-term investments 1,824.4 1,045.0 0 2,869.4 2,869.4 Total portfolio $ 11,950.4 $ 15,319.0 $ 5.3 $ 27,274.7 $ 25,276.9 Debt $ 0 $ 3,606.5 $ 37.1 $ 3,643.6 $ 3,306.3 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 2018 or 2017 . Our short-term security holdings classified as Level 1 are highly liquid, actively marketed, and have a very short duration, primarily 90 days or less to redemption. These securities are held at their original cost, adjusted for any accretion of discount, since that value very closely approximates what an active market participant would be willing to pay for such securities. The remainder of our short-term securities are classified as Level 2 and are not priced externally since these securities continually trade at par value. These securities are classified as Level 2 since they are primarily longer-dated securities issued by municipalities that contain either liquidity facilities or mandatory put features within one year. At September 30, 2018 , vendor-quoted prices represented 76% of our Level 1 classifications (excluding short-term investments), compared to 59% and 66% at September 30, 2017 and December 31, 2017 , 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, 2018 and 2017 , and December 31, 2017 , vendor-quoted prices comprised 99% , 98% , and 98% , respectively, of our Level 2 classifications (excluding short-term investments), while dealer-quoted prices represented 1% , 2% , and 2% , respectively. In our process for selecting a source (e.g., dealer or 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 30 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, 2018 and 2017 , and December 31, 2017 , we did not have any securities in our fixed-maturity portfolio listed as Level 3. At September 30, 2018 and 2017 , and December 31, 2017 , we held one private nonredeemable preferred security with a value of $5.0 million that was priced internally. The security was purchased during the third quarter 2017 and the value at all periods equals the cost at acquisition. A review of their latest available financial statements did not reveal any significant changes that would impact the security’s fair value. We 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 2018 or 2017 , 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. We did not have any material changes in fair value associated with Level 3 assets for the three and nine months ended September 30, 2018 and 2017 . 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, 2018
Debt Disclosure [Abstract]
Debt	Debt — Debt at each of the balance sheet periods consisted of: September 30, 2018 September 30, 2017 December 31, 2017 (millions) Carrying Value Fair Value Carrying Value Fair Value Carrying Value Fair Value 3.75% Senior Notes due 2021 $ 499.0 $ 503.9 $ 498.7 $ 525.7 $ 498.8 $ 520.7 2.45% Senior Notes due 2027 496.4 453.3 496.0 475.3 496.1 477.9 6 5/8% Senior Notes due 2029 296.3 360.3 296.0 388.3 296.1 382.3 6.25% Senior Notes due 2032 395.4 481.8 395.3 518.7 395.3 516.9 4.35% Senior Notes due 2044 346.6 349.7 346.5 379.8 346.5 388.7 3.70% Senior Notes due 2045 395.3 359.1 395.2 392.6 395.2 402.9 4.125% Senior Notes due 2047 841.3 828.4 841.2 894.2 841.2 917.1 4.20% Senior Notes due 2048 589.6 590.4 0 0 0 0 Other debt instruments 0 0 43.3 43.3 37.1 37.1 Total $ 3,859.9 $ 3,926.9 $ 3,312.2 $ 3,617.9 $ 3,306.3 $ 3,643.6 The Progressive Corporation issued $600 million of 4.20% Senior Notes due 2048 (the “ 4.20% Senior Notes”) in March 2018, and $850 million of 4.125% Senior Notes due 2047 (the “ 4.125% Senior Notes”) in April 2017, in underwritten public offerings. The net proceeds from these issuances, after deducting underwriters’ discounts, commissions, and other issuance costs, were approximately $589.5 million and $841.1 million , respectively. 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 unrealized loss as part of accumulated other comprehensive income (loss), which is being amortized as an adjustment to interest expense over the life of the 4.125% Senior Notes. Consistent with the other senior notes issued by Progressive, interest on the 4.20% Senior Notes and 4.125% Senior Notes is payable semiannually and both are redeemable, in whole or in part, at any time. See Note 16 – Subsequent Event , for a discussion of senior notes issued in October 2018. During the first quarter 2018, ARX repaid its term loans, in their entirety, in the aggregate principal amount of $37.1 million and, during the third quarter 2017, redeemed its junior subordinated notes and senior notes, in their entirety, in the aggregate principal amount of $65.2 million . Both the repayment and the redemptions were funded in part with proceeds from fixed-rate loans made by The Progressive Corporation. These intercompany transactions were eliminated in consolidation. During the second quarter 2018, The Progressive Corporation entered into a new line of credit with PNC Bank, National Association (PNC) in the maximum principal amount of $250 million . The prior line of credit, entered into in April 2017, had expired. The line of credit is on the same terms and conditions as the previous line of credit. 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, 2019, the expiration date of the line of credit. Prepayments are permitted without penalty. The line of credit is uncommitted and, as such, all advances are subject to PNC’s discretion. We had no borrowings under either line of credit during any of the periods presented.

Income Taxes

Income Taxes	9 Months Ended
	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes — Deferred tax assets and liabilities are recorded based on the difference between the financial statement and tax bases of assets and liabilities at the enacted tax rates. We review our deferred tax assets regularly for recoverability. At September 30, 2018 and 2017 , and December 31, 2017 , 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 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 deferred taxes on items that are reported in accumulated other comprehensive income, our policy is to release the income tax effects related to these items on an aggregate portfolio approach. For this purpose, we consider our available-for-sale fixed maturity securities and hedges on forecasted transactions as separate portfolios. For the three and nine months ended September 30, 2018 , there have been no material changes in our uncertain tax positions. For the third quarter 2018, our effective tax rate was 17.7% compared to 14.6% for the same period last year. On a year-to-date basis, the effective tax rate was 19.2% , compared to 29.7% for the same period last year. The federal corporate income tax rate decreased to 21% in 2018, from the previous rate of 35%, under the legislation commonly known as the Tax Cuts and Jobs Act of 2017. Despite the decrease in the federal rate, the effective tax rate for the third quarter 2018 was higher than the prior year quarter as the relative impact of the tax benefits decreased as pre-tax income increased quarter-over-prior-year quarter. As of September 30, 2018, we have not fully completed our accounting for the tax effects of the enactment of the legislation commonly known as the Tax Cuts and Jobs Act of 2017, with regard to the deductibility of compensation expense for certain covered executives, due to uncertainty surrounding the appropriate tax treatment of outstanding performance-based awards, and with regard to loss reserve discounting due to uncertainty surrounding the discount factors to be applied. Based on an Internal Revenue Service issued notice of proposed rule-making, related to the compensation issue, we determined no adjustment was necessary during the nine months ended September 30, 2018; however, we are waiting for definitive guidance to be published on both items.

Loss and Loss Adjustment Expens

Loss and Loss Adjustment Expense Reserves	9 Months Ended
	Sep. 30, 2018
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 is summarized as follows: September 30, (millions) 2018 2017 Balance, Beginning of period $ 13,086.9 $ 11,368.0 Less reinsurance recoverables on unpaid losses 2,170.1 1,801.0 Net balance, Beginning of period 10,916.8 9,567.0 Incurred related to: Current year 15,722.2 13,886.3 Prior years 47.0 42.5 Total incurred 15,769.2 13,928.8 Paid related to: Current year 9,409.7 8,379.4 Prior years 5,031.9 4,387.8 Total paid 14,441.6 12,767.2 Net balance, End of period 12,244.4 10,728.6 Plus reinsurance recoverables on unpaid losses 2,376.4 2,624.7 Balance, End of period $ 14,620.8 $ 13,353.3 We experienced unfavorable reserve development of $47.0 million and $42.5 million for the first nine months of 2018 and 2017, respectively, which is reflected as “Incurred related to prior years ” in the table above. Year-to-date September 30, 2018 • Accident year 2016 had approximately $52 million of unfavorable prior year reserve development, which was in part offset by favorable development in 2017 as well as 2015 and prior accident years. • Our personal auto business incurred about $41 million of unfavorable loss and loss adjustment expense (LAE) reserve development, with the Agency and Direct auto businesses contributing about $30 million and $11 million , respectively, of unfavorable development. The unfavorable development was primarily due to an increase in reopened personal injury protection claims. • Our Property business recognized unfavorable development of about $5 million . • Our special lines products and Commercial Lines business had minimal development during the first nine months of the year. Year-to-date September 30, 2017 • Accident years 2016 and 2015 combined has approximately $51 million of unfavorable prior year reserve development. 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 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.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	9 Months Ended
	Sep. 30, 2018
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	Supplemental Cash Flow Information — Cash and cash equivalents include bank demand deposits and daily overnight reverse repurchase commitments of funds held in bank demand deposit accounts on ARX’s subsidiaries, which are primarily collateralized by U.S. Treasury notes. The amount of reverse repurchase commitments held by ARX’s subsidiaries at September 30, 2018 and 2017 , and December 31, 2017 , were $152.4 million , $189.2 million , and $247.2 million , respectively. 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. We paid the following in the respective periods: Nine Months Ended September 30, (millions) 2018 2017 Income taxes $ 535.8 $ 538.7 Interest 116.4 107.4

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2018
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 include 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, among other products, 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, 2018 2017 2018 2017 (millions) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Personal Lines Agency $ 3,318.2 $ 372.2 $ 2,840.0 $ 69.5 $ 9,607.7 $ 1,142.2 $ 8,224.0 $ 524.7 Direct 3,337.2 294.4 2,734.8 128.7 9,565.3 880.3 7,908.5 466.7 Total Personal Lines 1 6,655.4 666.6 5,574.8 198.2 19,173.0 2,022.5 16,132.5 991.4 Commercial Lines 939.6 112.7 714.0 42.8 2,632.5 307.8 2,031.2 166.4 Property 2 335.5 (7.8 ) 255.2 (69.0 ) 933.2 (31.2 ) 720.3 (57.5 ) Other indemnity 0 (0.1 ) 0 0 0 0.1 0 (0.3 ) Total underwriting operations 7,930.5 771.4 6,544.0 172.0 22,738.7 2,299.2 18,884.0 1,100.0 Fees and other revenues 3 122.6 NA 96.3 NA 342.4 NA 270.3 NA Service businesses 42.5 6.7 33.3 4.4 119.6 17.5 94.5 12.7 Investments 4 400.2 394.4 118.2 112.4 743.2 725.2 470.2 452.2 Other gains (losses) 0 0 0 0 0 0 0.2 0.2 Interest expense NA (42.0 ) NA (37.4 ) NA (120.5 ) NA (117.6 ) Consolidated total $ 8,495.8 $ 1,130.5 $ 6,791.8 $ 251.4 $ 23,943.9 $ 2,921.4 $ 19,719.2 $ 1,447.5 NA = Not applicable 1 Personal auto insurance accounted for 94% of the total Personal Lines segment net premiums earned during the three and nine months ended September 30, 2018 , and 93% for the same periods in 2017 ; insurance for our special lines products (e.g., motorcycles, ATVs, RVs, watercraft, and snowmobiles) accounted for the balance of the Personal Lines net premiums earned. 2 For the three and nine months ended September 30, 2018 , pretax profit (loss) includes $18.0 million and $54.0 million , respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX and $17.2 million and $48.2 million for the same periods in 2017 . Although this expense is included in our Property segment, it is not reported in the consolidated results of ARX and, therefore, will not affect the value of net income (loss) attributable to noncontrolling interest. 3 Pretax profit (loss) for fees and other revenues is attributable to operating segments. 4 Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit is net of investment expense. Our management uses underwriting margin and combined ratio as primary measures of underwriting profitability. Underwriting profitability is calculated by subtracting losses and loss adjustment expenses, policy acquisition costs, and other underwriting expenses from the total of net premiums earned and fees and other revenues. The underwriting margin is the pretax underwriting profit (loss) expressed as a percentage of net premiums earned (i.e., revenues from underwriting operations). Combined ratio is the complement of the underwriting margin. Following are the underwriting margins/combined ratios for our underwriting operations for the respective periods: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Under-writing Combined Ratio Under-writing Combined Ratio Under-writing Margin Combined Ratio Under-writing Margin Combined Ratio Personal Lines Agency 11.2 % 88.8 2.4 % 97.6 11.9 % 88.1 6.4 % 93.6 Direct 8.8 91.2 4.7 95.3 9.2 90.8 5.9 94.1 Total Personal Lines 10.0 90.0 3.6 96.4 10.5 89.5 6.1 93.9 Commercial Lines 12.0 88.0 6.0 94.0 11.7 88.3 8.2 91.8 Property 1 (2.3 ) 102.3 (27.0 ) 127.0 (3.3 ) 103.3 (8.0 ) 108.0 Total underwriting operations 9.7 90.3 2.6 97.4 10.1 89.9 5.8 94.2 1 Included in the three and nine months ended September 30, 2018 , is 5.4 points and 5.8 points, respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX and 6.7 points for the three and nine months ended September 30, 2017 .

Dividends

Dividends	9 Months Ended
	Sep. 30, 2018
Text Block [Abstract]
Dividends	Dividends Common Share Dividends We maintain a policy of paying an annual variable dividend on our common shares 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 (using the statutory tax rate) multiplied by a performance factor (Gainshare factor), which is determined based on the results of the Agency auto, Direct auto, special lines, Commercial Lines, and Property business units, with minor exclusions and adjustments, and subject to the limitations discussed below. The target percentage is determined by our Board of Directors on an annual basis as part of their review of the dividend policy and announced to shareholders and the public. In December 2017 , the Board determined the target percentage for 2018 to be 33-1/3 % of annual after-tax underwriting income. The Gainshare factor can range from zero to two and is determined by comparing our operating performance for the specified business units for the year to certain predetermined profitability and growth objectives approved by the Compensation Committee of the Board. This Gainshare factor is also used in the annual cash incentive program currently in place for our employees (our “Gainsharing program”). On a year-to-date basis, as of September 30, 2018 , the Gainshare factor was 1.91 . 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 2018 dividend program is 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. In addition, the ultimate decision on whether or not a dividend will be paid is in the discretion of the Board of Directors. The Board could decide to alter our policy, or not to pay the annual variable dividend, at any time prior to the declaration of the dividend for the year. Such an action by the Board could result from, among other reasons, changes in the insurance marketplace, changes in our performance or capital needs, changes in the U.S. federal income tax laws, disruptions of national or international capital markets, or other events affecting our business, liquidity, or financial position. Following is a summary of our common shareholder dividends that were declared in the last two years: (millions, except per share amounts) Amount of Common Share Dividends Dividend Type Declared Paid Per Share Accrued 1 Paid 1 Annual – Variable December 2017 February 2018 $ 1.1247 $ 655.1 $ 654.9 Annual – Variable December 2016 February 2017 0.6808 395.4 395.4 1 Variance between accrued and paid, if any, reflects the difference between the number of estimated and actual shares outstanding as of the record date. Preferred Share Dividends In the first quarter 2018, we issued 500,000 Series B Fixed-to-Floating Rate Cumulative Perpetual Serial Preferred Shares, without par value (the “Series B Preferred Shares”), with a liquidation preference of $1,000 per share (the “stated amount”). Holders of the Series B Preferred Shares will be entitled to receive cumulative cash dividends semi-annually in March and September, if and when declared by the Board of Directors. Until March 15, 2023 (the “fixed-rate period”), the annual dividend rate is fixed at 5.375% of the stated amount per share. Beginning March 15, 2023, the annual dividend rate switches to a floating rate equal to the three-month LIBOR rate plus a spread of 2.539% applied to the stated amount per share. After the fixed-rate period and up until redemption of the Series B Preferred Shares, the dividends would be payable quarterly, if and when declared by the Board of Directors. The Series B Preferred Shares are perpetual and have no stated maturity date. After the fixed-rate period, we may redeem the Series B Preferred Shares at the stated amount plus all accrued and unpaid dividends. During the third quarter of 2018, the Board declared a $27.024 per share, or $13.5 million , dividend on the Series B Preferred Shares, which was paid within the quarter.

