The Progressive Corporation (PGR) 10-Q 2 May 18 2018 Q1 Quarterly report Financial data

Document and Entity Information

Document and Entity Information	3 Months Ended
	Mar. 31, 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	Mar. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q1
Amendment Flag	false
Trading Symbol	PGR
Entity Common Stock, Shares Outstanding	582,376,344

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) shares in Millions, $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Revenues
Net premiums earned		$ 7,174	$ 6,026.7
Period over prior year period percent change in net premiums earned		19.00%
Investment income		$ 166.3	129.2
Period over prior year period percent change in investment income		29.00%
Net realized gains (losses) on securities:
Gain (Loss) on Sale of Securities, Net		$ 107	52.1
Net Holding Period Realized Gain (Loss)		(155.2)	0.8
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net		0	(1)
Total net realized gains (losses) on securities		$ (48.2)	51.9
Period Over Prior Year Period, Percent Increase (Decrease), Net Realized Gains (Losses) On Security Sales		105.00%
Period Over Prior Year Period, Percent Increase (Decrease), Net Impairment Losses Recognized In Earnings		(100.00%)
Period Over Prior Year Period, Percent Increase (Decrease), Total Net Realized Gains (Losses) On Securities		(193.00%)
Fees and other revenues		$ 103.8	85.2
Period over prior year period percent change in fees and other revenues		22.00%
Service revenues		$ 34.2	28.5
Period over prior year period percent change in service revenues		20.00%
Other gains (losses)		$ 0	0.2
Period Over Prior Year Period, Percent Increase (Decrease), Other Gains (Losses)		(100.00%)
Total revenues		$ 7,430.1	6,321.7
Period over prior year period percent change in total revenues		18.00%
Expenses
Losses and loss adjustment expenses		$ 4,870.8	4,263.4
Period over prior year period percent change in losses and loss adjustment expenses		14.00%
Policy acquisition costs		$ 596.2	502.9
Period over prior year period percent change in policy acquisition costs		19.00%
Other underwriting expenses		$ 980.2	845.6
Period over prior year period percent change in other underwriting expenses		16.00%
Investment expenses		$ 6	5.6
Period over prior year period percent change in investment expenses		7.00%
Service expenses		$ 29.3	25.9
Period over prior year period percent change in service expenses		13.00%
Interest expense		$ 36.8	36.8
Period over prior year period percent change in interest expense		0.00%
Total expenses		$ 6,519.3	5,680.2
Period over prior year period percent change in total expenses		15.00%
Net Income
Income before income taxes		$ 910.8	641.5
Period over prior year period percent change in income before income taxes		42.00%
Provision for income taxes		$ 181	211.2
Period over prior year period percent change in provision for income taxes		(14.00%)
Net income		$ 729.8	430.3
Period over prior year period percent change in net income		70.00%
Net (income) loss attributable to noncontrolling interest (NCI)		$ (11.8)	(6)
Period over prior year period percent change in net (income) loss attributable to noncontrolling interest		97.00%
Net income attributable to Progressive		$ 718	424.3
Period over prior year period percent change in net income attributable to Progressive		69.00%
Other Comprehensive Income (Loss)
Total net unrealized gains (losses) on securities		$ (154.5)	132.4
Period over prior year period percent change in total net unrealized gains (losses) on securities		(217.00%)
Net unrealized losses on forecasted transactions		$ 0.2	2.3
Period Over Prior Year Period, Percent Increase (Decrease), Unrealized Gain (Loss) On Forecasted Transactions		(91.00%)
Foreign currency translation adjustment		$ 0	0.3
Period over prior year period percent change in foreign currency translation adjustment		(100.00%)
Other comprehensive income (loss)		$ (154.3)	135
Period over prior year period percent change in other comprehensive income		(214.00%)
Other comprehensive (income) loss attributable to NCI		$ 4	(0.9)
Comprehensive income attributable to Progressive		$ 567.7	558.4
Period over prior year period percent change in comprehensive income		2.00%
Computation of Per Share Earnings Available to Progressive Common Shareholders
Preferred Stock Dividends, Income Statement Impact		$ 1.2	0
Net Income (Loss) Available to Common Stockholders, Basic		$ 716.8	$ 424.3
Period Over Prior Year Period, Percent Increase (Decrease), Net Income (Loss) Available to Common Stockholders		69.00%
Average common shares outstanding - Basic		582	580.3
Period over prior year period percent change in average shares outstanding - basic		0.00%
Net effect of dilutive stock-based compensation		3.6	3
Period over prior year period percent change in net effect of dilutive stock-based compensation		20.00%
Total average equivalent common shares - Diluted		585.6	583.3
Period over prior year period percent change in total equivalent shares - diluted		0.00%
Basic: Earnings per common share		$ 1.23	$ 0.73
Period over prior year period percent change in basic: earnings per share		68.00%
Diluted: Earnings per common share		$ 1.22	0.73
Period over prior year period percent change in diluted: earnings per share		68.00%
Dividends declared per share	[1]	$ 0	$ 0
[1]	Progressive maintains an annual dividend program. See Note 9 – Dividends for further discussion.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Available-for-sale securities, at fair value:
Fixed maturities (amortized cost: $22,627.2, $17,444.8, and $20,209.9)		$ 22,420.1	$ 20,201.7	$ 17,474.3
Short-term investments (amortized cost: $3,052.4, $3,040.8, and $2,869.4)		3,052.4	2,869.4	3,040.8
Total investments		25,472.5	23,071.1	20,515.1
Equity securities, at fair value:
Equity Securities FV NI Nonredeemable Preferred Stock		745.9	803.8	809.8
Equity Securities FV NI Common Equities		3,033.2	3,399.8	2,978.1
Equity Securities, FV-NI		3,779.1	4,203.6	3,787.9
Investments, Fair Value Disclosure		29,251.6	27,274.7	24,303
Cash and cash equivalents		190.1	265	214.5
Restricted Cash		6.5	10.3	0.5
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents		196.6	275.3	215
Accrued investment income		139.2	119.7	110.1
Premiums receivable, net of allowance for doubtful accounts of $205.4, $174.5, and $210.9		6,043.8	5,422.5	4,850.7
Reinsurance recoverables, including $102.1, $88.6, and $103.3 on paid losses and loss adjustment expenses		2,239.1	2,273.4	1,939
Prepaid reinsurance premiums		342.8	203.3	182.7
Deferred acquisition costs		842.3	780.5	679.5
Property and equipment, net of accumulated depreciation of $973.8, $881.7, and $940.6		1,112	1,119.6	1,166
Goodwill		452.7	452.7	449.4
Intangible assets, net of accumulated amortization of $193.7, $125.0, and $175.7		348.6	366.6	417.3
Other assets		362.2	412.9	497.8
Total assets		41,330.9	38,701.2	34,810.5
Liabilities
Unearned premiums		9,837.8	8,903.5	7,945
Loss and loss adjustment expense reserves		13,329	13,086.9	11,628.9
Net deferred income taxes		53.2	135	168.4
Accounts payable, accrued expenses, and other liabilities		3,414.3	3,481	2,939.4
Debt	[1]	3,859.2	3,306.3	3,111.7
Total liabilities		30,493.5	28,912.7	25,793.4
Redeemable noncontrolling interest (NCI)2	[2]	514.2	503.7	494.2
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 215.1, 216.6, and 215.8)		582.4	581.7	580.9
Paid-in capital		1,401.6	1,389.2	1,331.1
Retained earnings		8,017.9	6,031.7	5,543.4
Accumulated other comprehensive income:
Net unrealized gains (losses) on securities		(160.8)	1,295	1,072
Net unrealized losses on forecasted transactions		(17.8)	(14.8)	(7.1)
Foreign currency translation adjustment		0	0	(0.8)
Accumulated other comprehensive (income) loss attributable to NCI		6	2	3.4
Total accumulated other comprehensive income attributable to Progressive		(172.6)	1,282.2	1,067.5
Total shareholders’ equity		10,323.2	9,284.8	8,522.9
Total liabilities, redeemable NCI, and shareholders’ equity		$ 41,330.9	$ 38,701.2	$ 34,810.5
[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	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Fixed maturities, amortized cost	$ 22,627.2	$ 20,209.9	$ 17,444.8
Nonredeemable preferred stocks, cost	659.6	698.6	710.3
Common equities, cost	1,265.5	1,499	1,451.5
Short-term investments, amortized cost	3,052.4	2,869.4	3,040.8
Premiums receivable, allowance for doubtful accounts	205.4	210.9	174.5
Reinsurance recoverables, paid losses and loss adjustment expenses	102.1	103.3	88.6
Intangible Assets, Accumulated Amortization	193.7	175.7	125
Property and equipment, accumulated depreciation	$ 973.8	$ 940.6	$ 881.7
Common Shares, par value (USD per share)	$ 1	$ 1	$ 1
Common Shares, authorized (shares)	900,000,000	900,000,000	900,000,000
Common Shares, issued (shares)	797,500,000	797,500,000	797,500,000
Common Shares, treasury shares (shares)	215,100,000	215,800,000	216,600,000
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, Shares Authorized (shares)	500,000
Preferred Stock, Shares Issued (shares)	500,000
Preferred Stock, Shares Outstanding (shares)	500,000
Preferred Stock, Liquidation Preference Per Share (USD per share)	$ 1,000	$ 0	0
Series B Preferred Stock [Member] | Serial Preferred Stock [Member]
Preferred Stock, No Par Value	$ 0	$ 0	$ 0
Preferred Stock, Shares Authorized (shares)		0	0
Preferred Stock, Shares Issued (shares)		0	0
Preferred Stock, Shares Outstanding (shares)		0	0

Consolidated Statements of Chan

Consolidated Statements of Changes in Shareholders' Equity Statement - USD ($) $ in Millions		Total	Serial Preferred Stock [Member]	Preferred Shares, No Par Value	Common Shares, $1.00 Par Value	Paid-in Capital	Retained Earnings		AOCI Attributable to Parent [Member]		Series B Preferred Stock [Member]	Series B Preferred Stock [Member]Serial Preferred Stock [Member]	Voting Preference Shares [Member]
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]
Stock Issued During Period, Value, New Issues				0
Net income attributable to Progressive		$ 424.3					424.3
Treasury shares purchased					(0.6)	(1.3)	(19.5)
Stock Issued During Period, Value, Restricted Stock Award, Gross					1.6	(1.6)
Amortization of equity-based compensation						30.5
Reinvested dividends on restricted stock units						0.3	(0.3)
Adjustment to carrying amount of redeemable noncontrolling interest		0.2				(0.2)
Attributable to noncontrolling interest		(0.9)							(0.9)
Other comprehensive income									135
Other, net							(1.5)
Balance, End of period at Mar. 31, 2017		8,522.9		0	580.9	1,331.1	5,543.4		1,067.5
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	$ 0
Preferred Stock, Shares Authorized (shares)			20,000,000									0	5,000,000
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 Stranded Amounts in AOCI to Retained Earnings | Accounting Standards Update 2018-02 [Member]	[1]						0		0
Preferred Stock, Shares Issued (shares)												0	0
Preferred Stock, Shares Outstanding (shares)												0
Preferred Stock, No Par Value												$ 0
Preferred Stock, Shares Authorized (shares)			20,000,000									0
Preferred Stock, Shares Issued (shares)												0
Preferred Stock, Shares Outstanding (shares)												0
Stock Issued During Period, Value, New Issues				493.9
Net income attributable to Progressive		718					718
Treasury shares purchased					(0.7)	(1.6)	(34.9)
Stock Issued During Period, Value, Restricted Stock Award, Gross					1.4	(1.4)
Amortization of equity-based compensation						17.2
Reinvested dividends on restricted stock units						(0.5)	0.5
Adjustment to carrying amount of redeemable noncontrolling interest		1.3				(1.3)
Attributable to noncontrolling interest		4							4
Other comprehensive income									(154.3)
Other, net							(1.9)
Balance, End of period at Mar. 31, 2018		10,323.2		$ 493.9	$ 582.4	$ 1,401.6	8,017.9		(172.6)
Preferred Stock, No Par Value											$ 0	$ 0
Preferred Stock, Shares Authorized (shares)			20,000,000								500,000		5,000,000
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 Stranded Amounts in AOCI to Retained Earnings | Accounting Standards Update 2018-02 [Member]		$ 4.3					$ 4.3	[1]	$ (4.3)	[1]
Preferred Stock, Shares Issued (shares)											500,000		0
Preferred Stock, Shares Outstanding (shares)											500,000
[1]	See Note 14 – New Accounting Standards for further discussion.

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Statement of Cash Flows [Abstract]
Net income	$ 729.8	$ 430.3
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	40.9	41
Amortization of intangible assets	18	15.5
Net amortization of fixed-income securities	15	22.1
Amortization of equity-based compensation	17.4	30.9
Net realized (gains) losses on securities	48.2	(51.9)
Net (gains) losses on disposition of property and equipment	1.2	1.3
Other gains (losses)	0	0.2
Changes in:
Premiums receivable	(621.3)	(341.4)
Reinsurance recoverables	34.3	(54.2)
Prepaid reinsurance premiums	(139.5)	(12.2)
Deferred acquisition costs	(61.8)	(28.3)
Income taxes	181.1	212.7
Unearned premiums	934.3	476.3
Loss and loss adjustment expense reserves	242.1	260.8
Accounts payable, accrued expenses, and other liabilities	283.2	137.4
Other, net	24.2	(35.7)
Net cash provided by operating activities	1,747.1	1,104.4
Purchases:
Fixed maturities	(5,563)	(3,203.9)
Equity securities	(39.9)	(52.5)
Sales:
Fixed maturities	1,692.4	1,014.1
Equity securities	428	86.5
Maturities, paydowns, calls, and other:
Fixed maturities	1,422.6	1,006.5
Equity securities	0	26.6
Net sales (purchases) of short-term investments	(175.8)	536.8
Net unsettled security transactions	89.4	83.2
Collateral on derivative instruments	0	(136.1)
Purchases of property and equipment	(35.8)	(35)
Sales of property and equipment	1.5	11.2
Net cash used in investing activities	(2,180.6)	(662.6)
Cash Flows From Financing Activities
Proceeds from exercise of equity options	0.6	0.5
Net proceeds from issuance of Serial Preferred Shares, Series B	493.9	0
Net proceeds from debt issuance	589.5	0
Payments of debt	(37.1)	(6.2)
Redemption/reacquisition of subordinated debt	0	(30.7)
Dividends paid to common shareholders	(654.9)	(395.4)
Acquisition of treasury shares for restricted stock tax liabilities	(36.8)	(21.4)
Acquisition of treasury shares acquired in open market	(0.4)	0
Net cash provided by (used in) financing activities	354.8	(453.2)
Decrease in cash, cash equivalents, and restricted cash	(78.7)	(11.4)
Cash, cash equivalents, and restricted cash - January 1	275.3	226.4
Cash, cash equivalents, and restricted cash - March 31	$ 196.6	$ 215

Basis of Presentation

Basis of Presentation	3 Months Ended
	Mar. 31, 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 68.9% of the outstanding capital stock of ARX at March 31, 2018 and 69.0% at March 31, 2017 and December 31, 2017 . The decrease reflects ARX employee stock options that were exercised during the first quarter 2018. In April 2018, the minority ARX shareholders “put” 204,527 of their ARX shares, including exercised stock options, to Progressive pursuant to the stockholders' agreement, bringing Progressive's ownership interest in ARX to 86.7% . See Note 12 – Redeemable Noncontrolling Interest for additional information. 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 March 31, 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 $5.3 million at both March 31, 2018 and December 31, 2017. No properties were held for sale at March 31, 2017.