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	9 Months Ended
	Sep. 30, 2018
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 Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income (loss) Total tax (provision) benefit After tax total accumulated other comprehensive income (loss) Total net unrealized gains (losses) on securities Net unrealized gains (losses) on forecasted transactions Foreign currency translation adjustment (Income) loss attributable to NCI Balance at June 30, 2018 $ (280.8 ) $ 58.9 $ (221.9 ) $ (210.9 ) $ (17.6 ) $ 0 $ 6.6 Reclassification of disproportionate amounts 1 (4.3 ) 0.9 (3.4 ) 0 0 0 (3.4 ) Adjusted balance at June 30, 2018 (285.1 ) 59.8 (225.3 ) (210.9 ) (17.6 ) 0 3.2 Other comprehensive income (loss) before reclassifications: Investment securities (58.7 ) 12.3 (46.4 ) (46.4 ) 0 0 0 Forecasted transactions 0 0 0 0 0 0 0 Foreign currency translation adjustment 0 0 0 0 0 0 0 Loss attributable to noncontrolling interest (NCI) 0.6 (0.2 ) 0.4 0 0 0 0.4 Total other comprehensive income (loss) before reclassifications (58.1 ) 12.1 (46.0 ) (46.4 ) 0 0 0.4 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings 0 0 0 0 0 0 0 Net realized gains (losses) on securities (9.3 ) 1.9 (7.4 ) (7.4 ) 0 0 0 Interest expense (0.2 ) 0 (0.2 ) 0 (0.2 ) 0 0 Total reclassification adjustment for amounts realized in net income (9.5 ) 1.9 (7.6 ) (7.4 ) (0.2 ) 0 0 Total other comprehensive income (loss) (48.6 ) 10.2 (38.4 ) (39.0 ) 0.2 0 0.4 Balance at September 30, 2018 $ (333.7 ) $ 70.0 $ (263.7 ) $ (249.9 ) $ (17.4 ) $ 0 $ 3.6 1 Reflects the change in value on (income) loss attributable to NCI in conjunction with the "put" transaction (See Note 12 – Redeemable Noncontrolling Interest for additional information). Components of Changes in Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income (loss) Total tax (provision) benefit After tax total accumulated other comprehensive income (loss) Total net unrealized gains (losses) on securities Net unrealized gains (losses) on forecasted transactions Foreign currency translation adjustment (Income) loss attributable to NCI Balance at December 31, 2017 $ 1,977.8 $ (695.6 ) $ 1,282.2 $ 1,295.0 $ (14.8 ) $ 0 $ 2.0 Cumulative effect adjustment 1 (2,006.0 ) 705.8 (1,300.2 ) (1,300.2 ) 0 0 0 Reclassification of disproportionate amounts 2 (4.3 ) (3.4 ) (7.7 ) (1.1 ) (3.2 ) 0 (3.4 ) Adjusted balance at December 31, 2017 (32.5 ) 6.8 (25.7 ) (6.3 ) (18.0 ) 0 (1.4 ) Other comprehensive income (loss) before reclassifications: Investment securities (352.5 ) 73.9 (278.6 ) (278.6 ) 0 0 0 Forecasted transactions 0 0 0 0 0 0 0 Foreign currency translation adjustment 0 0 0 0 0 0 0 Loss attributable to noncontrolling interest (NCI) 6.4 (1.4 ) 5.0 0 0 0 5.0 Total other comprehensive income (loss) before reclassifications (346.1 ) 72.5 (273.6 ) (278.6 ) 0 0 5.0 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings 0 0 0 0 0 0 0 Net realized gains (losses) on securities (44.2 ) 9.2 (35.0 ) (35.0 ) 0 0 0 Interest expense (0.7 ) 0.1 (0.6 ) 0 (0.6 ) 0 0 Total reclassification adjustment for amounts realized in net income (44.9 ) 9.3 (35.6 ) (35.0 ) (0.6 ) 0 0 Total other comprehensive income (loss) (301.2 ) 63.2 (238.0 ) (243.6 ) 0.6 0 5.0 Balance at September 30, 2018 $ (333.7 ) $ 70.0 $ (263.7 ) $ (249.9 ) $ (17.4 ) $ 0 $ 3.6 1 Reflects the fair value changes on equity securities as of December 31, 2017 , which are reported as realized gains (losses) under the new accounting guidance. See Note 14 – New Accounting Standards for additional information. 2 Reflects the effect of the change in the U.S. federal tax rate on our available-for-sale fixed-maturity securities and our hedges on forecasted transactions as of December 31, 2017 (See Note 14 – New Accounting Standards for additional information) and the adjustment to reflect the change in value on (income) loss attributable to NCI in conjunction with the "put" transaction (See Note 12 – Redeemable Noncontrolling Interest for additional information). Components of Changes in Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income (loss) Total tax (provision) benefit After tax total accumulated other comprehensive income (loss) Total net unrealized gains (losses) on securities Net Foreign currency translation adjustment (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 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 (loss) Total tax (provision) benefit After tax total accumulated other comprehensive income (loss) Total net Net unrealized gains (losses) 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 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 In an effort to manage interest rate risk, we often enter into forecasted transactions on Progressive’s debt issuances. We expect to reclassify $1.0 million (pretax) into interest expense during the next 12 months, related to net unrealized losses on forecasted transactions (see Note 4 – Debt for further discussion).

Litigation

Litigation	9 Months Ended
	Sep. 30, 2018
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 subsidiaries are named as defendants in a number of class action or individual lawsuits that challenge certain of the operations of the 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, marketing, or sales 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 2017 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, 2018 or 2017, and there were no material settlements during the first nine months of 2018 or 2017. 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 2017 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 2017 Annual Report to Shareholders.

Redeemable Noncontrolling Inter

Redeemable Noncontrolling Interest	9 Months Ended
	Sep. 30, 2018
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 stockholders had the right to “put” a portion of their ARX shares to Progressive in 2018, and have the right to put all of their remaining shares to Progressive in 2021. During the second quarter 2018, minority ARX stockholders put 204,527 shares, including 5,483 shares that were issued upon the exercise of outstanding stock options. Progressive acquired these additional shares, in a cash transaction, for a total cost of $295.9 million . If ARX stockholders do not put all of their shares to Progressive in 2021, Progressive has the ability to “call ” all of the outstanding shares shortly thereafter and to bring its ownership stake to 100% in 2021. See Note 15 – Redeemable Noncontrolling Interest in our 2017 Annual Report to Shareholders for a discussion of the purchase price for shares to be purchased by Progressive pursuant to these put or call rights. At September 30, 2018, Progressive’s share ownership interest in ARX was 86.8% . 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, 2018 , ARX employees held options to purchase 16,067 ARX shares. These options and any shares issued upon exercise are subject to the stockholders’ agreement, including the “put ” and “call ” rights described above. Until the options are exercised, the underlying obligation of approximately $23.4 million is not recorded as part of redeemable NCI. See Note 9 – Employee Benefit Plans in our 2017 Annual Report to Shareholders for a discussion of our employee stock options. The changes in the components of redeemable NCI during the nine months ended September 30, 2018 and 2017 , and the year ended December 31, 2017 , were: (millions) September 30, 2018 September 30, 2017 December 31, 2017 Balance, Beginning of period $ 503.7 $ 483.7 $ 483.7 Net income attributable to NCI 10.6 1.9 5.9 Other comprehensive income (loss) attributable to NCI 1 (5.0 ) 2.9 2.3 Exercise of employee stock options 9.4 3.4 3.4 Purchase/change of ARX minority shares (298.2 ) 0 0 Change in redemption value of NCI (3.1 ) 6.3 8.4 Balance, End of period $ 217.4 $ 498.2 $ 503.7 1 Amount represents the other comprehensive income (loss) attributable to NCI, as reflected on the the Consolidated Statements of Comprehensive Income; any reclassification to accumulated other comprehensive income (loss) attributable to NCI due to a change in the minority ownership percentage does not impact the amount of redeemable NCI.

Goodwill and Intangible Assets

Goodwill and Intangible Assets	9 Months Ended
	Sep. 30, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	Goodwill and Intangible Assets Goodwill During the nine months ended September 30, 2018 , there were no changes to the carrying amount of goodwill. 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, 2018 and 2017 , and December 31, 2017 : (millions) September 30, 2018 September 30, 2017 December 31, 2017 Intangible assets subject to amortization $ 300.2 $ 372.2 $ 354.2 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 312.6 $ 384.6 $ 366.6 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 include $0.6 million of accumulated amortization for all periods presented. Intangible assets subject to amortization consisted of the following: (millions) September 30, 2018 September 30, 2017 December 31, 2017 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 $ 128.1 $ 128.1 $ 256.2 $ 91.5 $ 164.7 $ 256.2 $ 100.7 $ 155.5 Agency relationships 159.2 39.8 119.4 159.2 28.4 130.8 159.2 31.3 127.9 Software rights 79.1 37.4 41.7 79.1 26.8 52.3 79.1 29.4 49.7 Trade name 34.8 23.8 11.0 34.8 10.4 24.4 34.8 13.7 21.1 Total $ 529.3 $ 229.1 $ 300.2 $ 529.3 $ 157.1 $ 372.2 $ 529.3 $ 175.1 $ 354.2 Amortization expense was $18.0 million and $54.0 million for the three and nine months ended September 30, 2018 , respectively, compared to $17.2 million and $48.2 million during the same periods 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 transition the branding of our Property products from the ASI trade name to the Progressive brand. As of September 30, 2018 , the remaining average life of all of our intangible assets is 4.2 years .

New Accounting Standards

New Accounting Standards	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
New Accounting Standards	New Accounting Standards Issued In August 2018, the Financial Accounting Standards Board (FASB) issued an Accounting Standards Update (ASU), which provides additional guidance on the requirements for capitalizing and amortizing implementation costs incurred in a cloud computing arrangement that does not include a software license. This ASU will be effective for fiscal years (including interim periods within those fiscal years) beginning after December 15, 2019 (2020 for calendar-year companies). We do not expect this standard to have a material impact on our financial condition, cash flows, or results of operations. In August 2018, the FASB issued an ASU, which amends the disclosure requirements for fair value measurements. The ASU requires companies to disclose the changes in unrealized gains and losses for the period included in other comprehensive income for recurring Level 3 fair value measurements held at the end of the reporting period and the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements. The ASU also removes current disclosure requirements for the amount of and reasons for transfers between Level 1 and Level 2 of the fair value hierarchy, the policy for timing of transfers between levels, and the valuation processes for Level 3 fair value measurements. The ASU is effective for fiscal years beginning after December 15, 2019, and should be applied prospectively for the additions to the disclosure requirements and applied retrospectively to all periods presented for all other amendments. Early adoption of this ASU is permitted, including the ability to early adopt the removal of current disclosures while delaying the adoption of new disclosures until the effective date. We do not expect this standard to have an impact on our financial condition, cash flows, or results of operations. In July 2018, the FASB issued an ASU, which provides targeted improvements to the new lease accounting guidance issued by the FASB in February 2016. The ASU, which eliminates the off-balance-sheet accounting for leases, will require lessees to report their operating leases as both an asset and liability on the statement of financial position and to disclose key information about leasing arrangements in the financial statement footnotes. Under the ASU, there will be no change to the recognition of lease expense in our results of operations. The ASU will be effective for fiscal years (including interim periods within those fiscal years) beginning after December 15, 2018 (2019 for calendar-year companies). Under the ASU, companies will have the option to apply the new lease requirements either as of the effective date (i.e., January 1, 2019), with comparative information presented in accordance with the previous standard, or on a modified retrospective basis, which would restate all financial statement information as of the beginning of the earliest period presented. Based on our lease portfolio at September 30, 2018, and in accordance with the accounting elections available in the ASU, we would have recorded an increase to assets and liabilities of approximately $160 million , and there would have been no impact on our results of operations or cash flows. Therefore, we do not expect this standard to have a material impact on our financial condition. In March 2017, the FASB issued an ASU related to premium amortization on purchased callable debt securities. The intent of the standard is to shorten the amortization period for certain purchased callable debt securities held at a premium. Under the ASU, the premium is required to be amortized to the earliest call date. The ASU more closely aligns interest income recorded on bonds held at a premium with the economics of the underlying instrument. The ASU, which is required to be applied on a modified retrospective basis, is effective for fiscal years beginning after December 15, 2018 (2019 for calendar-year companies), and interim periods within those fiscal years. Since we have historically used a yield-to-worst scenario for our securities that were purchased at a premium, and the first call on a premium security most often produces the lowest and most conservative yield, we do not expect this standard to have a significant impact on our financial condition, cash flows, or results of operations. In January 2017, the FASB issued an ASU, which eliminates the requirement to determine the implied fair value of goodwill in measuring an impairment loss. Upon adoption, the measurement of a goodwill impairment will represent the excess of the reporting unit’s carrying value over fair value, limited to the carrying value of goodwill. This ASU is effective for goodwill impairment tests in fiscal years beginning after December 15, 2019 (2020 for calendar-year companies), with early adoption permitted. We do not expect this standard to have a material impact on our financial position or results of operations. In June 2016, the FASB issued an ASU intended to improve the timing, and enhance the accounting and disclosure, of credit losses on financial assets. Additionally, this update will modify the existing accounting guidance related to the impairment evaluation for available-for-sale debt securities and will result in the creation of an allowance for credit losses as a contra asset account. The ASU will require cumulative-effect changes to retained earnings in the period of adoption, if any occur, and will also require prospective changes on previously recorded impairments. This ASU is effective for fiscal years (including interim periods within those fiscal years) beginning after December 15, 2019 (2020 for calendar-year companies), with early adoption permissible (including interim periods within that fiscal year) beginning after December 15, 2018 (2019 for calendar-year companies). While the ASU creates additional accounting complexities related to the recognition of the impairment losses, and subsequent recoveries, through an allowance for credit losses account, we do not expect the ASU to have a material impact on our current method of evaluating securities for credit losses or the timing or recognition of the amounts of the impairment losses. Adopted On January 1, 2018, we adopted the ASU intended to improve the recognition and measurement of financial instruments. Under this update, the changes in fair value of equity securities are recognized as a component of net income. Upon adoption, we recorded a cumulative-effect adjustment of $1.3 billion , which is net of taxes. The cumulative-effect adjustment represents the amount of after-tax net unrealized gains on equity securities that was recorded as part of accumulated other comprehensive income at December 31, 2017. The adoption of this ASU had no impact on comprehensive income. Consistent with our historical presentation, cash flows on equity securities will be reflected as investing activities in the Consolidated Statements of Cash Flows. In the first quarter 2018, we adopted the ASU related to the reclassification of certain tax effects from accumulated other comprehensive income. This update provided companies with the option to reclassify disproportionate tax effects in accumulated other comprehensive income caused by the legislation commonly known as the Tax Cuts and Jobs Act of 2017 to retained earnings. We opted to make the reclassification, which resulted in a decrease to accumulated other comprehensive income and an offsetting increase to retained earnings of $4.3 million . This reclass was solely due to the effect of the change in the U.S. federal tax rate on our available-for-sale fixed-maturity securities and our hedges on forecasted transactions. There were no disproportionate tax effects related to our equity securities subsequent to adopting the ASU related to classification and measurement discussed above.