Investments

Investments	3 Months Ended
	Mar. 31, 2018
Investments Schedule [Abstract]
Investments	Investments — Our securities are reported at fair value, with the changes in fair value of these securities (other than hybrid securities, derivative instruments, and, in 2018 , equity securities) reported as a component of accumulated other comprehensive income, net of deferred income taxes in our Consolidated Balance Sheets. The changes in fair value of the hybrid securities, derivative instruments, and, in 2018 , equity securities are recorded as a component of net realized gains (losses) on securities in our Consolidated Statements of Comprehensive Income. The following tables present the composition of our investment portfolio by major security type, consistent with our classification of how we manage, monitor, and measure the portfolio. The net holding period gains (losses) on our equity securities represent the change in fair value recorded as realized gains (losses); all of our equity securities have a readily determinable fair value. The net holding period gains (losses) on our fixed-maturity securities represent the amounts realized on our hybrid securities only. ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value March 31, 2018 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 9,081.6 $ 3.9 $ (127.4 ) $ 0 $ 8,958.1 30.6 % State and local government obligations 1,732.3 4.3 (16.4 ) (0.1 ) 1,720.1 5.9 Foreign government obligations 0 0 0 0 0 0 Corporate debt securities 6,102.6 4.8 (64.8 ) (1.5 ) 6,041.1 20.7 Residential mortgage-backed securities 746.7 9.9 (4.8 ) 0 751.8 2.6 Commercial mortgage-backed securities 2,289.8 4.5 (24.8 ) 0 2,269.5 7.8 Other asset-backed securities 2,472.4 2.4 (12.9 ) 0.2 2,462.1 8.4 Redeemable preferred stocks 201.8 18.6 (1.4 ) (1.6 ) 217.4 0.7 Total fixed maturities 22,627.2 48.4 (252.5 ) (3.0 ) 22,420.1 76.7 Short-term investments 3,052.4 0 0 0 3,052.4 10.4 Total available-for-sale securities 25,679.6 48.4 (252.5 ) (3.0 ) 25,472.5 87.1 Equity securities: Nonredeemable preferred stocks 659.6 0 0 86.3 745.9 2.5 Common equities 1,265.5 0 0 1,767.7 3,033.2 10.4 Total equity securities 1,925.1 0 0 1,854.0 3,779.1 12.9 Total portfolio 1,2 $ 27,604.7 $ 48.4 $ (252.5 ) $ 1,851.0 $ 29,251.6 100.0 % ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value March 31, 2017 Fixed maturities: U.S. government obligations $ 3,345.5 $ 0.9 $ (20.6 ) $ 0 $ 3,325.8 13.7 % State and local government obligations 2,475.8 23.3 (10.7 ) 0 2,488.4 10.2 Foreign government obligations 22.9 0 0 0 22.9 0.1 Corporate debt securities 5,199.6 23.1 (15.0 ) 0.3 5,208.0 21.4 Residential mortgage-backed securities 1,433.0 24.7 (11.7 ) 2.1 1,448.1 6.0 Commercial mortgage-backed securities 2,216.1 13.7 (16.3 ) 0 2,213.5 9.1 Other asset-backed securities 2,562.4 5.4 (3.1 ) 0.2 2,564.9 10.6 Redeemable preferred stocks 189.5 14.3 (1.1 ) 0 202.7 0.8 Total fixed maturities 17,444.8 105.4 (78.5 ) 2.6 17,474.3 71.9 Equity securities: Nonredeemable preferred stocks 710.3 111.3 (11.8 ) 0 809.8 3.3 Common equities 1,451.5 1,531.4 (4.8 ) 0 2,978.1 12.3 Short-term investments 3,040.8 0 0 0 3,040.8 12.5 Total portfolio 1,2 $ 22,647.4 $ 1,748.1 $ (95.1 ) $ 2.6 $ 24,303.0 100.0 % ($ 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 and collateral on open derivative positions; at March 31, 2018 and 2017 , we had $83.6 million and $111.0 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 March 31, 2018 and 2017 , and December 31, 2017 , included $2.1 billion , $0.7 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 March 31, 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 and equity securities are hybrid securities, which are reported at fair value: March 31, December 31, (millions) 2018 2017 Fixed maturities: State and local government obligations $ 3.7 $ 0 $ 6.1 Corporate debt securities 86.4 44.5 99.8 Residential mortgage-backed securities 0 186.6 0 Other asset-backed securities 6.1 8.3 6.7 Redeemable preferred stocks 37.8 0 30.3 Total hybrid securities $ 134.0 $ 239.4 $ 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 derivative (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) does not have an observable intrinsic value, we have elected to record the change in fair value of the entire security through income as a realized gain or loss. Fixed Maturities The composition of fixed maturities by maturity at March 31, 2018 , was: (millions) Cost Fair Value Less than one year $ 3,757.9 $ 3,767.7 One to five years 14,498.9 14,337.4 Five to ten years 4,327.3 4,272.2 Ten years or greater 43.1 42.8 Total $ 22,627.2 $ 22,420.1 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 March 31, 2018 , we had $252.5 million of gross unrealized losses in our fixed-maturity securities. We currently do not intend to sell the fixed-maturity securities and determined that it is more likely that we will not be required to sell these securities for the period of time necessary to recover their cost bases. A review of our fixed-maturity securities indicated that the issuers were current with respect to their interest obligations and that there was no evidence of any deterioration of the current cash flow projections that would indicate we would not receive the remaining principal at maturity. The following tables show the composition of gross unrealized losses by major security type and by the length of time that individual securities have been in a continuous unrealized loss position: Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses March 31, 2018 Fixed maturities: U.S. government obligations 64 $ 7,933.8 $ (127.4 ) 37 $ 5,954.6 $ (90.7 ) 27 $ 1,979.2 $ (36.7 ) State and local government obligations 426 1,263.7 (16.4 ) 301 890.2 (9.5 ) 125 373.5 (6.9 ) Corporate debt securities 348 5,082.9 (64.8 ) 290 4,515.1 (56.1 ) 58 567.8 (8.7 ) Residential mortgage-backed securities 211 329.3 (4.8 ) 36 112.8 (0.5 ) 175 216.5 (4.3 ) Commercial mortgage-backed securities 131 1,974.8 (24.8 ) 89 1,510.5 (16.2 ) 42 464.3 (8.6 ) Other asset-backed securities 208 1,958.5 (12.9 ) 129 1,439.1 (8.7 ) 79 519.4 (4.2 ) Redeemable preferred stocks 3 26.8 (1.4 ) 2 15.3 (0.4 ) 1 11.5 (1.0 ) Total fixed maturities 1,391 $ 18,569.8 $ (252.5 ) 884 $ 14,437.6 $ (182.1 ) 507 $ 4,132.2 $ (70.4 ) Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses March 31, 2017 Fixed maturities: U.S. government obligations 34 $ 2,747.0 $ (20.6 ) 34 $ 2,747.0 $ (20.6 ) 0 $ 0 $ 0 State and local government obligations 340 986.3 (10.7 ) 310 906.5 (9.5 ) 30 79.8 (1.2 ) Corporate debt securities 147 2,494.7 (15.0 ) 139 2,415.1 (14.9 ) 8 79.6 (0.1 ) Residential mortgage-backed securities 247 916.3 (11.7 ) 138 297.1 (1.7 ) 109 619.2 (10.0 ) Commercial mortgage-backed securities 92 1,131.9 (16.3 ) 73 985.7 (14.2 ) 19 146.2 (2.1 ) Other asset-backed securities 118 1,601.6 (3.1 ) 112 1,521.4 (2.8 ) 6 80.2 (0.3 ) Redeemable preferred stocks 1 11.4 (1.1 ) 0 0 0 1 11.4 (1.1 ) Total fixed maturities 979 9,889.2 (78.5 ) 806 8,872.8 (63.7 ) 173 1,016.4 (14.8 ) Equity securities: Nonredeemable preferred stocks 9 197.0 (11.8 ) 5 78.5 (0.7 ) 4 118.5 (11.1 ) Common equities 68 49.2 (4.8 ) 61 48.3 (4.8 ) 7 0.9 0 Total equity securities 77 246.2 (16.6 ) 66 126.8 (5.5 ) 11 119.4 (11.1 ) Total portfolio 1,056 $ 10,135.4 $ (95.1 ) 872 $ 8,999.6 $ (69.2 ) 184 $ 1,135.8 $ (25.9 ) 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 March 31, 2017 and December 31, 2017 , the number of securities in our fixed-maturity portfolio with unrealized losses increased, primarily the result of rising interest rates during the latter part of 2017 and early 2018. We had no material decreases in valuation as a result of credit rating downgrades on our fixed-maturity securities. All of the fixed-maturity securities in an unrealized loss position at March 31, 2018 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): March 31, December 31, (millions) 2018 2017 Fixed maturities: Residential mortgage-backed securities $ (19.7 ) $ (43.3 ) $ (19.7 ) Commercial mortgage-backed securities (0.3 ) (0.5 ) (0.3 ) Total fixed maturities $ (20.0 ) $ (43.8 ) $ (20.0 ) The following tables provide rollforwards of the amounts related to credit losses recognized in earnings for the periods ended March 31, 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 March 31, 2018 Mortgage-Backed (millions) Residential Commercial Total Balance at December 31, 2017 $ 0 $ 0.5 $ 0.5 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 March 31, 2018 $ 0 $ 0.5 $ 0.5 Three Months Ended March 31, 2017 Mortgage-Backed (millions) Residential Commercial Total Balance at December 31, 2016 $ 11.1 $ 0.4 $ 11.5 Reductions for securities sold/matured 0 (0.3 ) (0.3 ) Change in recoveries of future cash flows expected to be collected 1 0.3 0 0.3 Balance at March 31, 2017 $ 11.4 $ 0.1 $ 11.5 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 three months ended March 31, 2018 or 2017 . Realized Gains (Losses) The components of net realized gains (losses) for the three months ended March 31, were: Three Months (millions) 2018 2017 Gross realized gains on security sales Available-for-sale securities: U.S. government obligations $ 0 $ 0.4 State and local government obligations 8.6 0.5 Corporate and other debt securities 0.1 4.1 Residential mortgage-backed securities 0 0.1 Commercial mortgage-backed securities 1.7 1.2 Other asset-backed securities 0 0.3 Redeemable preferred stocks 1.1 0.3 Total available-for-sale securities 11.5 6.9 Equity securities: Nonredeemable preferred stocks 3.6 45.4 Common equities 119.9 7.4 Total equity securities 123.5 52.8 Subtotal gross realized gains on security sales 135.0 59.7 Gross realized losses on security sales Available-for-sale securities: U.S. government obligations (8.9 ) (3.2 ) State and local government obligations (1.3 ) (0.1 ) Corporate and other debt securities (3.1 ) (0.9 ) Commercial mortgage-backed securities (6.3 ) (2.4 ) Other asset-backed securities (0.1 ) 0 Total available-for-sale securities (19.7 ) (6.6 ) Equity securities: Nonredeemable preferred stocks (0.4 ) (1.2 ) Common equities (7.9 ) (0.1 ) Total equity securities (8.3 ) (1.3 ) Subtotal gross realized losses on security sales (28.0 ) (7.9 ) Net realized gains (losses) on security sales Available-for-sale securities: U.S. government obligations (8.9 ) (2.8 ) State and local government obligations 7.3 0.4 Corporate and other debt securities (3.0 ) 3.2 Residential mortgage-backed securities 0 0.1 Commercial mortgage-backed securities (4.6 ) (1.2 ) Other asset-backed securities (0.1 ) 0.3 Redeemable preferred stocks 1.1 0.3 Total available-for-sale securities (8.2 ) 0.3 Equity securities: Nonredeemable preferred stocks 3.2 44.2 Common equities 112.0 7.3 Total equity securities 115.2 51.5 Subtotal net realized gains (losses) on security sales 107.0 51.8 Net holding period gains (losses) Hybrid securities (3.2 ) 0.8 Equity securities (152.0 ) 0 Subtotal net holding period gains (losses) (155.2 ) 0.8 Other-than-temporary impairment losses Equity securities: Common equities 0 (1.0 ) Subtotal other-than-temporary impairment losses 0 (1.0 ) Litigation settlements and other gains (losses) 0 0.3 Total net realized gains (losses) on securities $ (48.2 ) $ 51.9 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 the result of changes in credit spreads and interest rates. Also included are holding period change in valuation gains and losses on equity securities and hybrid securities, write-downs for securities determined to be other-than-temporarily impaired, and recoveries from litigation settlements related to investments. The following table reflects our holding period realized gains (losses) on equity securities recognized for the three months ended March 31, 2018 for equity securities held at quarter end: Three Months (millions) 2018 Total net gains (losses) recognized during the period on equity securities $ (36.8 ) Less: Net gains (losses) recognized on equity securities sold during the period 115.2 Net holding period gains (losses) recognized during the period on equity securities held at period end $ (152.0 ) Note: Comparative disclosure for the prior year period is not meaningful. Net Investment Income The components of net investment income for the three months ended March 31, were: Three Months (millions) 2018 2017 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 39.8 $ 12.7 State and local government obligations 10.0 13.3 Foreign government obligations 0 0.1 Corporate debt securities 36.2 29.6 Residential mortgage-backed securities 6.9 9.6 Commercial mortgage-backed securities 21.2 18.8 Other asset-backed securities 13.6 9.9 Redeemable preferred stocks 2.6 3.4 Total fixed maturities 130.3 97.4 Short-term investments 10.1 7.0 Total available-for-sale securities 140.4 104.4 Equity securities: Nonredeemable preferred stocks 10.9 11.5 Common equities 15.0 13.3 Total equity securities 25.9 24.8 Investment income 166.3 129.2 Investment expenses (6.0 ) (5.6 ) Net investment income $ 160.3 $ 123.6 The amount of investment income (interest and dividends) we recognize varies from year to year based on the average assets held during the year and the book yields of the securities in our portfolio. The increase in net investment income on a year-over-year basis for the three months ended March 31, 2018, was a combination of an increase in average assets, from strong underwriting growth and profitability, as well as the $850 million debt issuance in the second quarter 2017, and an increase in portfolio yields as a result of our decision to increase the portfolio duration from 2.3 years at the end of the first quarter 2017 to 2.6 years at the end of the first quarter 2018. Trading Securities At March 31, 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 months ended March 31, 2018 and 2017 . Derivative Instruments We had no open derivative positions at March 31, 2018 or December 31, 2017. During the first quarter 2017 , we entered into a forecasted debt issuance hedge, to hedge against a possible rise in interest rates, with a 30-year interest rate swap for which we were paying a fixed rate and receiving a variable rate, in anticipation of a debt issuance. At March 31, 2017, we recorded a $4.0 million pretax unrealized gain as part of accumulated other comprehensive income and the balance of our margin collateral at the clearinghouse was $136.1 million , which is included in other assets on our Consolidated Balance Sheets. Upon issuance of $850 million of 4.125% Senior Notes due 2047 in April 2017 , we closed the hedge and recognized, as part of accumulated other comprehensive income, a pretax unrealized loss of $8.0 million . See Note 4 – Debt for further discussion.

Fair Value

Fair Value	3 Months Ended
	Mar. 31, 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 March 31, 2018 Fixed maturities: U.S. government obligations $ 8,958.1 $ 0 $ 0 $ 8,958.1 $ 9,081.6 State and local government obligations 0 1,720.1 0 1,720.1 1,732.3 Foreign government obligations 0 0 0 0 0 Corporate debt securities 0 6,041.1 0 6,041.1 6,102.6 Subtotal 8,958.1 7,761.2 0 16,719.3 16,916.5 Asset-backed securities: Residential mortgage-backed 0 751.8 0 751.8 746.7 Commercial mortgage-backed 0 2,269.5 0 2,269.5 2,289.8 Other asset-backed 0 2,462.1 0 2,462.1 2,472.4 Subtotal asset-backed securities 0 5,483.4 0 5,483.4 5,508.9 Redeemable preferred stocks: Financials 0 64.0 0 64.0 61.2 Utilities 0 11.6 0 11.6 10.7 Industrials 0 141.8 0 141.8 129.9 Subtotal redeemable preferred stocks 0 217.4 0 217.4 201.8 Total fixed maturities 8,958.1 13,462.0 0 22,420.1 22,627.2 Short-term investments 2,569.5 482.9 0 3,052.4 3,052.4 Total available-for-sale securities 11,527.6 13,944.9 0 25,472.5 25,679.6 Equity securities: Nonredeemable preferred stocks: Financials 78.9 662.0 0 740.9 654.6 Industrials 0 0 5.0 5.0 5.0 Subtotal nonredeemable preferred stocks 78.9 662.0 5.0 745.9 659.6 Common equities: Common stocks 3,032.9 0 0 3,032.9 1,265.2 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 3,032.9 0 0.3 3,033.2 1,265.5 Total equity securities 3,111.8 662.0 5.3 3,779.1 1,925.1 Total portfolio $ 14,639.4 $ 14,606.9 $ 5.3 $ 29,251.6 $ 27,604.7 Debt $ 0 $ 4,048.3 $ 0 $ 4,048.3 $ 3,859.2 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost March 31, 2017 Fixed maturities: U.S. government obligations $ 3,325.8 $ 0 $ 0 $ 3,325.8 $ 3,345.5 State and local government obligations 0 2,488.4 0 2,488.4 2,475.8 Foreign government obligations 22.9 0 0 22.9 22.9 Corporate debt securities 0 5,208.0 0 5,208.0 5,199.6 Subtotal 3,348.7 7,696.4 0 11,045.1 11,043.8 Asset-backed securities: Residential mortgage-backed 0 1,448.1 0 1,448.1 1,433.0 Commercial mortgage-backed 0 2,213.5 0 2,213.5 2,216.1 Other asset-backed 0 2,564.9 0 2,564.9 2,562.4 Subtotal asset-backed securities 0 6,226.5 0 6,226.5 6,211.5 Redeemable preferred stocks: Financials 0 63.2 0 63.2 60.4 Utilities 0 31.4 0 31.4 30.6 Industrials 0 108.1 0 108.1 98.5 Subtotal redeemable preferred stocks 0 202.7 0 202.7 189.5 Total fixed maturities 3,348.7 14,125.6 0 17,474.3 17,444.8 Equity securities: Nonredeemable preferred stocks: Financials 131.1 678.7 0 809.8 710.3 Industrials 0 0 0 0 0 Subtotal nonredeemable preferred stocks 131.1 678.7 0 809.8 710.3 Common equities: Common stocks 2,977.8 0 0 2,977.8 1,451.2 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 2,977.8 0 0.3 2,978.1 1,451.5 Total fixed maturities and equity securities 6,457.6 14,804.3 0.3 21,262.2 19,606.6 Short-term investments 2,226.7 814.1 0 3,040.8 3,040.8 Total portfolio $ 8,684.3 $ 15,618.4 $ 0.3 $ 24,303.0 $ 22,647.4 Debt $ 0 $ 3,198.3 $ 121.1 $ 3,319.4 $ 3,111.7 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost December 31, 2017 Fixed maturities: U.S. government obligations $ 6,645.9 $ 0 $ 0 $ 6,645.9 $ 6,688.8 State and local government obligations 0 2,297.1 0 2,297.1 2,285.6 Foreign government obligations 0 0 0 0 0 Corporate debt securities 0 4,997.7 0 4,997.7 4,997.2 Subtotal 6,645.9 7,294.8 0 13,940.7 13,971.6 Asset-backed securities: Residential mortgage-backed 0 836.7 0 836.7 828.8 Commercial mortgage-backed 0 2,758.6 0 2,758.6 2,760.1 Other asset-backed 0 2,454.7 0 2,454.7 2,454.5 Subtotal asset-backed securities 0 6,050.0 0 6,050.0 6,043.4 Redeemable preferred stocks: Financials 0 64.1 0 64.1 61.3 Utilities 0 11.4 0 11.4 10.1 Industrials 0 135.5 0 135.5 123.5 Subtotal redeemable preferred stocks 0 211.0 0 211.0 194.9 Total fixed maturities 6,645.9 13,555.8 0 20,201.7 20,209.9 Equity securities: Nonredeemable preferred stocks: Financials 80.6 718.2 0 798.8 693.6 Industrials 0 0 5.0 5.0 5.0 Subtotal nonredeemable preferred stocks 80.6 718.2 5.0 803.8 698.6 Common equities: Common stocks 3,399.5 0 0 3,399.5 1,498.7 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 3,399.5 0 0.3 3,399.8 1,499.0 Total fixed maturities and equity securities 10,126.0 14,274.0 5.3 24,405.3 22,407.5 Short-term investments 1,824.4 1,045.0 0 2,869.4 2,869.4 Total portfolio $ 11,950.4 $ 15,319.0 $ 5.3 $ 27,274.7 $ 25,276.9 Debt $ 0 $ 3,606.5 $ 37.1 $ 3,643.6 $ 3,306.3 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 March 31, 2018 , vendor-quoted prices represented 75% of our Level 1 classifications (excluding short-term investments), compared to 54% and 66% at March 31, 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 March 31, 2018 and 2017 , and December 31, 2017 , vendor-quoted prices comprised 99% , 99% , and 98% , respectively, of our Level 2 classifications (excluding short-term investments), while dealer-quoted prices represented 1% , 1% , and 2% , respectively. In our process for selecting a source (e.g., dealer, pricing service) to provide pricing for securities in our portfolio, we reviewed documentation from the sources that detailed the pricing techniques and methodologies used by these sources and determined if their policies adequately considered market activity, either based on specific transactions for the particular security type or based on modeling of securities with similar credit quality, duration, yield, and structure that were recently transacted. Once a source is chosen, we continue to monitor any changes or modifications to their processes by reviewing their documentation on internal controls for pricing and market reviews. We review quality control measures of our sources as they become available to determine if any significant changes have occurred from period to period that might indicate issues or concerns regarding their evaluation or market coverage. As part of our pricing procedures, we obtain quotes from more than one source to help us fully evaluate the market price of securities. However, our internal pricing policy is to use a consistent source for individual securities in order to maintain the integrity of our valuation process. Quotes obtained from the sources are not considered binding offers to transact. Under our policy, when a review of the valuation received from our selected source appears to be outside of what is considered market level activity (which is defined as trading at spreads or yields significantly different than those of comparable securities or outside the general sector level movement without a reasonable explanation), we may use an alternate source’s price. To the extent we determine that it may be prudent to substitute one source’s price for another, we will contact the initial source to obtain an understanding of the factors that may be contributing to the significant price variance, which often leads the source to adjust their pricing input data for future pricing. To allow us to determine if our initial source is providing a price that is outside of a reasonable range, we review our portfolio pricing on a weekly basis. When necessary, we challenge prices from our sources when a price provided does not match our expectations based on our evaluation of market trends and activity. Initially, we perform a review of our portfolio by sector to identify securities whose prices appear outside of a reasonable range. We then perform a more detailed review of fair values for securities disclosed as Level 2. We review dealer bids and quotes for these and/or similar securities to determine the market level context for our valuations. We then evaluate inputs relevant for each class of securities disclosed in the preceding hierarchy tables. For our structured debt securities, including commercial, residential, and asset-backed securities, we evaluate available market-related data for these and similar securities related to collateral, delinquencies, and defaults for historical trends and reasonably estimable projections, as well as historical prepayment rates and current prepayment assumptions and cash flow estimates. We further stratify each class of our structured debt securities into more finite sectors (e.g., planned amortization class, first pay, second pay, senior, subordinated, etc.) and use duration, credit quality, and coupon to determine if the fair value is appropriate. For our corporate debt and preferred stock (redeemable and nonredeemable) portfolios, as well as the notes and debentures issued by The Progressive Corporation (see Note 4 – Debt ), we review securities by duration, coupon, and credit quality, as well as changes in interest rate and credit spread movements within that stratification. The review also includes recent trades, including: volume traded at various levels that establish a market, issuer specific fundamentals, and industry specific economic news as it comes to light. For our municipal securities (e.g., general obligations, revenue, and housing), we stratify the portfolio to evaluate securities by type, coupon, credit quality, and duration to review price changes relative to credit spread and interest rate changes. Additionally, we look to economic data as it relates to geographic location as an indication of price-to-call or maturity predictors. For municipal housing securities, we look to changes in cash flow projections, both historical and reasonably estimable projections, to understand yield changes and their effect on valuation. Lastly, for our short-term securities, we look at acquisition price relative to the coupon or yield. Since our short-term securities are typically 90 days or less to maturity, with the majority listed in Level 2 being seven days or less to redemption, we believe that acquisition price is the best estimate of fair value. We also review data assumptions as supplied by our sources to determine if that data is relevant to current market conditions. In addition, we independently review each sector for transaction volumes, new issuances, and changes in spreads, as well as the overall movement of interest rates along the yield curve to determine if sufficient activity and liquidity exists to provide a credible source for our market valuations. During each valuation period, we create internal estimations of portfolio valuation (performance returns), based on current market-related activity (i.e., interest rate and credit spread movements and other credit-related factors) within each major sector of our portfolio. We compare our internally generated portfolio results with those generated based on quotes we receive externally and research material valuation differences. We compare our results to index returns for each major sector adjusting for duration and credit quality differences to better understand our portfolio’s results. Additionally, we review on a monthly basis our external sales transactions and compare the actual final market sales prices to previous market valuation prices. This review provides us further validation that our pricing sources are providing market level prices, since we are able to explain significant price changes (i.e., greater than 2%) as known events occur in the marketplace and affect a particular security’s price at sale. This analysis provides us with additional comfort regarding the source’s process, the quality of its review, and its willingness to improve its analysis based on feedback from clients. We believe this effort helps ensure that we are reporting the most representative fair values for our securities. Except as described below, our Level 3 securities are also priced externally; however, due to several factors (e.g., nature of the securities, level of activity, and lack of similar securities trading to obtain observable market level inputs), these valuations are more subjective in nature. Certain private equity investments and fixed-income investments included in the Level 3 category are valued using external pricing supplemented by internal review and analysis. After all the valuations are received and our review is complete, if the inputs used by vendors are determined to not contain sufficient observable market information, we will reclassify the affected security valuations to Level 3. At March 31, 2018 and 2017 , and December 31, 2017 , we did not have any securities in our fixed-maturity portfolio listed as Level 3. At March 31, 2018 and December 31, 2017 , we held one private nonredeemable preferred security with a value of $5.0 million that was priced internally. The security was purchased during the third quarter 2017 and the value at both periods equals the cost at acquisition. A review of their latest available financial statements did not reveal any significant changes that would impact the security's fair value. We did not hold any internally priced securities at March 31, 2017 . We review the prices from our external sources for reasonableness using internally developed assumptions to derive prices for the securities, which are then compared to the prices we received. During 2018 or 2017 , there were no material assets or liabilities measured at fair value on a nonrecurring basis. Based on our review, all prices received from external sources remained unadjusted. We did not have any material changes in fair value associated with Level 3 assets for the three months ended March 31, 2018 and 2017 . The following tables provide a summary of the quantitative information about Level 3 fair value measurements for our applicable securities at March 31, 2018 and 2017 , and December 31, 2017 : Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at March 31, 2018 Valuation Technique Unobservable Input Unobservable Equity securities: Nonredeemable preferred stocks: Industrials 1 $ 5.0 Internal price Purchase price 3.9 Subtotal Level 3 securities 5.0 Pricing exemption securities 2 0.3 Total Level 3 securities $ 5.3 1 The security was internally-priced since it is privately held and it was valued at March 31, 2018 using the purchase price. 2 The fair values for these securities were determined with unobservable inputs not reasonably available to us. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at March 31, 2017 Valuation Technique Unobservable Input Unobservable Equity securities: Nonredeemable preferred stocks: Industrials $ 0 NA NA NA Subtotal Level 3 securities 0 Pricing exemption securities 1 0.3 Total Level 3 securities $ 0.3 NA= Not applicable, since we did not hold any nonredeemable preferred stock Level 3 securities at March 31, 2017 . 1 The fair values for these securities were determined with unobservable inputs not reasonably available to us. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at Dec. 31, 2017 Valuation Technique Unobservable Input Unobservable Equity securities: Nonredeemable preferred stocks: Industrials 1 $ 5.0 Internal price Purchase price 3.9 Subtotal Level 3 securities 5.0 Pricing exemption securities 2 0.3 Total Level 3 securities $ 5.3 1 The security was internally-priced since it is privately held and it was valued at December 31, 2017 using the purchase price. 2 The fair values for these securities were determined with unobservable inputs not reasonably available to us. Due to the relative size of the Level 3 securities’ fair values compared to the total portfolio’s fair value, any changes in pricing methodology would not have a significant change in valuation that would materially impact net or comprehensive income.

Debt

Debt	3 Months Ended
	Mar. 31, 2018
Debt Disclosure [Abstract]
Debt	Debt — Debt at each of the balance sheet periods consisted of: March 31, 2018 March 31, 2017 December 31, 2017 (millions) Carrying Value Fair Value Carrying Value Fair Value Carrying Value Fair Value 3.75% Senior Notes due 2021 $ 498.9 $ 512.1 $ 498.5 $ 527.9 $ 498.8 $ 520.7 2.45% Senior Notes due 2027 496.2 458.3 495.8 468.6 496.1 477.9 6 5/8% Senior Notes due 2029 296.2 370.4 296.0 386.3 296.1 382.3 6.25% Senior Notes due 2032 395.4 498.9 395.2 512.1 395.3 516.9 4.35% Senior Notes due 2044 346.5 360.2 346.5 366.7 346.5 388.7 3.70% Senior Notes due 2045 395.2 376.6 395.1 380.0 395.2 402.9 4.125% Senior Notes due 2047 841.3 858.6 0 0 841.2 917.1 4.20% Senior Notes due 2048 589.5 613.2 0 0 0 0 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 0 0 563.5 556.7 0 0 Other debt instruments 0 0 121.1 121.1 37.1 37.1 Total $ 3,859.2 $ 4,048.3 $ 3,111.7 $ 3,319.4 $ 3,306.3 $ 3,643.6 The other debt instruments reported in the table above were issued by ARX, prior to The Progressive Corporation acquiring a controlling interest in 2015. 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. The Progressive Corporation Debt 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 underwriter’s 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. During the second quarter 2017, we redeemed our 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067, at par, in the aggregate principal amount of $563.7 million . The Progressive Corporation Line of Credit During 2017, The Progressive Corporation entered into a new line of credit with PNC Bank, National Association (PNC) in the maximum principal amount of $250 million . This line of credit replaced a previous line of credit with a maximum principal amount of $100 million that expired in April 2017. Subject to the terms and conditions of the line of credit documents, advances under the line of credit (if any) will bear interest at a variable rate equal to the higher of PNC’s Prime Rate or the sum of the Federal Funds Open Rate plus 50 basis points. Each advance must be repaid on the 30 th day after the advance or, if earlier, on April 30, 2018, the expiration date of the line of credit. Prepayments are permitted without penalty. The Line of Credit is uncommitted and, as such, all advances are subject to PNC’s discretion. We had no borrowings under either line of credit during the first three months of 2018 or throughout 2017.

Income Taxes

Income Taxes	3 Months Ended
	Mar. 31, 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 March 31, 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 months ended March 31, 2018 , there have been no material changes in our uncertain tax positions. The effective tax rate for the three months ended March 31, 2018 was 19.9% , compared to 32.9% for the same period in 2017. The current year effective tax rate is lower due to the reduction in the U.S. corporate income tax rate from 35% to 21%, effective January 1, 2018. As of March 31, 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 uncertainty surrounding the discount factors to be applied for loss reserve discounting. We are waiting for the Internal Revenue Service to publish additional guidance or information for both items.