Reclassification (Notes)

Reclassification (Notes)	9 Months Ended
	Sep. 30, 2018
Accounting Policies [Abstract]
Reclassifications [Text Block]	Reclassification — For the three and nine months ended September 30, 2017, we reclassified the change in the net holding period gains (losses) on hybrid securities held at September 30, 2017 to “net holding period gains (losses) on securities” out of “net realized gains (losses) on security sales” to conform with the current-year Consolidated Statements of Comprehensive Income presentation. There was no overall impact on total net realized gains (losses) on securities. We also reclassified the classification and presentation of restricted cash in our Consolidated Statements of Cash Flows in accordance with the accounting guidance we adopted for the year ended December 31, 2017 relating to this item; there was no overall impact on the increase in cash, cash equivalents, and restricted cash as a result of this reclassification. In addition, on our Consolidated Balance Sheets for December 31, 2017, we reclassified our “dividends payable” into “accounts payable, accrued expenses, and other liabilities” to be consistent with the current period presentation.

Subsequent Event (Notes)

Subsequent Event (Notes)	9 Months Ended
	Sep. 30, 2018
Subsequent Events [Abstract]
Subsequent Events [Text Block]	Subsequent Event — On October 18, 2018, we issued $550.0 million of 4.00% Senior Notes due 2029 (the “ 4.00% Senior Notes”) in an underwritten public offering. We received net proceeds, which will be used for general corporate purposes, of about $544.5 million , after deducting underwriters’ discounts, commissions, and other issuance costs.

Investments (Tables)

Investments (Tables)	9 Months Ended
	Sep. 30, 2018
Investments Schedule [Abstract]
Schedule of Debt and Equity 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. Our securities are reported in our Consolidated Balance Sheets at fair value. The changes in fair value for our fixed-maturity securities (other than hybrid securities) are reported as a component of accumulated other comprehensive income, net of deferred income taxes, in our Consolidated Balance Sheets. The net holding period gains (losses) reported below represent the inception-to-date changes in fair value. The changes in the net holding period gains (losses) between periods for the hybrid securities and, beginning in 2018 , equity securities are recorded as a component of net realized gains (losses) on securities in our Consolidated Statements of Comprehensive Income. Prior to 2018, the change in fair value of our equity securities was part of accumulated other comprehensive income (see Note 14 – New Accounting Standards for further discussion). ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value September 30, 2018 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 9,942.0 $ 0 $ (189.5 ) $ 0 $ 9,752.5 30.2 % State and local government obligations 1,610.1 1.9 (23.2 ) 0 1,588.8 4.9 Foreign government obligations 0 0 0 0 0 0 Corporate debt securities 7,229.1 4.9 (85.2 ) (2.0 ) 7,146.8 22.1 Residential mortgage-backed securities 767.8 7.6 (5.6 ) 0 769.8 2.4 Commercial mortgage-backed securities 2,986.5 3.3 (30.5 ) 0 2,959.3 9.1 Other asset-backed securities 3,205.0 1.4 (13.6 ) 0.1 3,192.9 9.9 Redeemable preferred stocks 222.9 13.2 (1.4 ) (2.1 ) 232.6 0.7 Total fixed maturities 25,963.4 32.3 (349.0 ) (4.0 ) 25,642.7 79.3 Short-term investments 2,809.7 0 0 0 2,809.7 8.7 Total available-for-sale securities 28,773.1 32.3 (349.0 ) (4.0 ) 28,452.4 88.0 Equity securities: Nonredeemable preferred stocks 758.9 0 0 82.0 840.9 2.6 Common equities 1,135.0 0 0 1,922.3 3,057.3 9.4 Total equity securities 1,893.9 0 0 2,004.3 3,898.2 12.0 Total portfolio 1,2 $ 30,667.0 $ 32.3 $ (349.0 ) $ 2,000.3 $ 32,350.6 100.0 % ($ 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 947.0 12.2 (3.6 ) 0 955.6 3.5 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 December 31, 2017 Fixed maturities: U.S. government obligations $ 6,688.8 $ 1.1 $ (44.0 ) $ 0 $ 6,645.9 24.4 % State and local government obligations 2,285.6 20.7 (9.3 ) 0.1 2,297.1 8.4 Foreign government obligations 0 0 0 0 0 0 Corporate debt securities 4,997.2 14.8 (14.4 ) 0.1 4,997.7 18.3 Residential mortgage-backed securities 828.8 11.3 (3.4 ) 0 836.7 3.1 Commercial mortgage-backed securities 2,760.1 11.8 (13.3 ) 0 2,758.6 10.1 Other asset-backed securities 2,454.5 4.5 (4.5 ) 0.2 2,454.7 9.0 Redeemable preferred stocks 194.9 17.8 (1.5 ) (0.2 ) 211.0 0.8 Total fixed maturities 20,209.9 82.0 (90.4 ) 0.2 20,201.7 74.1 Equity securities: Nonredeemable preferred stocks 698.6 114.0 (8.8 ) 0 803.8 2.9 Common equities 1,499.0 1,901.0 (0.2 ) 0 3,399.8 12.5 Short-term investments 2,869.4 0 0 0 2,869.4 10.5 Total portfolio 1,2 $ 25,276.9 $ 2,097.0 $ (99.4 ) $ 0.2 $ 27,274.7 100.0 % 1 Our portfolio reflects the effect of unsettled security transactions; at September 30, 2018 and 2017 , we had $5.2 million and $238.3 million , respectively, included in “other liabilities,” compared to $5.8 million included in “other assets” at December 31, 2017 . 2 The total fair value of the portfolio at September 30, 2018 and 2017 , and December 31, 2017 , included $1.8 billion , $1.1 billion , and $1.6 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 securities are hybrid securities, which are reported at fair value: September 30, (millions) 2018 2017 December 31, 2017 Fixed maturities: State and local government obligations $ 3.6 $ 2.4 $ 6.1 Corporate debt securities 159.7 114.9 99.8 Other asset-backed securities 5.0 7.1 6.7 Redeemable preferred stocks 72.0 35.5 30.3 Total hybrid securities $ 240.3 $ 159.9 $ 142.9
Investments Classified by Contractual Maturity Date [Table Text Block]	The composition of fixed maturities by maturity at September 30, 2018 , was: (millions) Cost Fair Value Less than one year $ 4,192.3 $ 4,196.4 One to five years 16,545.0 16,315.6 Five to ten years 5,123.1 5,028.3 Ten years or greater 103.0 102.4 Total $ 25,963.4 $ 25,642.7
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, 2018 Fixed maturities: U.S. government obligations 91 $ 9,752.5 $ (189.5 ) 39 $ 5,025.0 $ (67.2 ) 52 $ 4,727.5 $ (122.3 ) State and local government obligations 459 1,394.3 (23.2 ) 227 680.5 (6.4 ) 232 713.8 (16.8 ) Corporate debt securities 369 5,959.2 (85.2 ) 221 3,963.9 (45.3 ) 148 1,995.3 (39.9 ) Residential mortgage-backed securities 234 521.7 (5.6 ) 44 325.0 (0.6 ) 190 196.7 (5.0 ) Commercial mortgage-backed securities 151 2,392.7 (30.5 ) 80 1,164.2 (8.3 ) 71 1,228.5 (22.2 ) Other asset-backed securities 219 2,828.6 (13.6 ) 111 1,821.8 (4.0 ) 108 1,006.8 (9.6 ) Redeemable preferred stocks 3 30.8 (1.4 ) 1 4.7 (0.1 ) 2 26.1 (1.3 ) Total fixed maturities 1,526 $ 22,879.8 $ (349.0 ) 723 $ 12,985.1 $ (131.9 ) 803 $ 9,894.7 $ (217.1 ) 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 191 341.5 (3.6 ) 26 39.2 (0.1 ) 165 302.3 (3.5 ) 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 December 31, 2017 Fixed maturities: U.S. government obligations 58 $ 5,817.0 $ (44.0 ) 41 $ 4,869.3 $ (34.6 ) 17 $ 947.7 $ (9.4 ) State and local government obligations 358 1,200.3 (9.3 ) 230 737.6 (4.4 ) 128 462.7 (4.9 ) Corporate debt securities 222 2,979.4 (14.4 ) 171 2,072.9 (9.1 ) 51 906.5 (5.3 ) Residential mortgage-backed securities 201 300.9 (3.4 ) 30 75.1 (0.2 ) 171 225.8 (3.2 ) Commercial mortgage-backed securities 105 1,682.3 (13.3 ) 63 1,221.2 (5.9 ) 42 461.1 (7.4 ) Other asset-backed securities 197 1,837.3 (4.5 ) 134 1,377.8 (3.3 ) 63 459.5 (1.2 ) Redeemable preferred stocks 2 21.8 (1.5 ) 1 10.8 (0.1 ) 1 11.0 (1.4 ) Total fixed maturities 1,143 13,839.0 (90.4 ) 670 10,364.7 (57.6 ) 473 3,474.3 (32.8 ) Equity securities: Nonredeemable preferred stocks 4 127.8 (8.8 ) 1 56.5 (0.5 ) 3 71.3 (8.3 ) Common equities 19 13.4 (0.2 ) 18 13.4 (0.2 ) 1 0 0 Total equity securities 23 141.2 (9.0 ) 19 69.9 (0.7 ) 4 71.3 (8.3 ) Total portfolio 1,166 $ 13,980.2 $ (99.4 ) 689 $ 10,434.6 $ (58.3 ) 477 $ 3,545.6 $ (41.1 )
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) 2018 2017 Fixed maturities: Residential mortgage-backed securities $ (19.7 ) $ (19.7 ) $ (19.7 ) Commercial mortgage-backed securities (0.1 ) (0.4 ) (0.3 ) Total fixed maturities $ (19.8 ) $ (20.1 ) $ (20.0 )
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, 2018 and 2017 , 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, 2018 Mortgage-Backed (millions) Residential Commercial Total Balance at June 30, 2018 $ 0.3 $ 0.5 $ 0.8 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 0 0 Balance at September 30, 2018 $ 0.3 $ 0.5 $ 0.8 Nine Months Ended September 30, 2018 Mortgage-Backed (millions) Residential Commercial Total Balance at December 31, 2017 $ 0 $ 0.5 $ 0.5 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, 2018 $ 0.3 $ 0.5 $ 0.8 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 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) 2018 2017 2018 2017 Gross realized gains on security sales Available-for-sale securities: U.S. government obligations $ 0.1 $ 0.9 $ 1.8 $ 5.8 State and local government obligations 0.2 4.0 9.4 7.1 Corporate and other debt securities 1.7 5.1 2.1 16.5 Residential mortgage-backed securities 0 2.8 0 23.8 Commercial mortgage-backed securities 0 0 2.0 2.4 Other asset-backed securities 0 0 0.1 0.3 Redeemable preferred stocks 0.2 7.7 4.5 8.0 Total available-for-sale securities 2.2 20.5 19.9 63.9 Equity securities: Nonredeemable preferred stocks 0.1 3.0 3.7 54.6 Common equities 126.8 5.7 265.2 23.0 Total equity securities 126.9 8.7 268.9 77.6 Subtotal gross realized gains on security sales 129.1 29.2 288.8 141.5 Gross realized losses on security sales Available-for-sale securities: U.S. government obligations (7.6 ) (1.0 ) (46.4 ) (4.6 ) State and local government obligations (0.6 ) 0 (2.5 ) (0.1 ) Corporate and other debt securities (3.2 ) (1.8 ) (7.3 ) (4.6 ) Residential mortgage-backed securities 0 (0.1 ) 0 (0.4 ) Commercial mortgage-backed securities 0 (0.5 ) (6.3 ) (3.6 ) Other asset-backed securities (0.1 ) 0 (1.1 ) 0 Redeemable preferred stocks 0 (6.4 ) 0 (6.4 ) Total available-for-sale securities (11.5 ) (9.8 ) (63.6 ) (19.7 ) Equity securities: Nonredeemable preferred stocks 0 (0.1 ) (2.3 ) (5.9 ) Common equities (9.2 ) (0.2 ) (17.1 ) (0.3 ) Total equity securities (9.2 ) (0.3 ) (19.4 ) (6.2 ) Subtotal gross realized losses on security sales (20.7 ) (10.1 ) (83.0 ) (25.9 ) Net realized gains (losses) on security sales Available-for-sale securities: U.S. government obligations (7.5 ) (0.1 ) (44.6 ) 1.2 State and local government obligations (0.4 ) 4.0 6.9 7.0 Corporate and other debt securities (1.5 ) 3.3 (5.2 ) 11.9 Residential mortgage-backed securities 0 2.7 0 23.4 Commercial mortgage-backed securities 0 (0.5 ) (4.3 ) (1.2 ) Other asset-backed securities (0.1 ) 0 (1.0 ) 0.3 Redeemable preferred stocks 0.2 1.3 4.5 1.6 Total available-for-sale securities (9.3 ) 10.7 (43.7 ) 44.2 Equity securities: Nonredeemable preferred stocks 0.1 2.9 1.4 48.7 Common equities 117.6 5.5 248.1 22.7 Total equity securities 117.7 8.4 249.5 71.4 Litigation settlements and other gains (losses) 0 0.1 0 1.2 Subtotal net realized gains (losses) on security sales 108.4 19.2 205.8 116.8 Net holding period gains (losses) Hybrid securities 1.3 (0.9 ) (4.2 ) 0.3 Equity securities 94.5 0 (1.7 ) 0 Subtotal net holding period gains (losses) 95.8 (0.9 ) (5.9 ) 0.3 Other-than-temporary impairment losses Fixed maturities: Commercial mortgage-backed securities 0 (0.4 ) 0 (0.4 ) Total fixed maturities 0 (0.4 ) 0 (0.4 ) Equity securities: Common equities 0 (8.9 ) 0 (12.5 ) Subtotal investment other-than-temporary impairment losses 0 (9.3 ) 0 (12.9 ) Other asset impairment (22.1 ) (33.7 ) (33.2 ) (44.9 ) Subtotal other-than-temporary impairment losses (22.1 ) (43.0 ) (33.2 ) (57.8 ) Total net realized gains (losses) on securities $ 182.1 $ (24.7 ) $ 166.7 $ 59.3
Components of Equity Realized Gains (Losses) [Table Text Block]	The following table reflects our holding period realized gains (losses) on equity securities recognized for the three and nine months ended September 30, 2018 for equity securities held at quarter end: Three Months Nine Months (millions) 2018 2018 Total net gains (losses) recognized during the period on equity securities $ 212.2 $ 247.8 Less: Net gains (losses) recognized on equity securities sold during the period 117.7 249.5 Net holding period gains (losses) recognized during the period on equity securities held at period end $ 94.5 $ (1.7 ) Note: Comparative disclosure for the prior year period is not meaningful.
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) 2018 2017 2018 2017 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 47.7 $ 18.4 $ 133.4 $ 48.1 State and local government obligations 9.0 12.9 28.1 39.3 Foreign government obligations 0 0.1 0 0.3 Corporate debt securities 62.1 32.2 150.1 93.1 Residential mortgage-backed securities 6.9 8.0 20.4 27.9 Commercial mortgage-backed securities 24.1 20.0 65.5 57.1 Other asset-backed securities 21.2 12.1 50.9 33.8 Redeemable preferred stocks 3.3 2.8 8.8 9.1 Total fixed maturities 174.3 106.5 457.2 308.7 Short-term investments 15.7 10.6 40.7 26.4 Total available-for-sale securities 190.0 117.1 497.9 335.1 Equity securities: Nonredeemable preferred stocks 11.2 10.9 32.5 33.0 Common equities 16.9 14.9 46.1 42.8 Total equity securities 28.1 25.8 78.6 75.8 Investment income 218.1 142.9 576.5 410.9 Investment expenses (5.8 ) (5.8 ) (18.0 ) (18.0 ) Net investment income $ 212.3 $ 137.1 $ 558.5 $ 392.9

Fair Value (Tables)