Loss and Loss Adjustment Expens

Loss and Loss Adjustment Expense Reserves	3 Months Ended
	Mar. 31, 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 during the first quarter is summarized as follows: March 31, (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 4,815.2 4,152.2 Prior years 55.6 111.2 Total incurred 4,870.8 4,263.4 Paid related to: Current year 2,217.8 1,933.2 Prior years 2,377.8 2,118.7 Total paid 4,595.6 4,051.9 Net balance, End of period 11,192.0 9,778.5 Plus reinsurance recoverables on unpaid losses 2,137.0 1,850.4 Balance, End of period $ 13,329.0 $ 11,628.9 We experienced unfavorable reserve development of $55.6 million and $111.2 million for the first quarter of 2018 and 2017, respectively, which is reflected as “Incurred related to prior years ” in the table above. First quarter 2018 • The unfavorable prior year reserve development was attributable to accident years 2017 and 2016, with accident years 2015 and prior recognizing $6 million of favorable development. • Our personal auto business incurred about $35 million of unfavorable loss and loss adjustment expense (LAE) reserve development, with Agency and Direct auto businesses contributing about $24 million and $11 million , respectively. The unfavorable development was primarily due to an increase in reopened personal injury protection (PIP) claims. • Our Commercial Lines business experienced $19 million of unfavorable development primarily due to late reported losses and higher LAE than anticipated. • Our Property business recognized minimal unfavorable development of $2 million . First quarter 2017 • Approximately $100 million of the unfavorable prior year reserve development was attributable to accident year 2016. • Our vehicle businesses incurred about $124 million of unfavorable LAE reserve development for the first quarter 2017, partially offset by the favorable loss and LAE reserve development in our Property business. • Our Agency and Direct auto businesses incurred about $68 million and $37 million , respectively, of the total unfavorable reserve development, primarily due to an increase in costs related to property damage, unfavorable PIP case reserve development, mainly in Michigan, and more late reported bodily injury claims than anticipated. • Our Commercial Lines business experienced unfavorable development primarily due to more late reported injury claims than anticipated.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	3 Months Ended
	Mar. 31, 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 March 31, 2018 and 2017 , and December 31, 2017 , were $166.6 million , $140.0 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: Three Months Ended March 31, (millions) 2018 2017 Income taxes $ 0 $ 0.7 Interest 33.3 33.1

Segment Information

Segment Information	3 Months Ended
	Mar. 31, 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 March 31, 2018 2017 (millions) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Personal Lines Agency $ 3,063.8 $ 409.1 $ 2,631.5 $ 249.7 Direct 3,016.3 298.0 2,523.7 175.3 Total Personal Lines 1 6,080.1 707.1 5,155.2 425.0 Commercial Lines 808.6 94.8 645.5 67.4 Property 2 285.3 28.5 226.0 7.8 Other indemnity 0 0.2 0 (0.2 ) Total underwriting operations 7,174.0 830.6 6,026.7 500.0 Fees and other revenues 3 103.8 NA 85.2 NA Service businesses 34.2 4.9 28.5 2.6 Investments 4 118.1 112.1 181.1 175.5 Other gains (losses) 0 0 0.2 0.2 Interest expense NA (36.8 ) NA (36.8 ) Consolidated total $ 7,430.1 $ 910.8 $ 6,321.7 $ 641.5 NA = Not applicable 1 Personal auto insurance accounted for 94% and 93% of the total Personal Lines segment net premiums earned in the first quarter of 2018 and 2017 , respectively; 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 months ended March 31, 2018 and 2017, pretax profit (loss) includes $18.0 million and $15.5 million , respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX. Although this expense is included in our Property segment, it is not reported in the consolidated results of ARX and, therefore, will not affect the value of the net income attributable to the noncontrolling interest. 3 Pretax profit (loss) for fees and other revenues are attributable to operating segments. 4 Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit is net of investment 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 March 31, 2018 2017 Underwriting Combined Ratio Underwriting Combined Ratio Personal Lines Agency 13.4 % 86.6 9.5 % 90.5 Direct 9.9 90.1 6.9 93.1 Total Personal Lines 11.6 88.4 8.2 91.8 Commercial Lines 11.7 88.3 10.4 89.6 Property 1 10.0 90.0 3.4 96.6 Total underwriting operations 11.6 88.4 8.3 91.7 1 Included in the three months ended March 31, 2018 and 2017, is 6.3 points and 6.9 points, respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX.

Dividends

Dividends	3 Months Ended
	Mar. 31, 2018
Text Block [Abstract]
Dividends	Dividends Annual Variable 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 multiplied by a performance factor (Gainshare factor), which is determined by reference to the Agency auto, Direct auto, special lines, Commercial Lines, and Property business units, with minor exclusions and adjustments, and subject to the limitations discussed below. The target percentage is determined by our Board of Directors on an annual basis and announced to shareholders and the public. In December 2017 , the Board determined the target percentage for 2018 to be 33-1/3 % of annual after-tax underwriting income, which is unchanged from the 2017 target percentage. Since the inception of our variable dividend, we have applied a tax rate of 35% to calculate the after-tax underwriting income. Beginning in 2018, we will apply a rate of 21% to calculate the after-tax underwriting income. The Gainshare factor can range from zero to two and is determined by comparing our operating performance for the specified business units for the year to certain predetermined profitability and growth objectives approved by the Compensation Committee of the Board. This Gainshare factor is also used in the annual cash incentive program currently in place for our employees (our “Gainsharing program”). On a year-to-date basis, as of March 31, 2018 , the Gainshare factor was 1.99 . Since the final factor will be determined based on our results for the full year, the final factor may vary from the current factor. Our annual dividend program will result in a variable payment to shareholders each year, subject to certain limitations. If the Gainshare factor is zero or if our comprehensive income is less than after-tax underwriting income, no dividend would be payable under our annual variable dividend policy. In addition, the ultimate decision on whether or not a dividend will be paid is in the discretion of the Board of Directors. The Board could decide to alter our policy, or not to pay the annual variable dividend, at any time prior to the declaration of the dividend for the year. Such an action by the Board could result from, among other reasons, changes in the insurance marketplace, changes in our performance or capital needs, changes in the U.S. federal income tax laws, disruptions of national or international capital markets, or other events affecting our business, liquidity, or financial position. Following is a summary of our shareholder dividends that were declared in the last two years: (millions, except per share amounts) Amount Dividend Type Declared Paid Per Share Total 1 Annual – Variable December 2017 February 2018 $ 1.1247 $ 655.1 Annual – Variable December 2016 February 2017 $ 0.6808 $ 395.4 1 Based on an estimate of shares outstanding as of the record date. For the dividends declared in December 2017 and 2016, we paid $654.9 million and $395.4 million , respectively. Preferred Share Dividends In the first quarter 2018, we issued 500,000 of our 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.

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	3 Months Ended
	Mar. 31, 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 Total tax (provision) benefit After tax total accumulated other comprehensive income Total net unrealized gains (losses) on securities Net unrealized gains on forecasted transactions Foreign currency translation adjustment (Income)loss attributable to NCI Balance at December 31, 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 tax effect 2 0 (4.3 ) (4.3 ) (1.1 ) (3.2 ) 0 0 Adjusted balance at December 31, 2017 (28.2 ) 5.9 (22.3 ) (6.3 ) (18.0 ) 0 2.0 Other comprehensive income (loss) before reclassifications: Investment securities (203.4 ) 42.8 (160.6 ) (160.6 ) 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) 5.1 (1.1 ) 4.0 0 0 0 4.0 Total other comprehensive income (loss) before reclassifications (198.3 ) 41.7 (156.6 ) (160.6 ) 0 0 4.0 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 (7.7 ) 1.6 (6.1 ) (6.1 ) 0 0 0 Interest expense (0.2 ) 0 (0.2 ) 0 (0.2 ) 0 0 Total reclassification adjustment for amounts realized in net income (7.9 ) 1.6 (6.3 ) (6.1 ) (0.2 ) 0 0 Total other comprehensive income (loss) (190.4 ) 40.1 (150.3 ) (154.5 ) 0.2 0 4.0 Balance at March 31, 2018 $ (218.6 ) $ 46.0 $ (172.6 ) $ (160.8 ) $ (17.8 ) $ 0 $ 6.0 Components of Changes in Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income Total tax (provision) benefit After tax total accumulated other comprehensive income Total net Net unrealized gains on forecasted transactions Foreign currency translation adjustment (Income)loss attributable to NCI Balance at December 31, 2016 $ 1,439.5 $ (506.1 ) $ 933.4 $ 939.6 $ (9.4 ) $ (1.1 ) $ 4.3 Other comprehensive income (loss) before reclassifications: Investment securities 254.5 (89.2 ) 165.3 165.3 0 0 0 Forecasted transactions 4.0 (1.4 ) 2.6 0 2.6 0 0 Foreign currency translation adjustment 0.5 (0.2 ) 0.3 0 0 0.3 0 Loss attributable to noncontrolling interest (NCI) (1.5 ) 0.6 (0.9 ) 0 0 0 (0.9 ) Total other comprehensive income (loss) before reclassifications 257.5 (90.2 ) 167.3 165.3 2.6 0.3 (0.9 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (1.0 ) 0.4 (0.6 ) (0.6 ) 0 0 0 Net realized gains (losses) on securities 51.6 (18.1 ) 33.5 33.5 0 0 0 Interest expense 0.5 (0.2 ) 0.3 0 0.3 0 0 Total reclassification adjustment for amounts realized in net income 51.1 (17.9 ) 33.2 32.9 0.3 0 0 Total other comprehensive income (loss) 206.4 (72.3 ) 134.1 132.4 2.3 0.3 (0.9 ) Balance at March 31, 2017 $ 1,645.9 $ (578.4 ) $ 1,067.5 $ 1,072.0 $ (7.1 ) $ (0.8 ) $ 3.4 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. 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	3 Months Ended
	Mar. 31, 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 March 31, 2018 or 2017, and there were no material settlements during the first quarter 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	3 Months Ended
	Mar. 31, 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 have the right to “put” their ARX shares to Progressive in two installments, one in early 2018 and one in early 2021, and Progressive has the ability to “call ” a portion of the outstanding shares shortly thereafter. If these rights are exercised in full when available, our ownership stake in ARX capital stock would exceed 80% in 2018 and would reach 100% in 2021. See Note 15 – Redeemable Noncontrolling Interest in our 2017 Annual Report to Shareholders for a discussion of the purchase prices for shares to be purchased by Progressive pursuant to these put or call rights. Since these securities are redeemable upon the occurrence of an event that is not solely within the control of Progressive, we have recorded the redeemable noncontrolling interest (NCI) as mezzanine equity on our consolidated balance sheets, which represents the minority shares at the current estimated purchase price pursuant to the put and call provisions of the stockholders’ agreement. The estimated purchase price is based, in part, on the change in tangible net book value of ARX from December 31, 2014 to the balance sheet dates. In addition to these minority shares, at March 31, 2018 , ARX employees held options to purchase 21,550 ARX shares. These options and any shares issued upon exercise are subject to the stockholders’ agreement, including the right to “put ” these shares to Progressive, as described above. Until the options are exercised, the underlying obligation of approximately $31.6 million is not recorded as part of redeemable NCI. The changes in the components of redeemable NCI during the three months ended March 31, 2018 and 2017 , and the year ended December 31, 2017 , were: (millions) March 31, 2018 March 31, 2017 December 31, 2017 Balance, Beginning of period $ 503.7 $ 483.7 $ 483.7 Net income attributable to NCI 11.8 6.0 5.9 Other comprehensive income (loss) attributable to NCI (4.0 ) 0.9 2.3 Exercise of employee stock options 1.4 3.4 3.4 Change in redemption value of NCI 1.3 0.2 8.4 Balance, End of period $ 514.2 $ 494.2 $ 503.7 Subsequent Event In April 2018, pursuant to the stockholders’ agreement, the minority ARX stockholders put 204,527 shares, including 5,483 shares that were issued upon the exercise of the outstanding stock options, bringing Progressive's total ownership interest in ARX to 86.7% . The total cost to Progressive to acquire these additional shares was $295.9 million .

Goodwill and Intangible Assets

Goodwill and Intangible Assets	3 Months Ended
	Mar. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	Goodwill and Intangible Assets Goodwill During the three months ended March 31, 2018 , there were no changes to the carrying amount of goodwill. Goodwill recorded at March 31, 2018 and December 31, 2017 was $452.7 million and $449.4 million at March 31, 2017. The increase since the first quarter 2017 was the result of the acquisition of a small excess and surplus lines insurance company during the second quarter 2017. 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 March 31, 2018 and 2017 , and December 31, 2017 : (millions) March 31, 2018 March 31, 2017 December 31, 2017 Intangible assets subject to amortization $ 336.2 $ 404.9 $ 354.2 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 348.6 $ 417.3 $ 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 have $0.6 million of accumulated amortization for all periods presented. Intangible assets subject to amortization consisted of the following: (millions) March 31, 2018 March 31, 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 $ 109.8 $ 146.4 $ 256.2 $ 73.2 $ 183.0 $ 256.2 $ 100.7 $ 155.5 Agency relationships 159.2 34.2 125.0 159.2 22.7 136.5 159.2 31.3 127.9 Software rights 79.1 32.1 47.0 79.1 21.5 57.6 79.1 29.4 49.7 Trade name 34.8 17.0 17.8 34.8 7.0 27.8 34.8 13.7 21.1 Total $ 529.3 $ 193.1 $ 336.2 $ 529.3 $ 124.4 $ 404.9 $ 529.3 $ 175.1 $ 354.2 Amortization expense was $18.0 million and $15.5 million for the three months ended March 31, 2018 and 2017 , respectively. During the third quarter 2017, we revised our estimate of the economic useful life of our trade name intangible asset from an original life of 10 years to a remaining life of 2 years . The decrease in the useful life represents the estimated length of time that it is expected to take to transition the branding of our Property products from the ASI trade name to the Progressive brand. As of March 31, 2018 , the remaining average life of all of our intangible assets is 4.7 years .

New Accounting Standards

New Accounting Standards	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
New Accounting Standards	New Accounting Standards Issued In January 2018, the Financial Accounting Standards Board (FASB) proposed an Accounting Standards Update (ASU), which would provide targeted improvements to the new lease accounting guidance issued by the FASB in February 2016 (the “2016 ASU”). The 2016 ASU, which eliminates the off-balance-sheet accounting for leases, will require lessees to report their operating leases as both an asset and liability on the statement of financial position and to disclose key information about leasing arrangements in the financial statement footnotes. Under the 2016 ASU, there will be no change to the recognition of lease expense in our results of operations. The 2016 ASU will be effective for fiscal years (including interim periods within those fiscal years) beginning after December 15, 2018 (2019 for calendar-year companies). Under the proposed guidance, companies would have the option to apply the new lease requirements either as of the effective date (i.e., January 1, 2019), with comparative information presented in accordance with the previous standard, or on a modified retrospective basis, which would restate all financial statement information as of the beginning of the earliest period presented and is the transition method under the 2016 ASU. Based on our lease portfolio at March 31, 2018, and in accordance with the accounting elections available in the 2016 ASU, we would have recorded an increase to assets and liabilities of approximately $140 million , and there would have been no impact on our results of operations or cash flows. Therefore, we do not expect this standard to have a material impact on our financial condition. In March 2017, the FASB issued an ASU related to premium amortization on purchased callable debt securities. The intent of the standard is to shorten the amortization period for certain purchased callable debt securities held at a premium. Under the ASU, the premium is required to be amortized to the earliest call date. The ASU more closely aligns interest income recorded on bonds held at a premium with the economics of the underlying instrument. The ASU, which is required to be applied on a modified retrospective basis, is effective for fiscal years beginning after December 15, 2018 (2019 for calendar-year companies), and interim periods within those fiscal years. Since we have historically used a yield-to-worst scenario for our securities that were purchased at a premium, and the first call on a premium security most often produces the lowest and most conservative yield, we do not expect this standard to have a significant impact on our financial condition, cash flows, or results of operations. In January 2017, the FASB issued an ASU, which eliminates the requirement to determine the implied value of goodwill in measuring an impairment loss. Upon adoption, the measurement of a goodwill impairment will represent the excess of the reporting unit’s carrying value over fair value, limited to the carrying value of goodwill. This ASU is effective for goodwill impairment tests in fiscal years beginning after December 15, 2019 (2020 for calendar-year companies), with early adoption permitted. We do not expect this standard to have a material impact on our financial position or results of operations. In June 2016, the FASB issued an ASU intended to improve the timing, and enhance the accounting and disclosure, of credit losses on financial assets. Additionally, this update will modify the existing accounting guidance related to the impairment evaluation for available-for-sale debt securities and will result in the creation of an allowance for credit losses as a contra asset account. The ASU will require cumulative-effect changes to retained earnings in the period of adoption, if any occur, and will also require prospective changes on previously recorded impairments. This ASU is effective for fiscal years (including interim periods within those fiscal years) beginning after December 15, 2019 (2020 for calendar-year companies), with early adoption permissible (including interim periods within that fiscal year) beginning after December 15, 2018 (2019 for calendar-year companies). While the ASU creates additional accounting complexities related to the recognition of the impairment losses, and subsequent recoveries, through an allowance for credit losses account, we do not expect that the ASU will have a material impact on our current method of evaluating securities for credit losses or the timing or recognition of the amounts of the impairment losses. 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)	3 Months Ended
	Mar. 31, 2018
Accounting Policies [Abstract]
Reclassifications [Text Block]	Reclassification — For the period ended March 31, 2017, we reclassified the change in the net holding period gains (losses) on hybrid securities held at March 31, 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 year end 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.

Investments (Tables)

Investments (Tables)	3 Months Ended
	Mar. 31, 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. The net holding period gains (losses) on our equity securities represent the change in fair value recorded as realized gains (losses); all of our equity securities have a readily determinable fair value. The net holding period gains (losses) on our fixed-maturity securities represent the amounts realized on our hybrid securities only. ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value March 31, 2018 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 9,081.6 $ 3.9 $ (127.4 ) $ 0 $ 8,958.1 30.6 % State and local government obligations 1,732.3 4.3 (16.4 ) (0.1 ) 1,720.1 5.9 Foreign government obligations 0 0 0 0 0 0 Corporate debt securities 6,102.6 4.8 (64.8 ) (1.5 ) 6,041.1 20.7 Residential mortgage-backed securities 746.7 9.9 (4.8 ) 0 751.8 2.6 Commercial mortgage-backed securities 2,289.8 4.5 (24.8 ) 0 2,269.5 7.8 Other asset-backed securities 2,472.4 2.4 (12.9 ) 0.2 2,462.1 8.4 Redeemable preferred stocks 201.8 18.6 (1.4 ) (1.6 ) 217.4 0.7 Total fixed maturities 22,627.2 48.4 (252.5 ) (3.0 ) 22,420.1 76.7 Short-term investments 3,052.4 0 0 0 3,052.4 10.4 Total available-for-sale securities 25,679.6 48.4 (252.5 ) (3.0 ) 25,472.5 87.1 Equity securities: Nonredeemable preferred stocks 659.6 0 0 86.3 745.9 2.5 Common equities 1,265.5 0 0 1,767.7 3,033.2 10.4 Total equity securities 1,925.1 0 0 1,854.0 3,779.1 12.9 Total portfolio 1,2 $ 27,604.7 $ 48.4 $ (252.5 ) $ 1,851.0 $ 29,251.6 100.0 % ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value March 31, 2017 Fixed maturities: U.S. government obligations $ 3,345.5 $ 0.9 $ (20.6 ) $ 0 $ 3,325.8 13.7 % State and local government obligations 2,475.8 23.3 (10.7 ) 0 2,488.4 10.2 Foreign government obligations 22.9 0 0 0 22.9 0.1 Corporate debt securities 5,199.6 23.1 (15.0 ) 0.3 5,208.0 21.4 Residential mortgage-backed securities 1,433.0 24.7 (11.7 ) 2.1 1,448.1 6.0 Commercial mortgage-backed securities 2,216.1 13.7 (16.3 ) 0 2,213.5 9.1 Other asset-backed securities 2,562.4 5.4 (3.1 ) 0.2 2,564.9 10.6 Redeemable preferred stocks 189.5 14.3 (1.1 ) 0 202.7 0.8 Total fixed maturities 17,444.8 105.4 (78.5 ) 2.6 17,474.3 71.9 Equity securities: Nonredeemable preferred stocks 710.3 111.3 (11.8 ) 0 809.8 3.3 Common equities 1,451.5 1,531.4 (4.8 ) 0 2,978.1 12.3 Short-term investments 3,040.8 0 0 0 3,040.8 12.5 Total portfolio 1,2 $ 22,647.4 $ 1,748.1 $ (95.1 ) $ 2.6 $ 24,303.0 100.0 % ($ 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 and collateral on open derivative positions; at March 31, 2018 and 2017 , we had $83.6 million and $111.0 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 March 31, 2018 and 2017 , and December 31, 2017 , included $2.1 billion , $0.7 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 and equity securities are hybrid securities, which are reported at fair value: March 31, December 31, (millions) 2018 2017 Fixed maturities: State and local government obligations $ 3.7 $ 0 $ 6.1 Corporate debt securities 86.4 44.5 99.8 Residential mortgage-backed securities 0 186.6 0 Other asset-backed securities 6.1 8.3 6.7 Redeemable preferred stocks 37.8 0 30.3 Total hybrid securities $ 134.0 $ 239.4 $ 142.9
Investments Classified by Contractual Maturity Date [Table Text Block]	The composition of fixed maturities by maturity at March 31, 2018 , was: (millions) Cost Fair Value Less than one year $ 3,757.9 $ 3,767.7 One to five years 14,498.9 14,337.4 Five to ten years 4,327.3 4,272.2 Ten years or greater 43.1 42.8 Total $ 22,627.2 $ 22,420.1
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 March 31, 2018 Fixed maturities: U.S. government obligations 64 $ 7,933.8 $ (127.4 ) 37 $ 5,954.6 $ (90.7 ) 27 $ 1,979.2 $ (36.7 ) State and local government obligations 426 1,263.7 (16.4 ) 301 890.2 (9.5 ) 125 373.5 (6.9 ) Corporate debt securities 348 5,082.9 (64.8 ) 290 4,515.1 (56.1 ) 58 567.8 (8.7 ) Residential mortgage-backed securities 211 329.3 (4.8 ) 36 112.8 (0.5 ) 175 216.5 (4.3 ) Commercial mortgage-backed securities 131 1,974.8 (24.8 ) 89 1,510.5 (16.2 ) 42 464.3 (8.6 ) Other asset-backed securities 208 1,958.5 (12.9 ) 129 1,439.1 (8.7 ) 79 519.4 (4.2 ) Redeemable preferred stocks 3 26.8 (1.4 ) 2 15.3 (0.4 ) 1 11.5 (1.0 ) Total fixed maturities 1,391 $ 18,569.8 $ (252.5 ) 884 $ 14,437.6 $ (182.1 ) 507 $ 4,132.2 $ (70.4 ) Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses March 31, 2017 Fixed maturities: U.S. government obligations 34 $ 2,747.0 $ (20.6 ) 34 $ 2,747.0 $ (20.6 ) 0 $ 0 $ 0 State and local government obligations 340 986.3 (10.7 ) 310 906.5 (9.5 ) 30 79.8 (1.2 ) Corporate debt securities 147 2,494.7 (15.0 ) 139 2,415.1 (14.9 ) 8 79.6 (0.1 ) Residential mortgage-backed securities 247 916.3 (11.7 ) 138 297.1 (1.7 ) 109 619.2 (10.0 ) Commercial mortgage-backed securities 92 1,131.9 (16.3 ) 73 985.7 (14.2 ) 19 146.2 (2.1 ) Other asset-backed securities 118 1,601.6 (3.1 ) 112 1,521.4 (2.8 ) 6 80.2 (0.3 ) Redeemable preferred stocks 1 11.4 (1.1 ) 0 0 0 1 11.4 (1.1 ) Total fixed maturities 979 9,889.2 (78.5 ) 806 8,872.8 (63.7 ) 173 1,016.4 (14.8 ) Equity securities: Nonredeemable preferred stocks 9 197.0 (11.8 ) 5 78.5 (0.7 ) 4 118.5 (11.1 ) Common equities 68 49.2 (4.8 ) 61 48.3 (4.8 ) 7 0.9 0 Total equity securities 77 246.2 (16.6 ) 66 126.8 (5.5 ) 11 119.4 (11.1 ) Total portfolio 1,056 $ 10,135.4 $ (95.1 ) 872 $ 8,999.6 $ (69.2 ) 184 $ 1,135.8 $ (25.9 ) 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): March 31, December 31, (millions) 2018 2017 Fixed maturities: Residential mortgage-backed securities $ (19.7 ) $ (43.3 ) $ (19.7 ) Commercial mortgage-backed securities (0.3 ) (0.5 ) (0.3 ) Total fixed maturities $ (20.0 ) $ (43.8 ) $ (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 March 31, 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 March 31, 2018 Mortgage-Backed (millions) Residential Commercial Total Balance at December 31, 2017 $ 0 $ 0.5 $ 0.5 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 March 31, 2018 $ 0 $ 0.5 $ 0.5 Three Months Ended March 31, 2017 Mortgage-Backed (millions) Residential Commercial Total Balance at December 31, 2016 $ 11.1 $ 0.4 $ 11.5 Reductions for securities sold/matured 0 (0.3 ) (0.3 ) Change in recoveries of future cash flows expected to be collected 1 0.3 0 0.3 Balance at March 31, 2017 $ 11.4 $ 0.1 $ 11.5 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 months ended March 31, were: Three Months (millions) 2018 2017 Gross realized gains on security sales Available-for-sale securities: U.S. government obligations $ 0 $ 0.4 State and local government obligations 8.6 0.5 Corporate and other debt securities 0.1 4.1 Residential mortgage-backed securities 0 0.1 Commercial mortgage-backed securities 1.7 1.2 Other asset-backed securities 0 0.3 Redeemable preferred stocks 1.1 0.3 Total available-for-sale securities 11.5 6.9 Equity securities: Nonredeemable preferred stocks 3.6 45.4 Common equities 119.9 7.4 Total equity securities 123.5 52.8 Subtotal gross realized gains on security sales 135.0 59.7 Gross realized losses on security sales Available-for-sale securities: U.S. government obligations (8.9 ) (3.2 ) State and local government obligations (1.3 ) (0.1 ) Corporate and other debt securities (3.1 ) (0.9 ) Commercial mortgage-backed securities (6.3 ) (2.4 ) Other asset-backed securities (0.1 ) 0 Total available-for-sale securities (19.7 ) (6.6 ) Equity securities: Nonredeemable preferred stocks (0.4 ) (1.2 ) Common equities (7.9 ) (0.1 ) Total equity securities (8.3 ) (1.3 ) Subtotal gross realized losses on security sales (28.0 ) (7.9 ) Net realized gains (losses) on security sales Available-for-sale securities: U.S. government obligations (8.9 ) (2.8 ) State and local government obligations 7.3 0.4 Corporate and other debt securities (3.0 ) 3.2 Residential mortgage-backed securities 0 0.1 Commercial mortgage-backed securities (4.6 ) (1.2 ) Other asset-backed securities (0.1 ) 0.3 Redeemable preferred stocks 1.1 0.3 Total available-for-sale securities (8.2 ) 0.3 Equity securities: Nonredeemable preferred stocks 3.2 44.2 Common equities 112.0 7.3 Total equity securities 115.2 51.5 Subtotal net realized gains (losses) on security sales 107.0 51.8 Net holding period gains (losses) Hybrid securities (3.2 ) 0.8 Equity securities (152.0 ) 0 Subtotal net holding period gains (losses) (155.2 ) 0.8 Other-than-temporary impairment losses Equity securities: Common equities 0 (1.0 ) Subtotal other-than-temporary impairment losses 0 (1.0 ) Litigation settlements and other gains (losses) 0 0.3 Total net realized gains (losses) on securities $ (48.2 ) $ 51.9
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 months ended March 31, 2018 for equity securities held at quarter end: Three Months (millions) 2018 Total net gains (losses) recognized during the period on equity securities $ (36.8 ) Less: Net gains (losses) recognized on equity securities sold during the period 115.2 Net holding period gains (losses) recognized during the period on equity securities held at period end $ (152.0 ) 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 months ended March 31, were: Three Months (millions) 2018 2017 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 39.8 $ 12.7 State and local government obligations 10.0 13.3 Foreign government obligations 0 0.1 Corporate debt securities 36.2 29.6 Residential mortgage-backed securities 6.9 9.6 Commercial mortgage-backed securities 21.2 18.8 Other asset-backed securities 13.6 9.9 Redeemable preferred stocks 2.6 3.4 Total fixed maturities 130.3 97.4 Short-term investments 10.1 7.0 Total available-for-sale securities 140.4 104.4 Equity securities: Nonredeemable preferred stocks 10.9 11.5 Common equities 15.0 13.3 Total equity securities 25.9 24.8 Investment income 166.3 129.2 Investment expenses (6.0 ) (5.6 ) Net investment income $ 160.3 $ 123.6

Fair Value (Tables)