Fair Value (Tables)	9 Months Ended
	Sep. 30, 2018
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, 2018 Fixed maturities: U.S. government obligations $ 9,752.5 $ 0 $ 0 $ 9,752.5 $ 9,942.0 State and local government obligations 0 1,588.8 0 1,588.8 1,610.1 Foreign government obligations 0 0 0 0 0 Corporate debt securities 0 7,146.8 0 7,146.8 7,229.1 Subtotal 9,752.5 8,735.6 0 18,488.1 18,781.2 Asset-backed securities: Residential mortgage-backed 0 769.8 0 769.8 767.8 Commercial mortgage-backed 0 2,959.3 0 2,959.3 2,986.5 Other asset-backed 0 3,192.9 0 3,192.9 3,205.0 Subtotal asset-backed securities 0 6,922.0 0 6,922.0 6,959.3 Redeemable preferred stocks: Financials 0 67.8 0 67.8 65.2 Utilities 0 4.7 0 4.7 4.8 Industrials 10.1 150.0 0 160.1 152.9 Subtotal redeemable preferred stocks 10.1 222.5 0 232.6 222.9 Total fixed maturities 9,762.6 15,880.1 0 25,642.7 25,963.4 Short-term investments 2,573.4 236.3 0 2,809.7 2,809.7 Total available-for-sale securities 12,336.0 16,116.4 0 28,452.4 28,773.1 Equity securities: Nonredeemable preferred stocks: Financials 76.3 724.0 0 800.3 718.9 Utilities 0 35.6 0 35.6 35.0 Industrials 0 0 5.0 5.0 5.0 Subtotal nonredeemable preferred stocks 76.3 759.6 5.0 840.9 758.9 Common equities: Common stocks 3,057.0 0 0 3,057.0 1,134.7 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 3,057.0 0 0.3 3,057.3 1,135.0 Total equity securities 3,133.3 759.6 5.3 3,898.2 1,893.9 Total portfolio $ 15,469.3 $ 16,876.0 $ 5.3 $ 32,350.6 $ 30,667.0 Debt $ 0 $ 3,926.9 $ 0 $ 3,926.9 $ 3,859.9 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 0 955.6 0 955.6 947.0 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 Utilities 0 0 0 0 0 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 December 31, 2017 Fixed maturities: U.S. government obligations $ 6,645.9 $ 0 $ 0 $ 6,645.9 $ 6,688.8 State and local government obligations 0 2,297.1 0 2,297.1 2,285.6 Foreign government obligations 0 0 0 0 0 Corporate debt securities 0 4,997.7 0 4,997.7 4,997.2 Subtotal 6,645.9 7,294.8 0 13,940.7 13,971.6 Asset-backed securities: Residential mortgage-backed 0 836.7 0 836.7 828.8 Commercial mortgage-backed 0 2,758.6 0 2,758.6 2,760.1 Other asset-backed 0 2,454.7 0 2,454.7 2,454.5 Subtotal asset-backed securities 0 6,050.0 0 6,050.0 6,043.4 Redeemable preferred stocks: Financials 0 64.1 0 64.1 61.3 Utilities 0 11.4 0 11.4 10.1 Industrials 0 135.5 0 135.5 123.5 Subtotal redeemable preferred stocks 0 211.0 0 211.0 194.9 Total fixed maturities 6,645.9 13,555.8 0 20,201.7 20,209.9 Equity securities: Nonredeemable preferred stocks: Financials 80.6 718.2 0 798.8 693.6 Utilities 0 0 0 0 0 Industrials 0 0 5.0 5.0 5.0 Subtotal nonredeemable preferred stocks 80.6 718.2 5.0 803.8 698.6 Common equities: Common stocks 3,399.5 0 0 3,399.5 1,498.7 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 3,399.5 0 0.3 3,399.8 1,499.0 Total fixed maturities and equity securities 10,126.0 14,274.0 5.3 24,405.3 22,407.5 Short-term investments 1,824.4 1,045.0 0 2,869.4 2,869.4 Total portfolio $ 11,950.4 $ 15,319.0 $ 5.3 $ 27,274.7 $ 25,276.9 Debt $ 0 $ 3,606.5 $ 37.1 $ 3,643.6 $ 3,306.3

Debt (Tables)

Debt (Tables)	9 Months Ended
	Sep. 30, 2018
Debt Table	Debt at each of the balance sheet periods consisted of: September 30, 2018 September 30, 2017 December 31, 2017 (millions) Carrying Value Fair Value Carrying Value Fair Value Carrying Value Fair Value 3.75% Senior Notes due 2021 $ 499.0 $ 503.9 $ 498.7 $ 525.7 $ 498.8 $ 520.7 2.45% Senior Notes due 2027 496.4 453.3 496.0 475.3 496.1 477.9 6 5/8% Senior Notes due 2029 296.3 360.3 296.0 388.3 296.1 382.3 6.25% Senior Notes due 2032 395.4 481.8 395.3 518.7 395.3 516.9 4.35% Senior Notes due 2044 346.6 349.7 346.5 379.8 346.5 388.7 3.70% Senior Notes due 2045 395.3 359.1 395.2 392.6 395.2 402.9 4.125% Senior Notes due 2047 841.3 828.4 841.2 894.2 841.2 917.1 4.20% Senior Notes due 2048 589.6 590.4 0 0 0 0 Other debt instruments 0 0 43.3 43.3 37.1 37.1 Total $ 3,859.9 $ 3,926.9 $ 3,312.2 $ 3,617.9 $ 3,306.3 $ 3,643.6

Loss and Loss Adjustment Expe_2

Loss and Loss Adjustment Expense Reserves (Tables)	9 Months Ended
	Sep. 30, 2018
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 is summarized as follows: September 30, (millions) 2018 2017 Balance, Beginning of period $ 13,086.9 $ 11,368.0 Less reinsurance recoverables on unpaid losses 2,170.1 1,801.0 Net balance, Beginning of period 10,916.8 9,567.0 Incurred related to: Current year 15,722.2 13,886.3 Prior years 47.0 42.5 Total incurred 15,769.2 13,928.8 Paid related to: Current year 9,409.7 8,379.4 Prior years 5,031.9 4,387.8 Total paid 14,441.6 12,767.2 Net balance, End of period 12,244.4 10,728.6 Plus reinsurance recoverables on unpaid losses 2,376.4 2,624.7 Balance, End of period $ 14,620.8 $ 13,353.3

Supplemental Cash Flow Inform_2

Supplemental Cash Flow Information (Tables)	9 Months Ended
	Sep. 30, 2018
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information Table	We paid the following in the respective periods: Nine Months Ended September 30, (millions) 2018 2017 Income taxes $ 535.8 $ 538.7 Interest 116.4 107.4

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2018
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, 2018 2017 2018 2017 (millions) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Personal Lines Agency $ 3,318.2 $ 372.2 $ 2,840.0 $ 69.5 $ 9,607.7 $ 1,142.2 $ 8,224.0 $ 524.7 Direct 3,337.2 294.4 2,734.8 128.7 9,565.3 880.3 7,908.5 466.7 Total Personal Lines 1 6,655.4 666.6 5,574.8 198.2 19,173.0 2,022.5 16,132.5 991.4 Commercial Lines 939.6 112.7 714.0 42.8 2,632.5 307.8 2,031.2 166.4 Property 2 335.5 (7.8 ) 255.2 (69.0 ) 933.2 (31.2 ) 720.3 (57.5 ) Other indemnity 0 (0.1 ) 0 0 0 0.1 0 (0.3 ) Total underwriting operations 7,930.5 771.4 6,544.0 172.0 22,738.7 2,299.2 18,884.0 1,100.0 Fees and other revenues 3 122.6 NA 96.3 NA 342.4 NA 270.3 NA Service businesses 42.5 6.7 33.3 4.4 119.6 17.5 94.5 12.7 Investments 4 400.2 394.4 118.2 112.4 743.2 725.2 470.2 452.2 Other gains (losses) 0 0 0 0 0 0 0.2 0.2 Interest expense NA (42.0 ) NA (37.4 ) NA (120.5 ) NA (117.6 ) Consolidated total $ 8,495.8 $ 1,130.5 $ 6,791.8 $ 251.4 $ 23,943.9 $ 2,921.4 $ 19,719.2 $ 1,447.5 NA = Not applicable 1 Personal auto insurance accounted for 94% of the total Personal Lines segment net premiums earned during the three and nine months ended September 30, 2018 , and 93% for the same periods in 2017 ; insurance for our special lines products (e.g., motorcycles, ATVs, RVs, watercraft, and snowmobiles) accounted for the balance of the Personal Lines net premiums earned. 2 For the three and nine months ended September 30, 2018 , pretax profit (loss) includes $18.0 million and $54.0 million , respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX and $17.2 million and $48.2 million for the same periods in 2017 . Although this expense is included in our Property segment, it is not reported in the consolidated results of ARX and, therefore, will not affect the value of net income (loss) attributable to noncontrolling interest. 3 Pretax profit (loss) for fees and other revenues is attributable to operating segments. 4 Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit is net of investment expense.
Underwriting Margins and Combined Ratios for our Underwriting Operations	Following are the underwriting margins/combined ratios for our underwriting operations for the respective periods: Three Months Ended September 30, Nine Months Ended September 30, 2018 2017 2018 2017 Under-writing Combined Ratio Under-writing Combined Ratio Under-writing Margin Combined Ratio Under-writing Margin Combined Ratio Personal Lines Agency 11.2 % 88.8 2.4 % 97.6 11.9 % 88.1 6.4 % 93.6 Direct 8.8 91.2 4.7 95.3 9.2 90.8 5.9 94.1 Total Personal Lines 10.0 90.0 3.6 96.4 10.5 89.5 6.1 93.9 Commercial Lines 12.0 88.0 6.0 94.0 11.7 88.3 8.2 91.8 Property 1 (2.3 ) 102.3 (27.0 ) 127.0 (3.3 ) 103.3 (8.0 ) 108.0 Total underwriting operations 9.7 90.3 2.6 97.4 10.1 89.9 5.8 94.2 1 Included in the three and nine months ended September 30, 2018 , is 5.4 points and 5.8 points, respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX and 6.7 points for the three and nine months ended September 30, 2017 .

Dividends Dividends (Tables)

Dividends Dividends (Tables)	9 Months Ended
	Sep. 30, 2018
Dividends [Abstract]
Dividends	Following is a summary of our common shareholder dividends that were declared in the last two years: (millions, except per share amounts) Amount of Common Share Dividends Dividend Type Declared Paid Per Share Accrued 1 Paid 1 Annual – Variable December 2017 February 2018 $ 1.1247 $ 655.1 $ 654.9 Annual – Variable December 2016 February 2017 0.6808 395.4 395.4 1 Variance between accrued and paid, if any, reflects the difference between the number of estimated and actual shares outstanding as of the record date.

Other Comprehensive Income (L_2

Other Comprehensive Income (Loss) (Tables)	9 Months Ended
	Sep. 30, 2018
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 Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income (loss) Total tax (provision) benefit After tax total accumulated other comprehensive income (loss) Total net unrealized gains (losses) on securities Net unrealized gains (losses) on forecasted transactions Foreign currency translation adjustment (Income) loss attributable to NCI Balance at June 30, 2018 $ (280.8 ) $ 58.9 $ (221.9 ) $ (210.9 ) $ (17.6 ) $ 0 $ 6.6 Reclassification of disproportionate amounts 1 (4.3 ) 0.9 (3.4 ) 0 0 0 (3.4 ) Adjusted balance at June 30, 2018 (285.1 ) 59.8 (225.3 ) (210.9 ) (17.6 ) 0 3.2 Other comprehensive income (loss) before reclassifications: Investment securities (58.7 ) 12.3 (46.4 ) (46.4 ) 0 0 0 Forecasted transactions 0 0 0 0 0 0 0 Foreign currency translation adjustment 0 0 0 0 0 0 0 Loss attributable to noncontrolling interest (NCI) 0.6 (0.2 ) 0.4 0 0 0 0.4 Total other comprehensive income (loss) before reclassifications (58.1 ) 12.1 (46.0 ) (46.4 ) 0 0 0.4 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings 0 0 0 0 0 0 0 Net realized gains (losses) on securities (9.3 ) 1.9 (7.4 ) (7.4 ) 0 0 0 Interest expense (0.2 ) 0 (0.2 ) 0 (0.2 ) 0 0 Total reclassification adjustment for amounts realized in net income (9.5 ) 1.9 (7.6 ) (7.4 ) (0.2 ) 0 0 Total other comprehensive income (loss) (48.6 ) 10.2 (38.4 ) (39.0 ) 0.2 0 0.4 Balance at September 30, 2018 $ (333.7 ) $ 70.0 $ (263.7 ) $ (249.9 ) $ (17.4 ) $ 0 $ 3.6 1 Reflects the change in value on (income) loss attributable to NCI in conjunction with the "put" transaction (See Note 12 – Redeemable Noncontrolling Interest for additional information). Components of Changes in Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income (loss) Total tax (provision) benefit After tax total accumulated other comprehensive income (loss) Total net unrealized gains (losses) on securities Net unrealized gains (losses) on forecasted transactions Foreign currency translation adjustment (Income) loss attributable to NCI Balance at December 31, 2017 $ 1,977.8 $ (695.6 ) $ 1,282.2 $ 1,295.0 $ (14.8 ) $ 0 $ 2.0 Cumulative effect adjustment 1 (2,006.0 ) 705.8 (1,300.2 ) (1,300.2 ) 0 0 0 Reclassification of disproportionate amounts 2 (4.3 ) (3.4 ) (7.7 ) (1.1 ) (3.2 ) 0 (3.4 ) Adjusted balance at December 31, 2017 (32.5 ) 6.8 (25.7 ) (6.3 ) (18.0 ) 0 (1.4 ) Other comprehensive income (loss) before reclassifications: Investment securities (352.5 ) 73.9 (278.6 ) (278.6 ) 0 0 0 Forecasted transactions 0 0 0 0 0 0 0 Foreign currency translation adjustment 0 0 0 0 0 0 0 Loss attributable to noncontrolling interest (NCI) 6.4 (1.4 ) 5.0 0 0 0 5.0 Total other comprehensive income (loss) before reclassifications (346.1 ) 72.5 (273.6 ) (278.6 ) 0 0 5.0 Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings 0 0 0 0 0 0 0 Net realized gains (losses) on securities (44.2 ) 9.2 (35.0 ) (35.0 ) 0 0 0 Interest expense (0.7 ) 0.1 (0.6 ) 0 (0.6 ) 0 0 Total reclassification adjustment for amounts realized in net income (44.9 ) 9.3 (35.6 ) (35.0 ) (0.6 ) 0 0 Total other comprehensive income (loss) (301.2 ) 63.2 (238.0 ) (243.6 ) 0.6 0 5.0 Balance at September 30, 2018 $ (333.7 ) $ 70.0 $ (263.7 ) $ (249.9 ) $ (17.4 ) $ 0 $ 3.6 1 Reflects the fair value changes on equity securities as of December 31, 2017 , which are reported as realized gains (losses) under the new accounting guidance. See Note 14 – New Accounting Standards for additional information. 2 Reflects the effect of the change in the U.S. federal tax rate on our available-for-sale fixed-maturity securities and our hedges on forecasted transactions as of December 31, 2017 (See Note 14 – New Accounting Standards for additional information) and the adjustment to reflect the change in value on (income) loss attributable to NCI in conjunction with the "put" transaction (See Note 12 – Redeemable Noncontrolling Interest for additional information). Components of Changes in Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income (loss) Total tax (provision) benefit After tax total accumulated other comprehensive income (loss) Total net unrealized gains (losses) on securities Net Foreign currency translation adjustment (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 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 (loss) Total tax (provision) benefit After tax total accumulated other comprehensive income (loss) Total net Net unrealized gains (losses) 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 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