Fair Value (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2018 Fixed maturities: U.S. government obligations $ 8,958.1 $ 0 $ 0 $ 8,958.1 $ 9,081.6 State and local government obligations 0 1,720.1 0 1,720.1 1,732.3 Foreign government obligations 0 0 0 0 0 Corporate debt securities 0 6,041.1 0 6,041.1 6,102.6 Subtotal 8,958.1 7,761.2 0 16,719.3 16,916.5 Asset-backed securities: Residential mortgage-backed 0 751.8 0 751.8 746.7 Commercial mortgage-backed 0 2,269.5 0 2,269.5 2,289.8 Other asset-backed 0 2,462.1 0 2,462.1 2,472.4 Subtotal asset-backed securities 0 5,483.4 0 5,483.4 5,508.9 Redeemable preferred stocks: Financials 0 64.0 0 64.0 61.2 Utilities 0 11.6 0 11.6 10.7 Industrials 0 141.8 0 141.8 129.9 Subtotal redeemable preferred stocks 0 217.4 0 217.4 201.8 Total fixed maturities 8,958.1 13,462.0 0 22,420.1 22,627.2 Short-term investments 2,569.5 482.9 0 3,052.4 3,052.4 Total available-for-sale securities 11,527.6 13,944.9 0 25,472.5 25,679.6 Equity securities: Nonredeemable preferred stocks: Financials 78.9 662.0 0 740.9 654.6 Industrials 0 0 5.0 5.0 5.0 Subtotal nonredeemable preferred stocks 78.9 662.0 5.0 745.9 659.6 Common equities: Common stocks 3,032.9 0 0 3,032.9 1,265.2 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 3,032.9 0 0.3 3,033.2 1,265.5 Total equity securities 3,111.8 662.0 5.3 3,779.1 1,925.1 Total portfolio $ 14,639.4 $ 14,606.9 $ 5.3 $ 29,251.6 $ 27,604.7 Debt $ 0 $ 4,048.3 $ 0 $ 4,048.3 $ 3,859.2 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost March 31, 2017 Fixed maturities: U.S. government obligations $ 3,325.8 $ 0 $ 0 $ 3,325.8 $ 3,345.5 State and local government obligations 0 2,488.4 0 2,488.4 2,475.8 Foreign government obligations 22.9 0 0 22.9 22.9 Corporate debt securities 0 5,208.0 0 5,208.0 5,199.6 Subtotal 3,348.7 7,696.4 0 11,045.1 11,043.8 Asset-backed securities: Residential mortgage-backed 0 1,448.1 0 1,448.1 1,433.0 Commercial mortgage-backed 0 2,213.5 0 2,213.5 2,216.1 Other asset-backed 0 2,564.9 0 2,564.9 2,562.4 Subtotal asset-backed securities 0 6,226.5 0 6,226.5 6,211.5 Redeemable preferred stocks: Financials 0 63.2 0 63.2 60.4 Utilities 0 31.4 0 31.4 30.6 Industrials 0 108.1 0 108.1 98.5 Subtotal redeemable preferred stocks 0 202.7 0 202.7 189.5 Total fixed maturities 3,348.7 14,125.6 0 17,474.3 17,444.8 Equity securities: Nonredeemable preferred stocks: Financials 131.1 678.7 0 809.8 710.3 Industrials 0 0 0 0 0 Subtotal nonredeemable preferred stocks 131.1 678.7 0 809.8 710.3 Common equities: Common stocks 2,977.8 0 0 2,977.8 1,451.2 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 2,977.8 0 0.3 2,978.1 1,451.5 Total fixed maturities and equity securities 6,457.6 14,804.3 0.3 21,262.2 19,606.6 Short-term investments 2,226.7 814.1 0 3,040.8 3,040.8 Total portfolio $ 8,684.3 $ 15,618.4 $ 0.3 $ 24,303.0 $ 22,647.4 Debt $ 0 $ 3,198.3 $ 121.1 $ 3,319.4 $ 3,111.7 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost December 31, 2017 Fixed maturities: U.S. government obligations $ 6,645.9 $ 0 $ 0 $ 6,645.9 $ 6,688.8 State and local government obligations 0 2,297.1 0 2,297.1 2,285.6 Foreign government obligations 0 0 0 0 0 Corporate debt securities 0 4,997.7 0 4,997.7 4,997.2 Subtotal 6,645.9 7,294.8 0 13,940.7 13,971.6 Asset-backed securities: Residential mortgage-backed 0 836.7 0 836.7 828.8 Commercial mortgage-backed 0 2,758.6 0 2,758.6 2,760.1 Other asset-backed 0 2,454.7 0 2,454.7 2,454.5 Subtotal asset-backed securities 0 6,050.0 0 6,050.0 6,043.4 Redeemable preferred stocks: Financials 0 64.1 0 64.1 61.3 Utilities 0 11.4 0 11.4 10.1 Industrials 0 135.5 0 135.5 123.5 Subtotal redeemable preferred stocks 0 211.0 0 211.0 194.9 Total fixed maturities 6,645.9 13,555.8 0 20,201.7 20,209.9 Equity securities: Nonredeemable preferred stocks: Financials 80.6 718.2 0 798.8 693.6 Industrials 0 0 5.0 5.0 5.0 Subtotal nonredeemable preferred stocks 80.6 718.2 5.0 803.8 698.6 Common equities: Common stocks 3,399.5 0 0 3,399.5 1,498.7 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 3,399.5 0 0.3 3,399.8 1,499.0 Total fixed maturities and equity securities 10,126.0 14,274.0 5.3 24,405.3 22,407.5 Short-term investments 1,824.4 1,045.0 0 2,869.4 2,869.4 Total portfolio $ 11,950.4 $ 15,319.0 $ 5.3 $ 27,274.7 $ 25,276.9 Debt $ 0 $ 3,606.5 $ 37.1 $ 3,643.6 $ 3,306.3
Summary of Quantitative Information about Level 3 Fair Value Measurements	The following tables provide a summary of the quantitative information about Level 3 fair value measurements for our applicable securities at March 31, 2018 and 2017 , and December 31, 2017 : Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at March 31, 2018 Valuation Technique Unobservable Input Unobservable Equity securities: Nonredeemable preferred stocks: Industrials 1 $ 5.0 Internal price Purchase price 3.9 Subtotal Level 3 securities 5.0 Pricing exemption securities 2 0.3 Total Level 3 securities $ 5.3 1 The security was internally-priced since it is privately held and it was valued at March 31, 2018 using the purchase price. 2 The fair values for these securities were determined with unobservable inputs not reasonably available to us. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at March 31, 2017 Valuation Technique Unobservable Input Unobservable Equity securities: Nonredeemable preferred stocks: Industrials $ 0 NA NA NA Subtotal Level 3 securities 0 Pricing exemption securities 1 0.3 Total Level 3 securities $ 0.3 NA= Not applicable, since we did not hold any nonredeemable preferred stock Level 3 securities at March 31, 2017 . 1 The fair values for these securities were determined with unobservable inputs not reasonably available to us. Quantitative Information about Level 3 Fair Value Measurements ($ in millions) Fair Value at Dec. 31, 2017 Valuation Technique Unobservable Input Unobservable Equity securities: Nonredeemable preferred stocks: Industrials 1 $ 5.0 Internal price Purchase price 3.9 Subtotal Level 3 securities 5.0 Pricing exemption securities 2 0.3 Total Level 3 securities $ 5.3 1 The security was internally-priced since it is privately held and it was valued at December 31, 2017 using the purchase price. 2 The fair values for these securities were determined with unobservable inputs not reasonably available to us.

Debt (Tables)

Debt (Tables)	3 Months Ended
	Mar. 31, 2018
Debt Table	Debt at each of the balance sheet periods consisted of: March 31, 2018 March 31, 2017 December 31, 2017 (millions) Carrying Value Fair Value Carrying Value Fair Value Carrying Value Fair Value 3.75% Senior Notes due 2021 $ 498.9 $ 512.1 $ 498.5 $ 527.9 $ 498.8 $ 520.7 2.45% Senior Notes due 2027 496.2 458.3 495.8 468.6 496.1 477.9 6 5/8% Senior Notes due 2029 296.2 370.4 296.0 386.3 296.1 382.3 6.25% Senior Notes due 2032 395.4 498.9 395.2 512.1 395.3 516.9 4.35% Senior Notes due 2044 346.5 360.2 346.5 366.7 346.5 388.7 3.70% Senior Notes due 2045 395.2 376.6 395.1 380.0 395.2 402.9 4.125% Senior Notes due 2047 841.3 858.6 0 0 841.2 917.1 4.20% Senior Notes due 2048 589.5 613.2 0 0 0 0 6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067 0 0 563.5 556.7 0 0 Other debt instruments 0 0 121.1 121.1 37.1 37.1 Total $ 3,859.2 $ 4,048.3 $ 3,111.7 $ 3,319.4 $ 3,306.3 $ 3,643.6

Loss and Loss Adjustment Expe25

Loss and Loss Adjustment Expense Reserves (Tables)	3 Months Ended
	Mar. 31, 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 during the first quarter is summarized as follows: March 31, (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 4,815.2 4,152.2 Prior years 55.6 111.2 Total incurred 4,870.8 4,263.4 Paid related to: Current year 2,217.8 1,933.2 Prior years 2,377.8 2,118.7 Total paid 4,595.6 4,051.9 Net balance, End of period 11,192.0 9,778.5 Plus reinsurance recoverables on unpaid losses 2,137.0 1,850.4 Balance, End of period $ 13,329.0 $ 11,628.9

Supplemental Cash Flow Inform26

Supplemental Cash Flow Information (Tables)	3 Months Ended
	Mar. 31, 2018
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information Table	We paid the following in the respective periods: Three Months Ended March 31, (millions) 2018 2017 Income taxes $ 0 $ 0.7 Interest 33.3 33.1

Segment Information (Tables)

Segment Information (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2018 2017 (millions) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Personal Lines Agency $ 3,063.8 $ 409.1 $ 2,631.5 $ 249.7 Direct 3,016.3 298.0 2,523.7 175.3 Total Personal Lines 1 6,080.1 707.1 5,155.2 425.0 Commercial Lines 808.6 94.8 645.5 67.4 Property 2 285.3 28.5 226.0 7.8 Other indemnity 0 0.2 0 (0.2 ) Total underwriting operations 7,174.0 830.6 6,026.7 500.0 Fees and other revenues 3 103.8 NA 85.2 NA Service businesses 34.2 4.9 28.5 2.6 Investments 4 118.1 112.1 181.1 175.5 Other gains (losses) 0 0 0.2 0.2 Interest expense NA (36.8 ) NA (36.8 ) Consolidated total $ 7,430.1 $ 910.8 $ 6,321.7 $ 641.5 NA = Not applicable 1 Personal auto insurance accounted for 94% and 93% of the total Personal Lines segment net premiums earned in the first quarter of 2018 and 2017 , respectively; 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 months ended March 31, 2018 and 2017, pretax profit (loss) includes $18.0 million and $15.5 million , respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX. Although this expense is included in our Property segment, it is not reported in the consolidated results of ARX and, therefore, will not affect the value of the net income attributable to the noncontrolling interest. 3 Pretax profit (loss) for fees and other revenues are attributable to operating segments. 4 Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit is net of investment 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 March 31, 2018 2017 Underwriting Combined Ratio Underwriting Combined Ratio Personal Lines Agency 13.4 % 86.6 9.5 % 90.5 Direct 9.9 90.1 6.9 93.1 Total Personal Lines 11.6 88.4 8.2 91.8 Commercial Lines 11.7 88.3 10.4 89.6 Property 1 10.0 90.0 3.4 96.6 Total underwriting operations 11.6 88.4 8.3 91.7 1 Included in the three months ended March 31, 2018 and 2017, is 6.3 points and 6.9 points, respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX.

Dividends Dividends (Tables)

Dividends Dividends (Tables)	3 Months Ended
	Mar. 31, 2018
Dividends [Abstract]
Dividends	Following is a summary of our shareholder dividends that were declared in the last two years: (millions, except per share amounts) Amount Dividend Type Declared Paid Per Share Total 1 Annual – Variable December 2017 February 2018 $ 1.1247 $ 655.1 Annual – Variable December 2016 February 2017 $ 0.6808 $ 395.4 1 Based on an estimate of shares outstanding as of the record date. For the dividends declared in December 2017 and 2016, we paid $654.9 million and $395.4 million , respectively. Preferred Share Dividends In the first quarter 2018, we issued 500,000 of our 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.

Other Comprehensive Income (L29

Other Comprehensive Income (Loss) (Tables)	3 Months Ended
	Mar. 31, 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 Total tax (provision) benefit After tax total accumulated other comprehensive income Total net unrealized gains (losses) on securities Net unrealized gains on forecasted transactions Foreign currency translation adjustment (Income)loss attributable to NCI Balance at December 31, 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 tax effect 2 0 (4.3 ) (4.3 ) (1.1 ) (3.2 ) 0 0 Adjusted balance at December 31, 2017 (28.2 ) 5.9 (22.3 ) (6.3 ) (18.0 ) 0 2.0 Other comprehensive income (loss) before reclassifications: Investment securities (203.4 ) 42.8 (160.6 ) (160.6 ) 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) 5.1 (1.1 ) 4.0 0 0 0 4.0 Total other comprehensive income (loss) before reclassifications (198.3 ) 41.7 (156.6 ) (160.6 ) 0 0 4.0 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 (7.7 ) 1.6 (6.1 ) (6.1 ) 0 0 0 Interest expense (0.2 ) 0 (0.2 ) 0 (0.2 ) 0 0 Total reclassification adjustment for amounts realized in net income (7.9 ) 1.6 (6.3 ) (6.1 ) (0.2 ) 0 0 Total other comprehensive income (loss) (190.4 ) 40.1 (150.3 ) (154.5 ) 0.2 0 4.0 Balance at March 31, 2018 $ (218.6 ) $ 46.0 $ (172.6 ) $ (160.8 ) $ (17.8 ) $ 0 $ 6.0 Components of Changes in Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income Total tax (provision) benefit After tax total accumulated other comprehensive income Total net Net unrealized gains on forecasted transactions Foreign currency translation adjustment (Income)loss attributable to NCI Balance at December 31, 2016 $ 1,439.5 $ (506.1 ) $ 933.4 $ 939.6 $ (9.4 ) $ (1.1 ) $ 4.3 Other comprehensive income (loss) before reclassifications: Investment securities 254.5 (89.2 ) 165.3 165.3 0 0 0 Forecasted transactions 4.0 (1.4 ) 2.6 0 2.6 0 0 Foreign currency translation adjustment 0.5 (0.2 ) 0.3 0 0 0.3 0 Loss attributable to noncontrolling interest (NCI) (1.5 ) 0.6 (0.9 ) 0 0 0 (0.9 ) Total other comprehensive income (loss) before reclassifications 257.5 (90.2 ) 167.3 165.3 2.6 0.3 (0.9 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (1.0 ) 0.4 (0.6 ) (0.6 ) 0 0 0 Net realized gains (losses) on securities 51.6 (18.1 ) 33.5 33.5 0 0 0 Interest expense 0.5 (0.2 ) 0.3 0 0.3 0 0 Total reclassification adjustment for amounts realized in net income 51.1 (17.9 ) 33.2 32.9 0.3 0 0 Total other comprehensive income (loss) 206.4 (72.3 ) 134.1 132.4 2.3 0.3 (0.9 ) Balance at March 31, 2017 $ 1,645.9 $ (578.4 ) $ 1,067.5 $ 1,072.0 $ (7.1 ) $ (0.8 ) $ 3.4

Redeemable Noncontrolling Int30

Redeemable Noncontrolling Interest (Tables)	3 Months Ended
	Mar. 31, 2018
Temporary Equity Disclosure [Abstract]
Redeemable Noncontrolling Interest	The changes in the components of redeemable NCI during the three months ended March 31, 2018 and 2017 , and the year ended December 31, 2017 , were: (millions) March 31, 2018 March 31, 2017 December 31, 2017 Balance, Beginning of period $ 503.7 $ 483.7 $ 483.7 Net income attributable to NCI 11.8 6.0 5.9 Other comprehensive income (loss) attributable to NCI (4.0 ) 0.9 2.3 Exercise of employee stock options 1.4 3.4 3.4 Change in redemption value of NCI 1.3 0.2 8.4 Balance, End of period $ 514.2 $ 494.2 $ 503.7

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	3 Months Ended
	Mar. 31, 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 March 31, 2018 and 2017 , and December 31, 2017 : (millions) March 31, 2018 March 31, 2017 December 31, 2017 Intangible assets subject to amortization $ 336.2 $ 404.9 $ 354.2 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 348.6 $ 417.3 $ 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 have $0.6 million of accumulated amortization for all periods presented.
Intangible Assets Subject to Amortization	Intangible assets subject to amortization consisted of the following: (millions) March 31, 2018 March 31, 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 $ 109.8 $ 146.4 $ 256.2 $ 73.2 $ 183.0 $ 256.2 $ 100.7 $ 155.5 Agency relationships 159.2 34.2 125.0 159.2 22.7 136.5 159.2 31.3 127.9 Software rights 79.1 32.1 47.0 79.1 21.5 57.6 79.1 29.4 49.7 Trade name 34.8 17.0 17.8 34.8 7.0 27.8 34.8 13.7 21.1 Total $ 529.3 $ 193.1 $ 336.2 $ 529.3 $ 124.4 $ 404.9 $ 529.3 $ 175.1 $ 354.2

Basis of Presentation Basis of

Basis of Presentation Basis of Presentation-Additional Information (Details) $ in Millions	Apr. 01, 2018shares	Mar. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Mar. 31, 2017USD ($)
Business Acquisition [Line Items]
Assets Held-for-sale, Property, Plant and Equipment | $		$ 5.3	$ 5.3	$ 0
ARX Holding Corp. [Member]
Business Acquisition [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage		68.90%	69.00%	69.00%
Subsequent Event [Member] | ARX Holding Corp. [Member]
Business Acquisition [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage	86.70%
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	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Schedule of Debt and Equity Securities [Line Items]
Fair Value	$ 25,472.5	$ 23,071.1	$ 20,515.1
Equity Securities, FV-NI	3,779.1	4,203.6	3,787.9
Debt and Equity Securities, Amortized Cost	27,604.7	25,276.9	22,647.4
Debt and Equity Securities, Accumulated Gross Unrealized Gain, Before Tax	48.4	2,097	1,748.1
Debt and Equity Securities, Accumulated Gross Unrealized Loss, Before Tax	(252.5)	(99.4)	(95.1)
Debt and Equity Securities, Net Holding Period Gain/(Loss), Before Tax	1,851	0.2	2.6
Investments, Fair Value Disclosure	$ 29,251.6	$ 27,274.7	$ 24,303
Debt and Equity Securities, Percent Of Total Fair Value	100.00%	100.00%	100.00%
Other Payables to Broker-Dealers and Clearing Organizations	$ 83.6		$ 111
Other Receivables from Broker-Dealers and Clearing Organizations		$ 5.8
Securities Held In Consolidated Non Insurance Subsidiary	2,100	1,600	700
Equity securities
Schedule of Debt and Equity Securities [Line Items]
Equity Securities, FV-NI, Amortized Cost	1,925.1	2,197.6	2,161.8
Equity Securities, FV-NI, Gross Unrealized Gains	0	2,015	1,642.7
Equity Securities, FV-NI, Gross Unrealized Losses	0	(9)	(16.6)
Equity Securities, Net Holding Period Gain/(Loss), Before Tax	1,854	0	0
Equity Securities, FV-NI	3,779.1	4,203.6	3,787.9
Investments, Fair Value Disclosure	$ 3,779.1	$ 4,203.6	$ 3,787.9
Debt and Equity Securities, Percent Of Total Fair Value	12.90%	15.40%	15.60%
Equity securities | Nonredeemable preferred stocks
Schedule of Debt and Equity Securities [Line Items]
Equity Securities, FV-NI, Amortized Cost	$ 659.6	$ 698.6	$ 710.3
Equity Securities, FV-NI, Gross Unrealized Gains	0	114	111.3
Equity Securities, FV-NI, Gross Unrealized Losses	0	(8.8)	(11.8)
Equity Securities, Net Holding Period Gain/(Loss), Before Tax	86.3	0	0
Equity Securities, FV-NI	745.9	803.8	809.8
Investments, Fair Value Disclosure	$ 745.9	$ 803.8	$ 809.8
Debt and Equity Securities, Percent Of Total Fair Value	2.50%	2.90%	3.30%
Equity securities | Common equities
Schedule of Debt and Equity Securities [Line Items]
Equity Securities, FV-NI, Amortized Cost	$ 1,265.5	$ 1,499	$ 1,451.5
Equity Securities, FV-NI, Gross Unrealized Gains	0	1,901	1,531.4
Equity Securities, FV-NI, Gross Unrealized Losses	0	(0.2)	(4.8)
Equity Securities, Net Holding Period Gain/(Loss), Before Tax	1,767.7	0	0
Equity Securities, FV-NI	3,033.2	3,399.8	2,978.1
Investments, Fair Value Disclosure	$ 3,033.2	$ 3,399.8	$ 2,978.1
Debt and Equity Securities, Percent Of Total Fair Value	10.40%	12.50%	12.30%
Available-for-sale Securities [Member]
Schedule of Debt and Equity Securities [Line Items]
Cost	$ 25,679.6	$ 23,079.3	$ 20,485.6
Gross Unrealized Gains	48.4	82	105.4
Gross Unrealized Losses	(252.5)	(90.4)	(78.5)
Hybrid Fixed Maturities, Net Holding Period Gain/(Loss)	(3)	0.2	2.6
Fair Value	25,472.5	23,071.1	20,515.1
Investments, Fair Value Disclosure	$ 25,472.5	$ 23,071.1	$ 20,515.1
Debt and Equity Securities, Percent Of Total Fair Value	87.10%	84.60%	84.40%
Available-for-sale Securities [Member] | Fixed maturities
Schedule of Debt and Equity Securities [Line Items]
Cost	$ 22,627.2	$ 20,209.9	$ 17,444.8
Gross Unrealized Gains	48.4	82	105.4
Gross Unrealized Losses	(252.5)	(90.4)	(78.5)
Hybrid Fixed Maturities, Net Holding Period Gain/(Loss)	(3)	0.2	2.6
Fair Value	22,420.1	20,201.7	17,474.3
Investments, Fair Value Disclosure	$ 22,420.1	$ 20,201.7	$ 17,474.3
Debt and Equity Securities, Percent Of Total Fair Value	76.70%	74.10%	71.90%
Available-for-sale Securities [Member] | Fixed maturities | U.S. government obligations
Schedule of Debt and Equity Securities [Line Items]
Cost	$ 9,081.6	$ 6,688.8	$ 3,345.5
Gross Unrealized Gains	3.9	1.1	0.9
Gross Unrealized Losses	(127.4)	(44)	(20.6)
Hybrid Fixed Maturities, Net Holding Period Gain/(Loss)	0	0	0
Fair Value	$ 8,958.1	$ 6,645.9	$ 3,325.8
Debt and Equity Securities, Percent Of Total Fair Value	30.60%	24.40%	13.70%
Available-for-sale Securities [Member] | Fixed maturities | State and local government obligations
Schedule of Debt and Equity Securities [Line Items]
Cost	$ 1,732.3	$ 2,285.6	$ 2,475.8
Gross Unrealized Gains	4.3	20.7	23.3
Gross Unrealized Losses	(16.4)	(9.3)	(10.7)
Hybrid Fixed Maturities, Net Holding Period Gain/(Loss)	(0.1)	0.1	0
Fair Value	$ 1,720.1	$ 2,297.1	$ 2,488.4
Debt and Equity Securities, Percent Of Total Fair Value	5.90%	8.40%	10.20%
Available-for-sale Securities [Member] | Fixed maturities | Foreign government obligations
Schedule of Debt and Equity Securities [Line Items]
Cost	$ 0	$ 0	$ 22.9
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Hybrid Fixed Maturities, Net Holding Period Gain/(Loss)	0	0	0
Fair Value	$ 0	$ 0	$ 22.9
Debt and Equity Securities, Percent Of Total Fair Value	0.00%	0.00%	0.10%
Available-for-sale Securities [Member] | Fixed maturities | Corporate debt securities
Schedule of Debt and Equity Securities [Line Items]
Cost	$ 6,102.6	$ 4,997.2	$ 5,199.6
Gross Unrealized Gains	4.8	14.8	23.1
Gross Unrealized Losses	(64.8)	(14.4)	(15)
Hybrid Fixed Maturities, Net Holding Period Gain/(Loss)	(1.5)	0.1	0.3
Fair Value	$ 6,041.1	$ 4,997.7	$ 5,208
Debt and Equity Securities, Percent Of Total Fair Value	20.70%	18.30%	21.40%
Available-for-sale Securities [Member] | Fixed maturities | Residential mortgage-backed securities
Schedule of Debt and Equity Securities [Line Items]
Cost	$ 746.7	$ 828.8	$ 1,433
Gross Unrealized Gains	9.9	11.3	24.7
Gross Unrealized Losses	(4.8)	(3.4)	(11.7)
Hybrid Fixed Maturities, Net Holding Period Gain/(Loss)	0	0	2.1
Fair Value	$ 751.8	$ 836.7	$ 1,448.1
Debt and Equity Securities, Percent Of Total Fair Value	2.60%	3.10%	6.00%
Available-for-sale Securities [Member] | Fixed maturities | Commercial mortgage-backed securities
Schedule of Debt and Equity Securities [Line Items]
Cost	$ 2,289.8	$ 2,760.1	$ 2,216.1
Gross Unrealized Gains	4.5	11.8	13.7
Gross Unrealized Losses	(24.8)	(13.3)	(16.3)
Hybrid Fixed Maturities, Net Holding Period Gain/(Loss)	0	0	0
Fair Value	$ 2,269.5	$ 2,758.6	$ 2,213.5
Debt and Equity Securities, Percent Of Total Fair Value	7.80%	10.10%	9.10%
Available-for-sale Securities [Member] | Fixed maturities | Other asset-backed securities
Schedule of Debt and Equity Securities [Line Items]
Cost	$ 2,472.4	$ 2,454.5	$ 2,562.4
Gross Unrealized Gains	2.4	4.5	5.4
Gross Unrealized Losses	(12.9)	(4.5)	(3.1)
Hybrid Fixed Maturities, Net Holding Period Gain/(Loss)	0.2	0.2	0.2
Fair Value	$ 2,462.1	$ 2,454.7	$ 2,564.9
Debt and Equity Securities, Percent Of Total Fair Value	8.40%	9.00%	10.60%
Available-for-sale Securities [Member] | Fixed maturities | Redeemable preferred stocks
Schedule of Debt and Equity Securities [Line Items]
Cost	$ 201.8	$ 194.9	$ 189.5
Gross Unrealized Gains	18.6	17.8	14.3
Gross Unrealized Losses	(1.4)	(1.5)	(1.1)
Hybrid Fixed Maturities, Net Holding Period Gain/(Loss)	(1.6)	(0.2)	0
Fair Value	217.4	211	202.7
Investments, Fair Value Disclosure	$ 217.4	$ 211	$ 202.7
Debt and Equity Securities, Percent Of Total Fair Value	0.70%	0.80%	0.80%
Available-for-sale Securities [Member] | Short-term investments
Schedule of Debt and Equity Securities [Line Items]
Cost	$ 3,052.4	$ 2,869.4	$ 3,040.8
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Hybrid Fixed Maturities, Net Holding Period Gain/(Loss)	0	0	0
Fair Value	3,052.4	2,869.4	3,040.8
Investments, Fair Value Disclosure	$ 3,052.4	$ 2,869.4	$ 3,040.8
Debt and Equity Securities, Percent Of Total Fair Value	10.40%	10.50%	12.50%

Investments Hybrid Securities (

Investments Hybrid Securities (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Hybrid Securities [Line Items]
Hybrid securities	$ 134	$ 142.9	$ 239.4
Fixed maturities | State and local government obligations
Hybrid Securities [Line Items]
Hybrid securities	3.7	6.1	0
Fixed maturities | Corporate debt securities
Hybrid Securities [Line Items]
Hybrid securities	86.4	99.8	44.5
Fixed maturities | Residential mortgage-backed securities
Hybrid Securities [Line Items]
Hybrid securities	0	0	186.6
Fixed maturities | Other asset-backed securities
Hybrid Securities [Line Items]
Hybrid securities	6.1	6.7	8.3
Fixed maturities | Redeemable preferred stocks
Hybrid Securities [Line Items]
Hybrid securities	$ 37.8	$ 30.3	$ 0

Investments Composition of Fixe

Investments Composition of Fixed Maturities by Maturity (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Schedule of Debt Securities by Contractual Maturity [Line Items]
Less than one year	$ 3,757.9
One to five years	14,498.9
Five to ten years	4,327.3
Ten years or greater	43.1
Fixed maturities, amortized cost	22,627.2	$ 20,209.9	$ 17,444.8
Less than one year	3,767.7
One to five years	14,337.4
Five to ten years	4,272.2
Ten years or greater	42.8
Fixed maturities, fair value	22,420.1	$ 20,201.7	$ 17,474.3
Fixed maturities
Schedule of Debt Securities by Contractual Maturity [Line Items]
Fixed maturities, amortized cost	22,627.2
Fixed maturities, fair value	$ 22,420.1

Investments Gross Unrealized lo

Investments Gross Unrealized losses by Major Security (Detail) $ in Millions	Mar. 31, 2018USD ($)security	Dec. 31, 2017USD ($)security	Mar. 31, 2017USD ($)security
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	1,391	1,166	1,056
Total Fair Value	$ 18,569.8	$ 13,980.2	$ 10,135.4
Gross Unrealized Losses	$ (252.5)	$ (99.4)	$ (95.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	884	689	872
Less than 12 Months Fair Value	$ 14,437.6	$ 10,434.6	$ 8,999.6
Less than 12 Months Unrealized Losses	$ (182.1)	$ (58.3)	$ (69.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	507	477	184
12 Months or Greater Fair Value	$ 4,132.2	$ 3,545.6	$ 1,135.8
12 Months or Greater Unrealized Losses	$ (70.4)	$ (41.1)	$ (25.9)
Fixed maturities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	1,391	1,143	979
Total Fair Value	$ 18,569.8	$ 13,839	$ 9,889.2
Gross Unrealized Losses	$ (252.5)	$ (90.4)	$ (78.5)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	884	670	806
Less than 12 Months Fair Value	$ 14,437.6	$ 10,364.7	$ 8,872.8
Less than 12 Months Unrealized Losses	$ (182.1)	$ (57.6)	$ (63.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	507	473	173
12 Months or Greater Fair Value	$ 4,132.2	$ 3,474.3	$ 1,016.4
12 Months or Greater Unrealized Losses	$ (70.4)	$ (32.8)	$ (14.8)
Fixed maturities | U.S. government obligations
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	64	58	34
Total Fair Value	$ 7,933.8	$ 5,817	$ 2,747
Gross Unrealized Losses	$ (127.4)	$ (44)	$ (20.6)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	37	41	34
Less than 12 Months Fair Value	$ 5,954.6	$ 4,869.3	$ 2,747
Less than 12 Months Unrealized Losses	$ (90.7)	$ (34.6)	$ (20.6)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	27	17	0
12 Months or Greater Fair Value	$ 1,979.2	$ 947.7	$ 0
12 Months or Greater Unrealized Losses	$ (36.7)	$ (9.4)	$ 0
Fixed maturities | State and local government obligations
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	426	358	340
Total Fair Value	$ 1,263.7	$ 1,200.3	$ 986.3
Gross Unrealized Losses	$ (16.4)	$ (9.3)	$ (10.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	301	230	310
Less than 12 Months Fair Value	$ 890.2	$ 737.6	$ 906.5
Less than 12 Months Unrealized Losses	$ (9.5)	$ (4.4)	$ (9.5)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	125	128	30
12 Months or Greater Fair Value	$ 373.5	$ 462.7	$ 79.8
12 Months or Greater Unrealized Losses	$ (6.9)	$ (4.9)	$ (1.2)
Fixed maturities | Corporate debt securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	348	222	147
Total Fair Value	$ 5,082.9	$ 2,979.4	$ 2,494.7
Gross Unrealized Losses	$ (64.8)	$ (14.4)	$ (15)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	290	171	139
Less than 12 Months Fair Value	$ 4,515.1	$ 2,072.9	$ 2,415.1
Less than 12 Months Unrealized Losses	$ (56.1)	$ (9.1)	$ (14.9)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	58	51	8
12 Months or Greater Fair Value	$ 567.8	$ 906.5	$ 79.6
12 Months or Greater Unrealized Losses	$ (8.7)	$ (5.3)	$ (0.1)
Fixed maturities | Residential mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	211	201	247
Total Fair Value	$ 329.3	$ 300.9	$ 916.3
Gross Unrealized Losses	$ (4.8)	$ (3.4)	$ (11.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	36	30	138
Less than 12 Months Fair Value	$ 112.8	$ 75.1	$ 297.1
Less than 12 Months Unrealized Losses	$ (0.5)	$ (0.2)	$ (1.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	175	171	109
12 Months or Greater Fair Value	$ 216.5	$ 225.8	$ 619.2
12 Months or Greater Unrealized Losses	$ (4.3)	$ (3.2)	$ (10)
Fixed maturities | Commercial mortgage-backed securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	131	105	92
Total Fair Value	$ 1,974.8	$ 1,682.3	$ 1,131.9
Gross Unrealized Losses	$ (24.8)	$ (13.3)	$ (16.3)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	89	63	73
Less than 12 Months Fair Value	$ 1,510.5	$ 1,221.2	$ 985.7
Less than 12 Months Unrealized Losses	$ (16.2)	$ (5.9)	$ (14.2)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	42	42	19
12 Months or Greater Fair Value	$ 464.3	$ 461.1	$ 146.2
12 Months or Greater Unrealized Losses	$ (8.6)	$ (7.4)	$ (2.1)
Fixed maturities | Other asset-backed securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	208	197	118
Total Fair Value	$ 1,958.5	$ 1,837.3	$ 1,601.6
Gross Unrealized Losses	$ (12.9)	$ (4.5)	$ (3.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	129	134	112
Less than 12 Months Fair Value	$ 1,439.1	$ 1,377.8	$ 1,521.4
Less than 12 Months Unrealized Losses	$ (8.7)	$ (3.3)	$ (2.8)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	79	63	6
12 Months or Greater Fair Value	$ 519.4	$ 459.5	$ 80.2
12 Months or Greater Unrealized Losses	$ (4.2)	$ (1.2)	$ (0.3)
Fixed maturities | Redeemable preferred stocks
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	3	2	1
Total Fair Value	$ 26.8	$ 21.8	$ 11.4
Gross Unrealized Losses	$ (1.4)	$ (1.5)	$ (1.1)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	2	1	0
Less than 12 Months Fair Value	$ 15.3	$ 10.8	$ 0
Less than 12 Months Unrealized Losses	$ (0.4)	$ (0.1)	$ 0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	1	1	1
12 Months or Greater Fair Value	$ 11.5	$ 11	$ 11.4
12 Months or Greater Unrealized Losses	$ (1)	$ (1.4)	$ (1.1)
Equity securities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	0	23	77
Total Fair Value	$ 0	$ 141.2	$ 246.2
Gross Unrealized Losses	$ 0	$ (9)	$ (16.6)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	0	19	66
Less than 12 Months Fair Value	$ 0	$ 69.9	$ 126.8
Less than 12 Months Unrealized Losses	$ 0	$ (0.7)	$ (5.5)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	0	4	11
12 Months or Greater Fair Value	$ 0	$ 71.3	$ 119.4
12 Months or Greater Unrealized Losses	$ 0	$ (8.3)	$ (11.1)
Equity securities | Nonredeemable preferred stocks
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	0	4	9
Total Fair Value	$ 0	$ 127.8	$ 197
Gross Unrealized Losses	$ 0	$ (8.8)	$ (11.8)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	0	1	5
Less than 12 Months Fair Value	$ 0	$ 56.5	$ 78.5
Less than 12 Months Unrealized Losses	$ 0	$ (0.5)	$ (0.7)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	0	3	4
12 Months or Greater Fair Value	$ 0	$ 71.3	$ 118.5
12 Months or Greater Unrealized Losses	$ 0	$ (8.3)	$ (11.1)
Equity securities | Common equities
Investments, Unrealized Loss Position [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	0	19	68
Total Fair Value	$ 0	$ 13.4	$ 49.2
Gross Unrealized Losses	$ 0	$ (0.2)	$ (4.8)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	0	18	61
Less than 12 Months Fair Value	$ 0	$ 13.4	$ 48.3
Less than 12 Months Unrealized Losses	$ 0	$ (0.2)	$ (4.8)
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	0	1	7
12 Months or Greater Fair Value	$ 0	$ 0	$ 0.9
12 Months or Greater Unrealized Losses	$ 0	$ 0	$ 0