Redeemable Noncontrolling Int_2

Redeemable Noncontrolling Interest (Tables)	9 Months Ended
	Sep. 30, 2018
Temporary Equity Disclosure [Abstract]
Redeemable Noncontrolling Interest	The changes in the components of redeemable NCI during the nine months ended September 30, 2018 and 2017 , and the year ended December 31, 2017 , were: (millions) September 30, 2018 September 30, 2017 December 31, 2017 Balance, Beginning of period $ 503.7 $ 483.7 $ 483.7 Net income attributable to NCI 10.6 1.9 5.9 Other comprehensive income (loss) attributable to NCI 1 (5.0 ) 2.9 2.3 Exercise of employee stock options 9.4 3.4 3.4 Purchase/change of ARX minority shares (298.2 ) 0 0 Change in redemption value of NCI (3.1 ) 6.3 8.4 Balance, End of period $ 217.4 $ 498.2 $ 503.7

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	9 Months Ended
	Sep. 30, 2018
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, 2018 and 2017 , and December 31, 2017 : (millions) September 30, 2018 September 30, 2017 December 31, 2017 Intangible assets subject to amortization $ 300.2 $ 372.2 $ 354.2 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 312.6 $ 384.6 $ 366.6 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 include $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, 2018 September 30, 2017 December 31, 2017 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 $ 128.1 $ 128.1 $ 256.2 $ 91.5 $ 164.7 $ 256.2 $ 100.7 $ 155.5 Agency relationships 159.2 39.8 119.4 159.2 28.4 130.8 159.2 31.3 127.9 Software rights 79.1 37.4 41.7 79.1 26.8 52.3 79.1 29.4 49.7 Trade name 34.8 23.8 11.0 34.8 10.4 24.4 34.8 13.7 21.1 Total $ 529.3 $ 229.1 $ 300.2 $ 529.3 $ 157.1 $ 372.2 $ 529.3 $ 175.1 $ 354.2

Basis of Presentation Basis of

Basis of Presentation Basis of Presentation-Additional Information (Details) $ in Millions	Sep. 30, 2018USD ($)shares	Dec. 31, 2017USD ($)	Sep. 30, 2017USD ($)
Business Acquisition [Line Items]
Assets Held-for-sale, Property, Plant and Equipment | $	$ 20.5	$ 5.3	$ 5.3
ARX Holding Corp. [Member]
Business Acquisition [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage	86.80%	69.00%	69.00%
Number of Shares Put Pursuant to Stockholders' Agreement | shares	204,527

Investments Investment Portfoli

Investments Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 28,773.1
Gross Unrealized Gains	32.3
Gross Unrealized Losses	(349)
Net Holding Period Gains (Losses)	(4)
Total available-for-sale securities	$ 28,452.4
% of Total Fair Value	88.00%
Equity securities, at Fair Value
Cost	$ 1,893.9
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Net Holding Period Gains (Losses)	2,004.3
Fair Value	$ 3,898.2
% of Total Fair Value	12.00%
Debt and Equity Securities, at Fair Value
Cost	$ 30,667
Gross Unrealized Gains	32.3
Gross Unrealized Losses	(349)
Net Holding Period Gains (Losses)	2,000.3
Investments, Fair Value	$ 32,350.6	$ 27,274.7	$ 26,994.7
% of Total Fair Value	100.00%
Available-for-Sale Securities
Cost		25,276.9	25,080.7
Gross Unrealized Gains		2,097	1,972.6
Gross Unrealized Losses		(99.4)	(60.7)
Net Holding Period Gains (Losses)		0.2	2.1
Fair Value		$ 27,274.7	$ 26,994.7
% of Total Fair Value		100.00%	100.00%
Unsettled security transactions, payable	$ 5.2		$ 238.3
Unsettled security transactions, receivable		$ 5.8
Total fair value of non-insurance subsidiary portfolio	1,800	1,600	1,100
Nonredeemable preferred stocks
Equity securities, at Fair Value
Cost	758.9
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Net Holding Period Gains (Losses)	82
Fair Value	$ 840.9
% of Total Fair Value	2.60%
Available-for-Sale Securities
Cost		698.6	700.6
Gross Unrealized Gains		114	120.4
Gross Unrealized Losses		(8.8)	(7.3)
Net Holding Period Gains (Losses)		0	0
Fair Value		$ 803.8	$ 813.7
% of Total Fair Value		2.90%	3.00%
Common equities
Equity securities, at Fair Value
Cost	$ 1,135
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Net Holding Period Gains (Losses)	1,922.3
Fair Value	$ 3,057.3
% of Total Fair Value	9.40%
Available-for-Sale Securities
Cost		$ 1,499	$ 1,485.5
Gross Unrealized Gains		1,901	1,729.5
Gross Unrealized Losses		(0.2)	(5.5)
Net Holding Period Gains (Losses)		0	0
Fair Value		$ 3,399.8	$ 3,209.5
% of Total Fair Value		12.50%	11.90%
Total fixed maturities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 25,963.4
Gross Unrealized Gains	32.3
Gross Unrealized Losses	(349)
Net Holding Period Gains (Losses)	(4)
Total available-for-sale securities	$ 25,642.7
% of Total Fair Value	79.30%
Available-for-Sale Securities
Cost		$ 20,209.9	$ 18,583.1
Gross Unrealized Gains		82	122.7
Gross Unrealized Losses		(90.4)	(47.9)
Net Holding Period Gains (Losses)		0.2	2.1
Fair Value		$ 20,201.7	$ 18,660
% of Total Fair Value		74.10%	69.10%
Total fixed maturities | U.S. government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 9,942
Gross Unrealized Gains	0
Gross Unrealized Losses	(189.5)
Net Holding Period Gains (Losses)	0
Total available-for-sale securities	$ 9,752.5
% of Total Fair Value	30.20%
Available-for-Sale Securities
Cost		$ 6,688.8	$ 4,612.2
Gross Unrealized Gains		1.1	3
Gross Unrealized Losses		(44)	(20.2)
Net Holding Period Gains (Losses)		0	0
Fair Value		$ 6,645.9	$ 4,595
% of Total Fair Value		24.40%	17.00%
Total fixed maturities | State and local government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 1,610.1
Gross Unrealized Gains	1.9
Gross Unrealized Losses	(23.2)
Net Holding Period Gains (Losses)	0
Total available-for-sale securities	$ 1,588.8
% of Total Fair Value	4.90%
Available-for-Sale Securities
Cost		$ 2,285.6	$ 2,332.2
Gross Unrealized Gains		20.7	35
Gross Unrealized Losses		(9.3)	(3.1)
Net Holding Period Gains (Losses)		0.1	0.1
Fair Value		$ 2,297.1	$ 2,364.2
% of Total Fair Value		8.40%	8.70%
Total fixed maturities | Foreign government obligations
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 0
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Net Holding Period Gains (Losses)	0
Total available-for-sale securities	$ 0
% of Total Fair Value	0.00%
Available-for-Sale Securities
Cost		$ 0	$ 24.2
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Net Holding Period Gains (Losses)		0	0
Fair Value		$ 0	$ 24.2
% of Total Fair Value		0.00%	0.10%
Total fixed maturities | Corporate debt securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 7,229.1
Gross Unrealized Gains	4.9
Gross Unrealized Losses	(85.2)
Net Holding Period Gains (Losses)	(2)
Total available-for-sale securities	$ 7,146.8
% of Total Fair Value	22.10%
Available-for-Sale Securities
Cost		$ 4,997.2	$ 5,195.7
Gross Unrealized Gains		14.8	32.7
Gross Unrealized Losses		(14.4)	(4.2)
Net Holding Period Gains (Losses)		0.1	1.4
Fair Value		$ 4,997.7	$ 5,225.6
% of Total Fair Value		18.30%	19.40%
Total fixed maturities | Residential mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 767.8
Gross Unrealized Gains	7.6
Gross Unrealized Losses	(5.6)
Net Holding Period Gains (Losses)	0
Total available-for-sale securities	$ 769.8
% of Total Fair Value	2.40%
Available-for-Sale Securities
Cost		$ 828.8	$ 947
Gross Unrealized Gains		11.3	12.2
Gross Unrealized Losses		(3.4)	(3.6)
Net Holding Period Gains (Losses)		0	0
Fair Value		$ 836.7	$ 955.6
% of Total Fair Value		3.10%	3.50%
Total fixed maturities | Commercial mortgage-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 2,986.5
Gross Unrealized Gains	3.3
Gross Unrealized Losses	(30.5)
Net Holding Period Gains (Losses)	0
Total available-for-sale securities	$ 2,959.3
% of Total Fair Value	9.10%
Available-for-Sale Securities
Cost		$ 2,760.1	$ 2,763.7
Gross Unrealized Gains		11.8	17
Gross Unrealized Losses		(13.3)	(12.8)
Net Holding Period Gains (Losses)		0	0
Fair Value		$ 2,758.6	$ 2,767.9
% of Total Fair Value		10.10%	10.30%
Total fixed maturities | Other asset-backed securities
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 3,205
Gross Unrealized Gains	1.4
Gross Unrealized Losses	(13.6)
Net Holding Period Gains (Losses)	0.1
Total available-for-sale securities	$ 3,192.9
% of Total Fair Value	9.90%
Available-for-Sale Securities
Cost		$ 2,454.5	$ 2,485.6
Gross Unrealized Gains		4.5	6.4
Gross Unrealized Losses		(4.5)	(2)
Net Holding Period Gains (Losses)		0.2	0.2
Fair Value		$ 2,454.7	$ 2,490.2
% of Total Fair Value		9.00%	9.20%
Total fixed maturities | Redeemable preferred stocks
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 222.9
Gross Unrealized Gains	13.2
Gross Unrealized Losses	(1.4)
Net Holding Period Gains (Losses)	(2.1)
Total available-for-sale securities	$ 232.6
% of Total Fair Value	0.70%
Available-for-Sale Securities
Cost		$ 194.9	$ 222.5
Gross Unrealized Gains		17.8	16.4
Gross Unrealized Losses		(1.5)	(2)
Net Holding Period Gains (Losses)		(0.2)	0.4
Fair Value		$ 211	$ 237.3
% of Total Fair Value		0.80%	0.90%
Short-term investments
Debt Securities, Available-for-Sale, at Fair Value
Cost	$ 2,809.7
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Net Holding Period Gains (Losses)	0
Total available-for-sale securities	$ 2,809.7
% of Total Fair Value	8.70%
Available-for-Sale Securities
Cost		$ 2,869.4	$ 4,311.5
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Net Holding Period Gains (Losses)		0	0
Fair Value		$ 2,869.4	$ 4,311.5
% of Total Fair Value		10.50%	16.00%

Investments Hybrid Securities (

Investments Hybrid Securities (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Hybrid Instruments [Line Items]
Hybrid securities	$ 240.3	$ 142.9	$ 159.9
State and local government obligations
Hybrid Instruments [Line Items]
Hybrid securities	3.6	6.1	2.4
Corporate debt securities
Hybrid Instruments [Line Items]
Hybrid securities	159.7	99.8	114.9
Other asset-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	5	6.7	7.1
Redeemable preferred stocks
Hybrid Instruments [Line Items]
Hybrid securities	$ 72	$ 30.3	$ 35.5

Investments Composition of Fixe

Investments Composition of Fixed Maturities by Maturity (Detail) - Total fixed maturities $ in Millions	Sep. 30, 2018USD ($)
Schedule of Debt Securities by Contractual Maturity [Line Items]
Less than one year	$ 4,192.3
One to five years	16,545
Five to ten years	5,123.1
Ten years or greater	103
Cost	25,963.4
Less than one year	4,196.4
One to five years	16,315.6
Five to ten years	5,028.3
Ten years or greater	102.4
Debt Securities, Available-for-Sale	$ 25,642.7

Investments Gross Unrealized lo

Investments Gross Unrealized losses by Major Security (Detail) $ in Millions	Sep. 30, 2018USD ($)security	Dec. 31, 2017USD ($)security	Sep. 30, 2017USD ($)security
No. of Sec.
Total | security	1,526
Less than 12 Months | security	723
12 Months or Greater | security	803
Total | security		1,166	852
Less than 12 Months | security		689	383
12 Months or Greater | security		477	469
Fair Value
Total	$ 22,879.8
Less than 12 Months	12,985.1
12 Months or Greater	9,894.7
Total		$ 13,980.2	$ 9,407.6
Less than 12 Months		10,434.6	5,172.9
12 Months or Greater		3,545.6	4,234.7
Unrealized Losses
Total	(349)
Less than 12 Months	(131.9)
12 Months or Greater	$ (217.1)
Total		(99.4)	(60.7)
Less than 12 Months		(58.3)	(19.5)
12 Months or Greater		$ (41.1)	$ (41.2)
U.S. government obligations
No. of Sec.
Total | security	91
Less than 12 Months | security	39
12 Months or Greater | security	52
Total | security		58	54
Less than 12 Months | security		41	31
12 Months or Greater | security		17	23
Fair Value
Total	$ 9,752.5
Less than 12 Months	5,025
12 Months or Greater	4,727.5
Total		$ 5,817	$ 3,804
Less than 12 Months		4,869.3	2,302.2
12 Months or Greater		947.7	1,501.8
Unrealized Losses
Total	(189.5)
Less than 12 Months	(67.2)
12 Months or Greater	$ (122.3)
Total		(44)	(20.2)
Less than 12 Months		(34.6)	(7.1)
12 Months or Greater		$ (9.4)	$ (13.1)
State and local government obligations
No. of Sec.
Total | security	459
Less than 12 Months | security	227
12 Months or Greater | security	232
Total | security		358	169
Less than 12 Months | security		230	40
12 Months or Greater | security		128	129
Fair Value
Total	$ 1,394.3
Less than 12 Months	680.5
12 Months or Greater	713.8
Total		$ 1,200.3	$ 610.4
Less than 12 Months		737.6	141
12 Months or Greater		462.7	469.4
Unrealized Losses
Total	(23.2)
Less than 12 Months	(6.4)
12 Months or Greater	$ (16.8)
Total		(9.3)	(3.1)
Less than 12 Months		(4.4)	(0.7)
12 Months or Greater		$ (4.9)	$ (2.4)
Corporate debt securities
No. of Sec.
Total | security	369
Less than 12 Months | security	221
12 Months or Greater | security	148
Total | security		222	113
Less than 12 Months | security		171	59
12 Months or Greater | security		51	54
Fair Value
Total	$ 5,959.2
Less than 12 Months	3,963.9
12 Months or Greater	1,995.3
Total		$ 2,979.4	$ 1,523.8
Less than 12 Months		2,072.9	636
12 Months or Greater		906.5	887.8
Unrealized Losses
Total	(85.2)
Less than 12 Months	(45.3)
12 Months or Greater	$ (39.9)
Total		(14.4)	(4.2)
Less than 12 Months		(9.1)	(1.1)
12 Months or Greater		$ (5.3)	$ (3.1)
Residential mortgage-backed securities
No. of Sec.
Total | security	234
Less than 12 Months | security	44
12 Months or Greater | security	190
Total | security		201	191
Less than 12 Months | security		30	26
12 Months or Greater | security		171	165
Fair Value
Total	$ 521.7
Less than 12 Months	325
12 Months or Greater	196.7
Total		$ 300.9	$ 341.5
Less than 12 Months		75.1	39.2
12 Months or Greater		225.8	302.3
Unrealized Losses
Total	(5.6)
Less than 12 Months	(0.6)
12 Months or Greater	$ (5)
Total		(3.4)	(3.6)
Less than 12 Months		(0.2)	(0.1)
12 Months or Greater		$ (3.2)	$ (3.5)
Commercial mortgage-backed securities
No. of Sec.
Total | security	151
Less than 12 Months | security	80
12 Months or Greater | security	71
Total | security		105	105
Less than 12 Months | security		63	61
12 Months or Greater | security		42	44
Fair Value
Total	$ 2,392.7
Less than 12 Months	1,164.2
12 Months or Greater	1,228.5
Total		$ 1,682.3	$ 1,588.9
Less than 12 Months		1,221.2	1,073.5
12 Months or Greater		461.1	515.4
Unrealized Losses
Total	(30.5)
Less than 12 Months	(8.3)
12 Months or Greater	$ (22.2)
Total		(13.3)	(12.8)
Less than 12 Months		(5.9)	(4.6)
12 Months or Greater		$ (7.4)	$ (8.2)
Other asset-backed securities
No. of Sec.
Total | security	219
Less than 12 Months | security	111
12 Months or Greater | security	108
Total | security		197	148
Less than 12 Months | security		134	103
12 Months or Greater | security		63	45
Fair Value
Total	$ 2,828.6
Less than 12 Months	1,821.8
12 Months or Greater	1,006.8
Total		$ 1,837.3	$ 1,393.2
Less than 12 Months		1,377.8	920.8
12 Months or Greater		459.5	472.4
Unrealized Losses
Total	(13.6)
Less than 12 Months	(4)
12 Months or Greater	$ (9.6)
Total		(4.5)	(2)
Less than 12 Months		(3.3)	(1)
12 Months or Greater		$ (1.2)	$ (1)
Redeemable preferred stocks
No. of Sec.
Total | security	3
Less than 12 Months | security	1
12 Months or Greater | security	2
Total | security		2	2
Less than 12 Months | security		1	1
12 Months or Greater | security		1	1
Fair Value
Total	$ 30.8
Less than 12 Months	4.7
12 Months or Greater	26.1
Total		$ 21.8	$ 21.4
Less than 12 Months		10.8	10.9
12 Months or Greater		11	10.5
Unrealized Losses
Total	(1.4)
Less than 12 Months	(0.1)
12 Months or Greater	$ (1.3)
Total		(1.5)	(2)
Less than 12 Months		(0.1)	0
12 Months or Greater		$ (1.4)	$ (2)
Total fixed maturities
No. of Sec.
Total | security		1,143	782
Less than 12 Months | security		670	321
12 Months or Greater | security		473	461
Fair Value
Total		$ 13,839	$ 9,283.2
Less than 12 Months		10,364.7	5,123.6
12 Months or Greater		3,474.3	4,159.6
Unrealized Losses
Total		(90.4)	(47.9)
Less than 12 Months		(57.6)	(14.6)
12 Months or Greater		$ (32.8)	$ (33.3)
Nonredeemable preferred stocks
No. of Sec.
Total | security		4	3
Less than 12 Months | security		1	0
12 Months or Greater | security		3	3
Fair Value
Total		$ 127.8	$ 72.4
Less than 12 Months		56.5	0
12 Months or Greater		71.3	72.4
Unrealized Losses
Total		(8.8)	(7.3)
Less than 12 Months		(0.5)	0
12 Months or Greater		$ (8.3)	$ (7.3)
Common equities
No. of Sec.
Total | security		19	67
Less than 12 Months | security		18	62
12 Months or Greater | security		1	5
Fair Value
Total		$ 13.4	$ 52
Less than 12 Months		13.4	49.3
12 Months or Greater		0	2.7
Unrealized Losses
Total		(0.2)	(5.5)
Less than 12 Months		(0.2)	(4.9)
12 Months or Greater		$ 0	$ (0.6)
Total equity securities
No. of Sec.
Total | security		23	70
Less than 12 Months | security		19	62
12 Months or Greater | security		4	8
Fair Value
Total		$ 141.2	$ 124.4
Less than 12 Months		69.9	49.3
12 Months or Greater		71.3	75.1
Unrealized Losses
Total		(9)	(12.8)
Less than 12 Months		(0.7)	(4.9)
12 Months or Greater		$ (8.3)	$ (7.9)

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) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
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	$ (19.8)	$ (20)	$ (20.1)
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)	(19.7)	(19.7)
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	$ (0.1)	$ (0.3)	$ (0.4)

Investments OTTI Credit Losses

Investments OTTI Credit Losses Recognized in Earnings (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	$ 800,000	$ 300,000	$ 500,000	$ 11,500,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	0	400,000	0	400,000
Reductions for securities sold/matured	0	0	0	(11,200,000)
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0	0	300,000	0
Ending balance	800,000	700,000	800,000	700,000
Residential mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	300,000	200,000	0	11,100,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	0	0	0	0
Reductions for securities sold/matured	0	0	0	(10,900,000)
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0	0	300,000	0
Ending balance	300,000	200,000	300,000	200,000
Commercial mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	500,000	100,000	500,000	400,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	0	400,000	0	400,000
Reductions for securities sold/matured	0	0	0	(300,000)
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0	0	0	0
Ending balance	$ 500,000	$ 500,000	$ 500,000	$ 500,000

Investments Components of Net R

Investments Components of Net Realized Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	$ 2.2		$ 19.9
Equity Securities, FV-NI, Gross Realized Gains on Sales	126.9		268.9
Debt and Equity Securities, Gross Realized Gains on Sales	129.1		288.8
Available-for-Sale Securities, Gross Realized Gains on Sales		$ 29.2		$ 141.5
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	(11.5)		(63.6)
Equity Securities, FV-NI, Gross Realized Losses on Sales	(9.2)		(19.4)
Debt and Equity Securities, Gross Realized Losses on Sales	(20.7)		(83)
Available-for-Sale Securities, Gross Realized Losses on Sales		(10.1)		(25.9)
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	(9.3)		(43.7)
Equity Securities, FV-NI, Net Realized Gain (Loss) on Sales	117.7		249.5
Litigation settlements and other gains (losses)	0	0.1	0	1.2
Gross Realized Gain (Loss), excluding Holding Period and OTTI	108.4		205.8
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments		19.2		116.8
Unrealized Gain (Loss) on Hybrid Instrument, Net	1.3	(0.9)	(4.2)	0.3
Net holding period gains (losses), equity securities	94.5		(1.7)
Net Holding Period Realized Gain (Loss)	95.8		(5.9)
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales, Debt and Equity Securities	0	(9.3)	0	(12.9)
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales	(22.1)	(43)	(33.2)	(57.8)
Total net realized gains (losses) on securities	182.1		166.7
Available-for-sale Securities, Gross Realized Gain (Loss)		(24.7)		59.3
U.S. government obligations
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	0.1		1.8
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	(7.6)		(46.4)
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	(7.5)		(44.6)
State and local government obligations
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	0.2		9.4
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	(0.6)		(2.5)
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	(0.4)		6.9
Corporate debt securities
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	1.7		2.1
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	(3.2)		(7.3)
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	(1.5)		(5.2)
Residential mortgage-backed securities
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	0		0
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	0		0
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	0		0
Commercial mortgage-backed securities
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	0		2
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	0		(6.3)
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	0		(4.3)
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales, Debt and Equity Securities	0	(0.4)	0	(0.4)
Other asset-backed securities
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	0		0.1
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	(0.1)		(1.1)
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	(0.1)		(1)
Redeemable preferred stocks
Gain (Loss) on Securities [Line Items]
Debt Securities, Available-for-Sale, Gross Realized Gains on Sales	0.2		4.5
Debt Securities, Available-for-Sale, Gross Realized Losses excluding Other-than-Temporary Impairment	0		0
Debt Securities, Available-for-Sale, Realized Gain (Loss) on Sales, Excluding Other-than-Temporary Impairment	0.2		4.5
Nonredeemable preferred stocks
Gain (Loss) on Securities [Line Items]
Equity Securities, FV-NI, Gross Realized Gains on Sales	0.1		3.7
Equity Securities, FV-NI, Gross Realized Losses on Sales	0		(2.3)
Equity Securities, FV-NI, Net Realized Gain (Loss) on Sales	0.1		1.4
Common equities
Gain (Loss) on Securities [Line Items]
Equity Securities, FV-NI, Gross Realized Gains on Sales	126.8		265.2
Equity Securities, FV-NI, Gross Realized Losses on Sales	(9.2)		(17.1)
Equity Securities, FV-NI, Net Realized Gain (Loss) on Sales	117.6		248.1
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales, Debt and Equity Securities	0	(8.9)	0	(12.5)
Total fixed maturities
Gain (Loss) on Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains on Sales		20.5		63.9
Available-for-Sale Securities, Gross Realized Losses on Sales		(9.8)		(19.7)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments		10.7		44.2
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales, Debt and Equity Securities	0	(0.4)	0	(0.4)
Total fixed maturities | U.S. government obligations
Gain (Loss) on Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains on Sales		0.9		5.8
Available-for-Sale Securities, Gross Realized Losses on Sales		(1)		(4.6)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments		(0.1)		1.2
Total fixed maturities | State and local government obligations
Gain (Loss) on Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains on Sales		4		7.1
Available-for-Sale Securities, Gross Realized Losses on Sales		0		(0.1)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments		4		7
Total fixed maturities | Corporate debt securities
Gain (Loss) on Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains on Sales		5.1		16.5
Available-for-Sale Securities, Gross Realized Losses on Sales		(1.8)		(4.6)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments		3.3		11.9
Total fixed maturities | Residential mortgage-backed securities
Gain (Loss) on Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains on Sales		2.8		23.8
Available-for-Sale Securities, Gross Realized Losses on Sales		(0.1)		(0.4)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments		2.7		23.4
Total fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains on Sales		0		2.4
Available-for-Sale Securities, Gross Realized Losses on Sales		(0.5)		(3.6)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments		(0.5)		(1.2)
Total fixed maturities | Other asset-backed securities
Gain (Loss) on Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains on Sales		0		0.3
Available-for-Sale Securities, Gross Realized Losses on Sales		0		0
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments		0		0.3
Total fixed maturities | Redeemable preferred stocks
Gain (Loss) on Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains on Sales		7.7		8
Available-for-Sale Securities, Gross Realized Losses on Sales		(6.4)		(6.4)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments		1.3		1.6
Total equity securities
Gain (Loss) on Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains on Sales		8.7		77.6
Available-for-Sale Securities, Gross Realized Losses on Sales		(0.3)		(6.2)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments		8.4		71.4
Total equity securities | Nonredeemable preferred stocks
Gain (Loss) on Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains on Sales		3		54.6
Available-for-Sale Securities, Gross Realized Losses on Sales		(0.1)		(5.9)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments		2.9		48.7
Total equity securities | Common equities
Gain (Loss) on Securities [Line Items]
Available-for-Sale Securities, Gross Realized Gains on Sales		5.7		23
Available-for-Sale Securities, Gross Realized Losses on Sales		(0.2)		(0.3)
Available-for-sale Securities, Gross Realized Gain (Loss), Excluding Other than Temporary Impairments		5.5		22.7
Other Assets
Gain (Loss) on Securities [Line Items]
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales	$ (22.1)	$ (33.7)	$ (33.2)	$ (44.9)

Investments Components of Equit

Investments Components of Equity Realized Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Components of Equity Realized Gains (Losses) [Line Items]
Realized Investment Gains (Losses)	$ 212.2	$ 247.8
Equity Securities, FV-NI, Net Realized Gain (Loss) on Sales	117.7	249.5
Equity Securities, FV-NI, Unrealized Gain (Loss)	$ 94.5	$ (1.7)

Investments Components of Net I

Investments Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net Investment Income [Line Items]
Gross investment income	$ 218.1	$ 142.9	$ 576.5	$ 410.9
Investment expenses	(5.8)	(5.8)	(18)	(18)
Net Investment Income	212.3	137.1	558.5	392.9
Total equity securities
Net Investment Income [Line Items]
Gross investment income	28.1	25.8	78.6	75.8
Total equity securities | Nonredeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	11.2	10.9	32.5	33
Total equity securities | Common equities
Net Investment Income [Line Items]
Gross investment income	16.9	14.9	46.1	42.8
Available-for-sale Securities
Net Investment Income [Line Items]
Gross investment income	190	117.1	497.9	335.1
Short-term investments
Net Investment Income [Line Items]
Gross investment income	15.7	10.6	40.7	26.4
Total fixed maturities
Net Investment Income [Line Items]
Gross investment income	174.3	106.5	457.2	308.7
Total fixed maturities | U.S. government obligations
Net Investment Income [Line Items]
Gross investment income	47.7	18.4	133.4	48.1
Total fixed maturities | State and local government obligations
Net Investment Income [Line Items]
Gross investment income	9	12.9	28.1	39.3
Total fixed maturities | Foreign government obligations
Net Investment Income [Line Items]
Gross investment income	0	0.1	0	0.3
Total fixed maturities | Corporate debt securities
Net Investment Income [Line Items]
Gross investment income	62.1	32.2	150.1	93.1
Total fixed maturities | Residential mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	6.9	8	20.4	27.9
Total fixed maturities | Commercial mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	24.1	20	65.5	57.1
Total fixed maturities | Other asset-backed securities
Net Investment Income [Line Items]
Gross investment income	21.2	12.1	50.9	33.8
Total fixed maturities | Redeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	$ 3.3	$ 2.8	$ 8.8	$ 9.1

Investments - Additional Inform

Investments - Additional Information (Detail) - USD ($)	Apr. 03, 2017	Apr. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Securities Sold under Agreements to Repurchase			$ 0	$ 0	$ 0	$ 0		$ 0
Open reverse repurchase commitments			0	0	0	0		0
Gross unrealized losses			349,000,000		349,000,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment			0	400,000	$ 0	$ 400,000
Investment Portfolio Duration					2 years 7 months 10 days	2 years 2 months 17 days
Trading securities			0	0	$ 0	$ 0		0
Trading Securities, Gain (Loss)			0	0	0	0
Derivative Assets (Liabilities), at Fair Value, Net			$ 0	$ 0	$ 0	$ 0		$ 0
4.20% Senior Notes due 2048
Schedule of Investments [Line Items]
Debt, face amount							$ 600,000,000
Debt, stated interest rate			4.20%		4.20%		4.20%
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)
Series B Preferred Stock [Member]
Schedule of Investments [Line Items]
Preferred Stock, Value, Issued							$ 500,000,000

Fair Value Composition of Inves

Fair Value Composition of Investment Portfolio by Major Security Type (Detail) - USD ($)	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	$ 32,350,600,000	$ 27,274,700,000	$ 26,994,700,000
Fair Value	3,926,900,000	3,643,600,000	3,617,900,000
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	15,469,300,000	11,950,400,000	11,086,000,000
Fair Value	0	0	0
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	16,876,000,000	15,319,000,000	15,903,400,000
Fair Value	3,926,900,000	3,606,500,000	3,574,600,000
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	5,300,000	5,300,000	5,300,000
Fair Value	0	37,100,000	43,300,000
Total Fixed Maturities And Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value		24,405,300,000	22,683,200,000
Total Fixed Maturities And Equity Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value		10,126,000,000	7,910,600,000
Total Fixed Maturities And Equity Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value		14,274,000,000	14,767,300,000
Total Fixed Maturities And Equity Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value		5,300,000	5,300,000
Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	28,452,400,000
Available-for-sale Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	12,336,000,000
Available-for-sale Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	16,116,400,000
Available-for-sale Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0
Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	3,898,200,000
Total equity securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	3,133,300,000
Total equity securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	759,600,000
Total equity securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	5,300,000
Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	840,900,000	803,800,000	813,700,000
Total equity securities | Nonredeemable preferred stocks | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	76,300,000	80,600,000	82,200,000
Total equity securities | Nonredeemable preferred stocks | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	759,600,000	718,200,000	726,500,000
Total equity securities | Nonredeemable preferred stocks | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	5,000,000	5,000,000	5,000,000
Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	800,300,000	798,800,000	808,700,000
Total equity securities | Nonredeemable preferred stocks | Financials | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	76,300,000	80,600,000	82,200,000
Total equity securities | Nonredeemable preferred stocks | Financials | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	724,000,000	718,200,000	726,500,000
Total equity securities | Nonredeemable preferred stocks | Financials | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total equity securities | Nonredeemable preferred stocks | Public Utility, Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	35,600,000	0	0
Total equity securities | Nonredeemable preferred stocks | Public Utility, Equities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total equity securities | Nonredeemable preferred stocks | Public Utility, Equities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	35,600,000	0	0
Total equity securities | Nonredeemable preferred stocks | Public Utility, Equities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	5,000,000	5,000,000	5,000,000
Total equity securities | Nonredeemable preferred stocks | Industrials | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total equity securities | Nonredeemable preferred stocks | Industrials | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total equity securities | Nonredeemable preferred stocks | Industrials | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	5,000,000	5,000,000	5,000,000
Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	3,057,300,000	3,399,800,000	3,209,500,000
Total equity securities | Common equities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	3,057,000,000	3,399,500,000	3,209,200,000
Total equity securities | Common equities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total equity securities | Common equities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	300,000	300,000	300,000
Total equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	3,057,000,000	3,399,500,000	3,209,200,000
Total equity securities | Common equities | Common equities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	3,057,000,000	3,399,500,000	3,209,200,000
Total equity securities | Common equities | Common equities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total equity securities | Common equities | Common equities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	300,000	300,000	300,000
Total equity securities | Common equities | Other risk investments | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total equity securities | Common equities | Other risk investments | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total equity securities | Common equities | Other risk investments | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	300,000	300,000	300,000
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	30,667,000,000	25,276,900,000	25,080,700,000
Long-term Debt	3,859,900,000	3,306,300,000	3,312,200,000
Carrying (Reported) Amount, Fair Value Disclosure | Total Fixed Maturities And Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value		22,407,500,000	20,769,200,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	28,773,100,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,893,900,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	758,900,000	698,600,000	700,600,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	718,900,000	693,600,000	695,600,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Public Utility, Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	35,000,000	0	0
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	5,000,000	5,000,000	5,000,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,135,000,000	1,499,000,000	1,485,500,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,134,700,000	1,498,700,000	1,485,200,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	300,000	300,000	300,000
Total fixed maturities | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	25,642,700,000	20,201,700,000	18,660,000,000
Total fixed maturities | Available-for-sale Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	9,762,600,000	6,645,900,000	4,619,200,000
Total fixed maturities | Available-for-sale Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	15,880,100,000	13,555,800,000	14,040,800,000
Total fixed maturities | Available-for-sale Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	18,488,100,000	13,940,700,000	12,209,000,000
Total fixed maturities | Available-for-sale Securities | Debt Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	9,752,500,000	6,645,900,000	4,619,200,000
Total fixed maturities | Available-for-sale Securities | Debt Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	8,735,600,000	7,294,800,000	7,589,800,000
Total fixed maturities | Available-for-sale Securities | Debt Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	6,922,000,000	6,050,000,000	6,213,700,000
Total fixed maturities | Available-for-sale Securities | Asset-backed Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Asset-backed Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	6,922,000,000	6,050,000,000	6,213,700,000
Total fixed maturities | Available-for-sale Securities | Asset-backed Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	9,752,500,000	6,645,900,000	4,595,000,000
Total fixed maturities | Available-for-sale Securities | U.S. government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	9,752,500,000	6,645,900,000	4,595,000,000
Total fixed maturities | Available-for-sale Securities | U.S. government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | U.S. government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,588,800,000	2,297,100,000	2,364,200,000
Total fixed maturities | Available-for-sale Securities | State and local government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | State and local government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,588,800,000	2,297,100,000	2,364,200,000
Total fixed maturities | Available-for-sale Securities | State and local government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	24,200,000
Total fixed maturities | Available-for-sale Securities | Foreign government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	24,200,000
Total fixed maturities | Available-for-sale Securities | Foreign government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Foreign government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	7,146,800,000	4,997,700,000	5,225,600,000
Total fixed maturities | Available-for-sale Securities | Corporate debt securities | Debt Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Corporate debt securities | Debt Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	7,146,800,000	4,997,700,000	5,225,600,000
Total fixed maturities | Available-for-sale Securities | Corporate debt securities | Debt Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	769,800,000	836,700,000	955,600,000
Total fixed maturities | Available-for-sale Securities | Residential mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Residential mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	769,800,000	836,700,000	955,600,000
Total fixed maturities | Available-for-sale Securities | Residential mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,959,300,000	2,758,600,000	2,767,900,000
Total fixed maturities | Available-for-sale Securities | Commercial mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Commercial mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,959,300,000	2,758,600,000	2,767,900,000
Total fixed maturities | Available-for-sale Securities | Commercial mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	3,192,900,000	2,454,700,000	2,490,200,000
Total fixed maturities | Available-for-sale Securities | Other asset-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Other asset-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	3,192,900,000	2,454,700,000	2,490,200,000
Total fixed maturities | Available-for-sale Securities | Other asset-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	232,600,000	211,000,000	237,300,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10,100,000	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	222,500,000	211,000,000	237,300,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	67,800,000	64,100,000	63,800,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Financials | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Financials | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	67,800,000	64,100,000	63,800,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Financials | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Public Utility, Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,700,000	11,400,000	31,900,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Public Utility, Equities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Public Utility, Equities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,700,000	11,400,000	31,900,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Public Utility, Equities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	160,100,000	135,500,000	141,600,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Industrials | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	10,100,000	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Industrials | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	150,000,000	135,500,000	141,600,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Industrials | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	25,963,400,000	20,209,900,000	18,583,100,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	18,781,200,000	13,971,600,000	12,164,300,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	6,959,300,000	6,043,400,000	6,196,300,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	9,942,000,000	6,688,800,000	4,612,200,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	1,610,100,000	2,285,600,000	2,332,200,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	24,200,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	7,229,100,000	4,997,200,000	5,195,700,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	767,800,000	828,800,000	947,000,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,986,500,000	2,760,100,000	2,763,700,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	3,205,000,000	2,454,500,000	2,485,600,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	222,900,000	194,900,000	222,500,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	65,200,000	61,300,000	61,400,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Redeemable preferred stocks | Public Utility, Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	4,800,000	10,100,000	30,500,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	152,900,000	123,500,000	130,600,000
Short-term investments | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,809,700,000	2,869,400,000	4,311,500,000
Short-term investments | Available-for-sale Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	2,573,400,000	1,824,400,000	3,175,400,000
Short-term investments | Available-for-sale Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	236,300,000	1,045,000,000	1,136,100,000
Short-term investments | Available-for-sale Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	0	0	0
Short-term investments | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value	$ 2,809,700,000	$ 2,869,400,000	$ 4,311,500,000

Fair Value Fair Value - Additio

Fair Value Fair Value - Additional Information (Detail)	Sep. 30, 2018USD ($)security	Dec. 31, 2017USD ($)security	Sep. 30, 2017USD ($)security
Fair Value Measurements [Line Items]
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	$ 0	$ 0	$ 0
Investments, Fair Value	$ 32,350,600,000	$ 27,274,700,000	$ 26,994,700,000
Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	76.00%	66.00%	59.00%
Investments, Fair Value	$ 15,469,300,000	$ 11,950,400,000	$ 11,086,000,000
Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	99.00%	98.00%	98.00%
Dealer Quoted Prices, Percent of FV Hierarchy Level	1.00%	2.00%	2.00%
Investments, Fair Value	$ 16,876,000,000	$ 15,319,000,000	$ 15,903,400,000
Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value	5,300,000	5,300,000	5,300,000
Available-for-sale Securities
Fair Value Measurements [Line Items]
Investments, Fair Value	28,452,400,000
Available-for-sale Securities | Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Investments, Fair Value	12,336,000,000
Available-for-sale Securities | Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Investments, Fair Value	16,116,400,000
Available-for-sale Securities | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value	0
Nonredeemable preferred stocks | Internal Pricing | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value	$ 5,000,000	$ 5,000,000	$ 5,000,000
Number of securities | security	1	1	1

Debt (Detail)

Debt (Detail) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Debt Instrument [Line Items]
Fair Value		$ 3,926.9	$ 3,643.6	$ 3,617.9
Debt, Long-term and Short-term, Combined Amount	[1]	3,859.9	3,306.3	3,312.2
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Long-term Debt		499	498.8	498.7
Fair Value		503.9	520.7	525.7
2.45% Senior Notes due 2027
Debt Instrument [Line Items]
Long-term Debt		496.4	496.1	496
Fair Value		453.3	477.9	475.3
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Long-term Debt		296.3	296.1	296
Fair Value		360.3	382.3	388.3
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Long-term Debt		395.4	395.3	395.3
Fair Value		481.8	516.9	518.7
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Long-term Debt		346.6	346.5	346.5
Fair Value		349.7	388.7	379.8
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Long-term Debt		395.3	395.2	395.2
Fair Value		359.1	402.9	392.6
4.125% Senior Notes due 2047
Debt Instrument [Line Items]
Long-term Debt		841.3	841.2	841.2
Fair Value		828.4	917.1	894.2
4.20% Senior Notes due 2048
Debt Instrument [Line Items]
Long-term Debt		589.6	0	0
Fair Value		590.4	0	0
Other debt instruments
Debt Instrument [Line Items]
Debt, Long-term and Short-term, Combined Amount		$ 0	$ 37.1	$ 43.3
[1]	Consists of long-term debt. See Note 4 – Debt for further discussion.

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($)	Apr. 03, 2017	Mar. 31, 2018	Apr. 30, 2017	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Repayments of Debt						$ 37,100,000	$ 42,800,000
Repayment of debt						0	$ 635,600,000
Discretionary Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 250,000,000
Debt term						30 days
Borrowings outstanding						$ 0		$ 0
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Debt, stated interest rate						3.75%
2.45% Senior Notes due 2027
Debt Instrument [Line Items]
Debt, stated interest rate						2.45%
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Debt, stated interest rate						6.625%
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Debt, stated interest rate						6.25%
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Debt, stated interest rate						4.35%
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Debt, stated interest rate						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%
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)
4.20% Senior Notes due 2048
Debt Instrument [Line Items]
Debt, face amount		$ 600,000,000		$ 600,000,000
Debt, stated interest rate		4.20%		4.20%		4.20%
Proceeds, net of underwriter's discounts and commissions		$ 589,500,000
Federal Funds Open Rate | Discretionary Line of Credit
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate						0.50%
Notes Payable, Other Payables | Other debt instruments
Debt Instrument [Line Items]
Repayments of Debt				$ 37,100,000
Trust Preferred Debt Securities [Member] | Other debt instruments
Debt Instrument [Line Items]
Repayments of Debt					$ 65,200,000

Income Taxes Additional Informa

Income Taxes Additional Information (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Valuation allowance	$ 0	$ 0	$ 0	$ 0	$ 0
Effective Income Tax Rate Reconciliation, Percent	17.70%	14.60%	19.20%	29.70%

Loss and Loss Adjustment Expe_3

Loss and Loss Adjustment Expense Reserves (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	$ 14,620.8	$ 13,353.3	$ 13,086.9	$ 11,368
Reinsurance Recoverable for Unpaid Claims and Claims Adjustments and Liability for Unpaid Claims and Claim Adjustment Expense, Other Reconciling Item	2,376.4	2,624.7	2,170.1	1,801
Liability for Unpaid Claims and Claims Adjustment Expense, Net	12,244.4	10,728.6	$ 10,916.8	$ 9,567
Current Year Claims and Claims Adjustment Expense	15,722.2	13,886.3
Prior Year Claims and Claims Adjustment Expense	47	42.5
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	15,769.2	13,928.8
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	9,409.7	8,379.4
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	5,031.9	4,387.8
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	14,441.6	12,767.2
Accident years 2017 and 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		8
Accident year 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	52
Short-duration Insurance Contracts, Accident Years 2016 and 2015 [Domain]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		51
Personal Lines | Personal auto businesses
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	41	76
Personal Lines | Agency Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	30
Personal Lines | Direct Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	11
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 5	$ 24

Supplemental Cash Flow Inform_3

Supplemental Cash Flow Information (Detail) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Open reverse repurchase commitments	$ 0	$ 0	$ 0
Income Taxes Paid	535,800,000	538,700,000
Interest Paid, Including Capitalized Interest, Operating and Investing Activities	116,400,000	107,400,000
Cash and Cash Equivalents [Member] | ARX Holding Corp. [Member]
Open reverse repurchase commitments	$ 152,400,000	$ 189,200,000	$ 247,200,000

Segment Information Operating R

Segment Information Operating Results (Detail) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2017USD ($)
Segment Reporting Information [Line Items]
Revenues	$ 8,495.8	$ 6,791.8	$ 23,943.9	$ 19,719.2
Pretax Profit (Loss)	1,130.5	251.4	2,921.4	1,447.5
Amortization of intangible assets	18	17.2	54	48.2
Personal Lines | Agency | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	3,318.2	2,840	9,607.7	8,224
Pretax Profit (Loss)	372.2	69.5	1,142.2	524.7
Personal Lines | Direct | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	3,337.2	2,734.8	9,565.3	7,908.5
Pretax Profit (Loss)	294.4	128.7	880.3	466.7
Operating Segments | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	7,930.5	6,544	22,738.7	18,884
Pretax Profit (Loss)	771.4	172	2,299.2	1,100
Operating Segments | Personal Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	6,655.4	5,574.8	19,173	16,132.5
Pretax Profit (Loss)	$ 666.6	$ 198.2	$ 2,022.5	$ 991.4
Personal auto insurance percentage of the total personal lines segment net premiums earned	94.00%	93.00%	94.00%	93.00%
Operating Segments | Commercial Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	$ 939.6	$ 714	$ 2,632.5	$ 2,031.2
Pretax Profit (Loss)	$ 112.7	$ 42.8	$ 307.8	$ 166.4
Operating Segments | Property | Underwriting operations
Segment Reporting Information [Line Items]
Combined ratio amortization points	0.054	0.067	0.058	0.067
Revenues	$ 335.5	$ 255.2	$ 933.2	$ 720.3
Pretax Profit (Loss)	(7.8)	(69)	(31.2)	(57.5)
Amortization of intangible assets	18	17.2	54	48.2
Operating Segments | Other Indemnity | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Pretax Profit (Loss)	(0.1)	0	0.1	(0.3)
Segment Reconciling Items | Fees and other revenues
Segment Reporting Information [Line Items]
Revenues	122.6	96.3	342.4	270.3
Segment Reconciling Items | Service businesses
Segment Reporting Information [Line Items]
Revenues	42.5	33.3	119.6	94.5
Pretax Profit (Loss)	6.7	4.4	17.5	12.7
Segment Reconciling Items | Investment income
Segment Reporting Information [Line Items]
Revenues	400.2	118.2	743.2	470.2
Pretax Profit (Loss)	394.4	112.4	725.2	452.2
Segment Reconciling Items | Other gains (losses)
Segment Reporting Information [Line Items]
Revenues	0	0	0	0.2
Pretax Profit (Loss)	0	0	0	0.2
Segment Reconciling Items | Interest expense
Segment Reporting Information [Line Items]
Pretax Profit (Loss)	$ (42)	$ (37.4)	$ (120.5)	$ (117.6)

Segment Information Underwritin

Segment Information Underwriting Margins and Combined Ratios for our Underwriting Operations (Detail) - Underwriting operations	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Personal Lines | Agency
Segment Reporting Information [Line Items]
Underwriting Margin	11.20%	2.40%	11.90%	6.40%
Combined Ratio	88.80%	97.60%	88.10%	93.60%
Personal Lines | Direct
Segment Reporting Information [Line Items]
Underwriting Margin	8.80%	4.70%	9.20%	5.90%
Combined Ratio	91.20%	95.30%	90.80%	94.10%
Operating Segments
Segment Reporting Information [Line Items]
Underwriting Margin	9.70%	2.60%	10.10%	5.80%
Combined Ratio	90.30%	97.40%	89.90%	94.20%
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Underwriting Margin	10.00%	3.60%	10.50%	6.10%
Combined Ratio	90.00%	96.40%	89.50%	93.90%
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Underwriting Margin	12.00%	6.00%	11.70%	8.20%
Combined Ratio	88.00%	94.00%	88.30%	91.80%
Operating Segments | Property
Segment Reporting Information [Line Items]
Underwriting Margin	(2.30%)	(27.00%)	(3.30%)	(8.00%)
Combined Ratio	102.30%	127.00%	103.30%	108.00%
Combined ratio amortization points	0.054	0.067	0.058	0.067

Dividends Dividends (Details)

Dividends Dividends (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		3 Months Ended	9 Months Ended
	Feb. 28, 2018	Feb. 28, 2017	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Payments of Ordinary Dividends, Common Stock	$ 654.9	$ 395.4		$ 654.9	$ 395.4
Dividends payable							$ 655.1	$ 395.4
Dividends, Preferred Stock, Cash			$ 13.5	$ 13.5	$ 0
Annual variable dividend [Member]
Common Stock, Dividends, Per Share, Cash Paid	$ 1.1247	$ 0.6808
Series B Preferred Stock [Member]
Preferred Stock, Dividends Per Share, Declared			$ 27.024	$ 27.024	$ 0
Preferred Stock, Shares Issued (shares)						500,000
Series B Preferred Stock [Member] | Serial Preferred Stock [Member]
Preferred Stock, Shares Issued (shares)			500,000	500,000	0		0

Dividends - Additional Informat

Dividends - Additional Information (Detail)	1 Months Ended	9 Months Ended	60 Months Ended
	Mar. 31, 2023	Sep. 30, 2018USD ($)$ / sharesshares	Mar. 14, 2023	Mar. 31, 2018$ / sharesshares	Dec. 31, 2017$ / sharesshares	Sep. 30, 2017$ / sharesshares
Class of Stock [Line Items]
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		1.91
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued (shares) | shares				500,000
Preferred Stock, Liquidation Preference Per Share (USD per share) | $ / shares				$ 1,000	$ 0	$ 0
Series B Preferred Stock [Member] | Serial Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued (shares) | shares		500,000			0	0
Preferred Stock, Liquidation Preference Per Share (USD per share) | $ / shares		$ 1,000
Scenario, Forecast [Member] | Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage			5.375%
London Interbank Offered Rate (LIBOR) [Member] | Scenario, Forecast [Member] | Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Basis Spread on Variable Rate	2.539%

Other Comprehensive Income (L_3

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, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
AOCI before Tax, Attributable to Parent, beginning balance	$ (280.8)	$ 1,774.1	$ 1,977.8	$ 1,439.5	$ 1,439.5
Pretax Cumulative Effect of New Accounting Principal in Period of Adoption			(2,006)
Pretax Reclassification of Stranded Amounts in AOCI to Retained Earnings	(4.3)		(4.3)
AOCI before Tax, Attributable to Parent, after Cumulative Effect Adjustment	(285.1)		(32.5)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, before Tax	(58.7)	122.7	(352.5)	554.4
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	0	0	0	(8)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, before Tax	0	0.9	0	1.2
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Noncontrolling Interest	0.6	(1.1)	6.4	(4.5)
OCI, before Reclassifications, before Tax, Attributable to Parent	(58.1)	122.5	(346.1)	543.1
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, before Tax	0	(9.3)	0	(12.9)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	(9.3)	15.7	(44.2)	104.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	(0.2)	(0.2)	(0.7)	0.6
Total reclassification adjustment for amounts realized in net income, pre tax	(9.5)	6.2	(44.9)	92.2
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	(48.6)	116.3	(301.2)	450.9
AOCI before Tax, Attributable to Parent, ending balance	(333.7)	1,890.4	(333.7)	1,890.4	1,977.8
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
AOCI Tax, Attributable to Parent, beginning balance	58.9	(623.4)	(695.6)	(506.1)	(506.1)
Tax, Cumulative Effect of New Accounting Principal in Period of Adoption			705.8
Tax, Reclassification of Stranded Amounts in AOCI to Retained Earnings	0.9		(3.4)
AOCI Tax, Attributable to Parent, After Cumulative Effect Adjustment	59.8		6.8
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax	12.3	(43)	73.9	(194.3)
Forecasted transactions, Total tax (provision) benefit	0	0	0	2.8
Other Comprehensive Income (Loss), Foreign Current Translation Adjustment, Tax	0	(0.3)	0	(0.4)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Noncontrolling Interest	(0.2)	0.4	(1.4)	1.6
Other Comprehensive Income (Loss), Available-for-sale Securities, before Reclassification Adjustments, Tax	12.1	(42.9)	72.5	(190.3)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Tax	0	3.3	0	4.6
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	1.9	(5.5)	9.2	(36.6)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	0	0.1	0.1	(0.2)
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	1.9	(2.1)	9.3	(32.2)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	10.2	(40.8)	63.2	(158.1)
AOCI Tax, Attributable to Parent, ending balance	70	(664.2)	70	(664.2)	(695.6)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(221.9)	1,150.7	1,282.2	933.4	933.4
Cumulative Effect of New Accounting Principle in Period of Adoption			(1,300.2)
Reclassification of Stranded Amounts in AOCI to Retained Earnings	(3.4)		(7.7)
AOCI, Net of Tax, After Cumulative Effect Adjustment	(225.3)		(25.7)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	(46.4)	79.7	(278.6)	360.1
Forecasted transactions, after tax	0	0	0	(5.2)
Foreign currency translation adjustment	0	0.6	0	0.8
Other comprehensive (income) loss attributable to NCI	0.4	(0.7)	5	(2.9)	(2.3)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(46)	79.6	(273.6)	352.8
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0	(6)	0	(8.3)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	(7.4)	10.2	(35)	67.9
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.2)	(0.1)	(0.6)	0.4
Total reclassification adjustment for amounts realized in net income, after tax	(7.6)	4.1	(35.6)	60
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(38.4)	75.5	(238)	292.8
After tax total accumulated other comprehensive income, ending balance	(263.7)	1,226.2	(263.7)	1,226.2	1,282.2
Net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(210.9)	1,164.6	1,295	939.6	939.6
Cumulative Effect of New Accounting Principle in Period of Adoption			(1,300.2)
Reclassification of Stranded Amounts in AOCI to Retained Earnings	0		(1.1)
AOCI, Net of Tax, After Cumulative Effect Adjustment	(210.9)		(6.3)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	(46.4)	79.7	(278.6)	360.1
Forecasted transactions, after tax	0	0	0	0
Foreign currency translation adjustment	0	0	0	0
Other comprehensive (income) loss attributable to NCI	0	0	0	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(46.4)	79.7	(278.6)	360.1
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0	(6)	0	(8.3)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	(7.4)	10.2	(35)	67.9
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0	0	0	0
Total reclassification adjustment for amounts realized in net income, after tax	(7.4)	4.2	(35)	59.6
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(39)	75.5	(243.6)	300.5
After tax total accumulated other comprehensive income, ending balance	(249.9)	1,240.1	(249.9)	1,240.1	1,295
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(17.6)	(15.1)	(14.8)	(9.4)	(9.4)
Cumulative Effect of New Accounting Principle in Period of Adoption			0
Reclassification of Stranded Amounts in AOCI to Retained Earnings	0		(3.2)
AOCI, Net of Tax, After Cumulative Effect Adjustment	(17.6)		(18)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	0	0	0	0
Forecasted transactions, after tax	0	0	0	(5.2)
Foreign currency translation adjustment	0	0	0	0
Other comprehensive (income) loss attributable to NCI	0	0	0	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	0	0	(5.2)
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	(0.2)	(0.1)	(0.6)	0.4
Total reclassification adjustment for amounts realized in net income, after tax	(0.2)	(0.1)	(0.6)	0.4
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.2	0.1	0.6	(5.6)
After tax total accumulated other comprehensive income, ending balance	(17.4)	(15)	(17.4)	(15)	(14.8)
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	0	(0.9)	0	(1.1)	(1.1)
Cumulative Effect of New Accounting Principle in Period of Adoption			0
Reclassification of Stranded Amounts in AOCI to Retained Earnings	0		0
AOCI, Net of Tax, After Cumulative Effect Adjustment	0		0
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	0	0	0	0
Forecasted transactions, after tax	0	0	0	0
Foreign currency translation adjustment	0	0.6	0	0.8
Other comprehensive (income) loss attributable to NCI	0	0	0	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	0.6	0	0.8
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
Total reclassification adjustment for amounts realized in net income, after tax	0	0	0	0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0	0.6	0	0.8
After tax total accumulated other comprehensive income, ending balance	0	(0.3)	0	(0.3)	0
Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	6.6	2.1	2	4.3	4.3
Cumulative Effect of New Accounting Principle in Period of Adoption			0
Reclassification of Stranded Amounts in AOCI to Retained Earnings	(3.4)		(3.4)
AOCI, Net of Tax, After Cumulative Effect Adjustment	3.2		(1.4)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	0	0	0	0
Forecasted transactions, after tax	0	0	0	0
Foreign currency translation adjustment	0	0	0	0
Other comprehensive (income) loss attributable to NCI	0.4	(0.7)	5	(2.9)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0.4	(0.7)	5	(2.9)
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
Total reclassification adjustment for amounts realized in net income, after tax	0	0	0	0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.4	(0.7)	5	(2.9)
After tax total accumulated other comprehensive income, ending balance	$ 3.6	$ 1.4	$ 3.6	$ 1.4	$ 2

Other Comprehensive Income (L_4

Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement, Additional Information (Detail) $ in Millions	Sep. 30, 2018USD ($)
Statement of Comprehensive Income [Abstract]
Net unrealized gains on forecasted transactions, expected to reclassify into income within the next 12 months	$ 1

Redeemable Noncontrolling Int_3

Redeemable Noncontrolling Interest (Details) $ in Millions		3 Months Ended		9 Months Ended			12 Months Ended
		Sep. 30, 2018USD ($)shares	Sep. 30, 2017USD ($)	Sep. 30, 2018USD ($)shares		Sep. 30, 2017USD ($)	Dec. 31, 2017USD ($)	Apr. 01, 2021
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance, Beginning of period				$ 503.7	[1]	$ 483.7	$ 483.7
Net income attributable to NCI		$ 1.8	$ (9.2)	10.6		1.9	5.9
Other comprehensive income (loss) attributable to NCI1		(0.4)	0.7	(5)		2.9	2.3
Exercise of employee stock options				9.4		3.4	3.4
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests				(298.2)		0	0
Change in redemption value of NCI				(3.1)		6.3	8.4
Balance, End of period	[1]	$ 217.4	$ 498.2	$ 217.4		$ 498.2	$ 503.7
ARX Holding Corp. [Member]
Redeemable Noncontrolling Interest [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage		86.80%	69.00%	86.80%		69.00%	69.00%
Shares held in Employee Stock Option Plan, Suspense Shares | shares		16,067		16,067
Employee Stock Ownership Plan (ESOP), Deferred Shares, Fair Value		$ 23.4		$ 23.4
Number of Shares Put Pursuant to Stockholders' Agreement | shares		204,527		204,527
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period | shares		5,483
Payments to Acquire Interest in Subsidiaries and Affiliates		$ 295.9
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%
[1]	See Note 12 – Redeemable Noncontrolling Interest for further discussion.

Goodwill and Intangible Asset_2

Goodwill and Intangible Assets Goodwill (Details) - USD ($)	3 Months Ended		6 Months Ended	9 Months Ended		15 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill, Acquired During Period				$ 0
Goodwill	$ 452,700,000	$ 452,700,000		452,700,000	$ 452,700,000	$ 452,700,000	$ 452,700,000
Goodwill, Impaired, Accumulated Impairment Loss	0			0		$ 0
Amortization of intangible assets	$ 18,000,000	$ 17,200,000		$ 54,000,000	$ 48,200,000
Weighted Average [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Remaining Amortization Period				4 years 1 month 25 days
Trade name [Member]
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life			10 years			2 years

Goodwill and Intangible Asset_3

Goodwill and Intangible Assets - Net Carrying Amount of Other Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Schedule of Other Intangible Assets [Line Items]
Intangible assets subject to amortization	$ 300.2	$ 354.2	$ 372.2
Indefinite lived intangible assets	12.4	12.4	12.4
Total	312.6	366.6	384.6
Accumulated amortization	229.7	175.7	157.7
Licensing Agreements [Member]
Schedule of Other Intangible Assets [Line Items]
Accumulated amortization	$ 0.6	$ 0.6	$ 0.6

Goodwill and Intangible Asset_4

Goodwill and Intangible Assets - Finite-lived Intangible Assets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	$ 229.7	$ 175.7	$ 157.7
Net Carrying Amount	300.2	354.2	372.2
Policies in force [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	256.2	256.2	256.2
Accumulated Amortization	128.1	100.7	91.5
Net Carrying Amount	128.1	155.5	164.7
Agency relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	159.2	159.2	159.2
Accumulated Amortization	39.8	31.3	28.4
Net Carrying Amount	119.4	127.9	130.8
Software rights [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	79.1	79.1	79.1
Accumulated Amortization	37.4	29.4	26.8
Net Carrying Amount	41.7	49.7	52.3
Trade name [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	34.8	34.8	34.8
Accumulated Amortization	23.8	13.7	10.4
Net Carrying Amount	11	21.1	24.4
Finite-Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	529.3	529.3	529.3
Accumulated Amortization	$ 229.1	$ 175.1	$ 157.1

New Accounting Standards Effect

New Accounting Standards Effect of Standard Change (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Item Effected [Line Items]
Liabilities		$ 33,467.5	$ 29,145	$ 33,467.5	$ 29,145				$ 28,912.7
Assets		45,543.7	38,932.6	45,543.7	38,932.6				38,701.2
Provision (benefit) for income taxes		200.3	36.6	560.2	429.7
Cumulative Effect of New Accounting Principle in Period of Adoption									1,300.2
Reclassification of disproportionate tax effects						$ 3.4			$ 7.7
Accounting Standards Update 2016-01 [Member]
Item Effected [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption								$ 1,300
Adjustments for New Accounting Pronouncement | Pro Forma [Member] | Property Subject to Operating Lease [Member]
Item Effected [Line Items]
Liabilities		160		160
Assets		160		160
Accounting Standards Update 2018-02 [Member]
Item Effected [Line Items]
Reclassification of disproportionate tax effects							$ 4.3
Retained Earnings | Accounting Standards Update 2016-01 [Member]
Item Effected [Line Items]
Cumulative Effect of New Accounting Principle in Period of Adoption	[1]	1,300.2	0	1,300.2	0
Retained Earnings | Accounting Standards Update 2018-02 [Member]
Item Effected [Line Items]
Reclassification of disproportionate tax effects	[1]	$ 4.3	$ 0	$ 4.3	$ 0
[1]	See Note 14 – New Accounting Standards for further discussion.

Subsequent Event (Details)

Subsequent Event (Details) - 4.20% Senior Notes due 2048 - USD ($)	Oct. 18, 2018	Mar. 31, 2018	Sep. 30, 2018
Subsequent Event [Line Items]
Debt, face amount		$ 600,000,000
Debt, stated interest rate		4.20%	4.20%
Proceeds from Debt, Net of Issuance Costs		$ 589,500,000
Subsequent Event [Member]
Subsequent Event [Line Items]
Debt, face amount	$ 550,000,000
Debt, stated interest rate	4.00%
Proceeds from Debt, Net of Issuance Costs	$ 544,500,000