Investments Total Non-Credit Po

Investments Total Non-Credit Portion of Other-Than-Temporary Impairment Recorded in Accumulated Other Comprehensive Income, Reflecting Original Non-Credit Loss at Time Credit Impairment (Detail) - Fixed maturities - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 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	$ (20)	$ (20)	$ (43.8)
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)	(43.3)
Commercial mortgage-backed securities
Other Than Temporary Impairment Non Credit Losses Recognized In Accumulated Other Comprehensive Income [Line Items]
Non-credit portion of other-than-temporary impairments recognized in accumulated other comprehensive income	$ (0.3)	$ (0.3)	$ (0.5)

Investments OTTI Credit Losses

Investments OTTI Credit Losses Recognized in Earnings (Detail) - Fixed maturities - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	$ 0.5	$ 11.5
Reductions for securities sold/matured	0	(0.3)
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0	0.3
Ending balance	0.5	11.5
Residential mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	0	11.1
Reductions for securities sold/matured	0	0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0	0.3
Ending balance	0	11.4
Commercial mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	0.5	0.4
Reductions for securities sold/matured	0	(0.3)
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0	0
Ending balance	$ 0.5	$ 0.1

Investments Components of Net R

Investments Components of Net Realized Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	$ 11.5	$ 6.9
Debt and Equity Securities, Gross Realized Gains	135	59.7
Equity Securities, Gross Realized Gain	123.5	52.8
Equity Securities, Gross Realized Loss	(8.3)	(1.3)
Debt and Equity Securities, Gross Realized Losses	(28)	(7.9)
Net realized gains (losses) on securities sales	(8.2)	0.3
Equity Securities, Gross Realized Gain (Loss), excluding Holding Period	115.2	51.5
Gross Realized Gain (Loss), excluding Holding Period, OTTI, and Litigation	107	51.8
Net Holding Period Realized Gain (Loss)	(155.2)	0.8
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales	0	(1)
Realized Gain on Litigation Settlements	0	0.3
Total net realized gains (losses) on securities	(48.2)	51.9
Fixed maturities
Gain (Loss) on Investments [Line Items]
Gross realized losses on securities sales	(19.7)	(6.6)
Fixed maturities | U.S. government obligations
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0	0.4
Gross realized losses on securities sales	(8.9)	(3.2)
Net realized gains (losses) on securities sales	(8.9)	(2.8)
Fixed maturities | State and local government obligations
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	8.6	0.5
Gross realized losses on securities sales	(1.3)	(0.1)
Net realized gains (losses) on securities sales	7.3	0.4
Fixed maturities | Corporate debt securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0.1	4.1
Gross realized losses on securities sales	(3.1)	(0.9)
Net realized gains (losses) on securities sales	(3)	3.2
Fixed maturities | Residential mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0	0.1
Net realized gains (losses) on securities sales	0	0.1
Fixed maturities | Commercial mortgage-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	1.7	1.2
Gross realized losses on securities sales	(6.3)	(2.4)
Net realized gains (losses) on securities sales	(4.6)	(1.2)
Fixed maturities | Other asset-backed securities
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	0	0.3
Gross realized losses on securities sales	(0.1)	0
Net realized gains (losses) on securities sales	(0.1)	0.3
Fixed maturities | Redeemable preferred stocks
Gain (Loss) on Investments [Line Items]
Gross realized gains on securities sales	1.1	0.3
Net realized gains (losses) on securities sales	1.1	0.3
Equity securities
Gain (Loss) on Investments [Line Items]
Net Holding Period Realized Gain (Loss)	(152)	0
Total net realized gains (losses) on securities	(36.8)
Equity securities | Nonredeemable preferred stocks
Gain (Loss) on Investments [Line Items]
Equity Securities, Gross Realized Gain	3.6	45.4
Equity Securities, Gross Realized Loss	(0.4)	(1.2)
Equity Securities, Gross Realized Gain (Loss), excluding Holding Period	3.2	44.2
Equity securities | Common equities
Gain (Loss) on Investments [Line Items]
Equity Securities, Gross Realized Gain	119.9	7.4
Equity Securities, Gross Realized Loss	(7.9)	(0.1)
Equity Securities, Gross Realized Gain (Loss), excluding Holding Period	112	7.3
Other than Temporary Impairment Losses, Portion Recognized in Earnings, Net of Sales, Available for Sale Securities	0	(1)
Hybrid Debt Securities [Member]
Gain (Loss) on Investments [Line Items]
Net Holding Period Realized Gain (Loss)	$ (3.2)	$ 0.8

Investments Components of Equit

Investments Components of Equity Realized Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Components of Equity Realized Gains (Losses) [Line Items]
Realized Investment Gains (Losses)	$ (48.2)	$ 51.9
Equity Securities, Gross Realized Gain (Loss), excluding Holding Period	115.2	51.5
Net Holding Period Realized Gain (Loss)	(155.2)	0.8
Equity securities
Components of Equity Realized Gains (Losses) [Line Items]
Realized Investment Gains (Losses)	(36.8)
Net Holding Period Realized Gain (Loss)	$ (152)	$ 0

Investments Components of Net I

Investments Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net Investment Income [Line Items]
Investment income	$ 166.3	$ 129.2
Gross Investment Income, Operating	166.3	129.2
Investment expenses	(6)	(5.6)
Net Investment Income	160.3	123.6
Available-for-sale Securities [Member]
Net Investment Income [Line Items]
Gross Investment Income, Operating	140.4	104.4
Available-for-sale Securities [Member] | Fixed maturities
Net Investment Income [Line Items]
Gross Investment Income, Operating	130.3	97.4
Available-for-sale Securities [Member] | Fixed maturities | U.S. government obligations
Net Investment Income [Line Items]
Gross Investment Income, Operating	39.8	12.7
Available-for-sale Securities [Member] | Fixed maturities | State and local government obligations
Net Investment Income [Line Items]
Gross Investment Income, Operating	10	13.3
Available-for-sale Securities [Member] | Fixed maturities | Foreign government obligations
Net Investment Income [Line Items]
Gross Investment Income, Operating	0	0.1
Available-for-sale Securities [Member] | Fixed maturities | Corporate debt securities
Net Investment Income [Line Items]
Gross Investment Income, Operating	36.2	29.6
Available-for-sale Securities [Member] | Fixed maturities | Residential mortgage-backed securities
Net Investment Income [Line Items]
Gross Investment Income, Operating	6.9	9.6
Available-for-sale Securities [Member] | Fixed maturities | Commercial mortgage-backed securities
Net Investment Income [Line Items]
Gross Investment Income, Operating	21.2	18.8
Available-for-sale Securities [Member] | Fixed maturities | Other asset-backed securities
Net Investment Income [Line Items]
Gross Investment Income, Operating	13.6	9.9
Available-for-sale Securities [Member] | Fixed maturities | Redeemable preferred stocks
Net Investment Income [Line Items]
Gross Investment Income, Operating	2.6	3.4
Available-for-sale Securities [Member] | Short-term investments
Net Investment Income [Line Items]
Gross Investment Income, Operating	10.1	7
Equity securities
Net Investment Income [Line Items]
Gross Investment Income, Operating	25.9	24.8
Equity securities | Nonredeemable preferred stocks
Net Investment Income [Line Items]
Gross Investment Income, Operating	10.9	11.5
Equity securities | Common equities
Net Investment Income [Line Items]
Gross Investment Income, Operating	$ 15	$ 13.3

Investments - Additional Inform

Investments - Additional Information (Detail) - USD ($)	Apr. 03, 2017	Apr. 30, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Schedule of Investments [Line Items]
Securities Sold under Agreements to Repurchase			$ 0	$ 0	$ 0
Open reverse repurchase commitments			0	0	0
Gross unrealized losses			$ 252,500,000	$ 95,100,000	99,400,000
Investment Portfolio Duration			2 years 7 months 10 days	2 years 3 months 20 days
Trading securities			$ 0	$ 0	0
Gain (Loss) on Sale of Securities, Net			107,000,000	52,100,000
Derivative Assets (Liabilities), at Fair Value, Net			$ 0	4,000,000	0
Derivative, Collateral, Right to Reclaim Cash				136,100,000
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%
Unrealized gain (loss) on interest rate cash flow hedges	$ (8,000,000)	$ (8,000,000)
Equity securities
Schedule of Investments [Line Items]
Gross unrealized losses			$ 0	16,600,000	9,000,000
Equity securities | Common equities
Schedule of Investments [Line Items]
Gross unrealized losses			0	4,800,000	200,000
Equity securities | Nonredeemable preferred stocks
Schedule of Investments [Line Items]
Gross unrealized losses			0	11,800,000	8,800,000
Fixed maturities
Schedule of Investments [Line Items]
Gross unrealized losses			252,500,000	78,500,000	90,400,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment			0	0
Fixed maturities | Corporate debt securities
Schedule of Investments [Line Items]
Gross unrealized losses			64,800,000	15,000,000	$ 14,400,000
Categories of Investments, Marketable Securities, Trading Securities
Schedule of Investments [Line Items]
Gain (Loss) on Sale of Securities, Net			$ 0	$ 0

Fair Value Composition of Inves

Fair Value Composition of Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	$ 29,251.6	$ 27,274.7	$ 24,303
Fair Value	4,048.3	3,643.6	3,319.4
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	14,639.4	11,950.4	8,684.3
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 Disclosure	14,606.9	15,319	15,618.4
Fair Value	4,048.3	3,606.5	3,198.3
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	5.3	5.3	0.3
Fair Value	0	37.1	121.1
Total Fixed Maturities And Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	26,199.2	24,405.3	21,262.2
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 Disclosure	12,069.9	10,126	6,457.6
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 Disclosure	14,124	14,274	14,804.3
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 Disclosure	5.3	5.3	0.3
Available-for-sale Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	25,472.5	23,071.1	20,515.1
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	11,527.6	8,470.3	5,575.4
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	13,944.9	14,600.8	14,939.7
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	22,420.1	20,201.7	17,474.3
Available-for-sale Securities [Member] | Fixed maturities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	8,958.1	6,645.9	3,348.7
Available-for-sale Securities [Member] | Fixed maturities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	13,462	13,555.8	14,125.6
Available-for-sale Securities [Member] | Fixed maturities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	16,719.3	13,940.7	11,045.1
Available-for-sale Securities [Member] | Fixed maturities | Debt Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	8,958.1	6,645.9	3,348.7
Available-for-sale Securities [Member] | Fixed maturities | Debt Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	7,761.2	7,294.8	7,696.4
Available-for-sale Securities [Member] | Fixed maturities | Debt Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	5,483.4	6,050	6,226.5
Available-for-sale Securities [Member] | Fixed maturities | Asset-backed Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | Asset-backed Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	5,483.4	6,050	6,226.5
Available-for-sale Securities [Member] | Fixed maturities | Asset-backed Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	8,958.1	6,645.9	3,325.8
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	8,958.1	6,645.9	3,325.8
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	1,720.1	2,297.1	2,488.4
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	1,720.1	2,297.1	2,488.4
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0	22.9
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	22.9
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	6,041.1	4,997.7	5,208
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	6,041.1	4,997.7	5,208
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	751.8	836.7	1,448.1
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	751.8	836.7	1,448.1
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	2,269.5	2,758.6	2,213.5
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	2,269.5	2,758.6	2,213.5
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	2,462.1	2,454.7	2,564.9
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	2,462.1	2,454.7	2,564.9
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	217.4	211	202.7
Available-for-sale Securities [Member] | Fixed maturities | Redeemable preferred stocks | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | Redeemable preferred stocks | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	217.4	211	202.7
Available-for-sale Securities [Member] | Fixed maturities | Redeemable preferred stocks | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	64	64.1	63.2
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	64	64.1	63.2
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	11.6	11.4	31.4
Available-for-sale Securities [Member] | Fixed maturities | Redeemable preferred stocks | Utilities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | Redeemable preferred stocks | Utilities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	11.6	11.4	31.4
Available-for-sale Securities [Member] | Fixed maturities | Redeemable preferred stocks | Utilities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	141.8	135.5	108.1
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	141.8	135.5	108.1
Available-for-sale Securities [Member] | Fixed maturities | 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 Disclosure	0	0	0
Available-for-sale Securities [Member] | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	3,052.4	2,869.4	3,040.8
Available-for-sale Securities [Member] | Short-term investments | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	2,569.5	1,824.4	2,226.7
Available-for-sale Securities [Member] | Short-term investments | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	482.9	1,045	814.1
Available-for-sale Securities [Member] | Short-term investments | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0	0
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	3,779.1	4,203.6	3,787.9
Equity securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	3,111.8	3,480.1	3,108.9
Equity securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	662	718.2	678.7
Equity securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	5.3	5.3	0.3
Equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	745.9	803.8	809.8
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 Disclosure	78.9	80.6	131.1
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 Disclosure	662	718.2	678.7
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 Disclosure	5	5	0
Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	740.9	798.8	809.8
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 Disclosure	78.9	80.6	131.1
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 Disclosure	662	718.2	678.7
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 Disclosure	0	0	0
Equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	5	5	0
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 Disclosure	0	0	0
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 Disclosure	0	0	0
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 Disclosure	5	5	0
Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	3,033.2	3,399.8	2,978.1
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 Disclosure	3,032.9	3,399.5	2,977.8
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 Disclosure	0	0	0
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 Disclosure	0.3	0.3	0.3
Equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	3,032.9	3,399.5	2,977.8
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 Disclosure	3,032.9	3,399.5	2,977.8
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 Disclosure	0	0	0
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 Disclosure	0	0	0
Equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0.3	0.3	0.3
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 Disclosure	0	0	0
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 Disclosure	0	0	0
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 Disclosure	0.3	0.3	0.3
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	27,604.7	25,276.9	22,647.4
Long-term Debt	3,859.2	3,306.3	3,111.7
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 Disclosure	24,552.3	22,407.5	19,606.6
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	25,679.6	23,079.3	20,485.6
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities [Member] | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	22,627.2	20,209.9	17,444.8
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities [Member] | Fixed maturities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	16,916.5	13,971.6	11,043.8
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities [Member] | Fixed maturities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	5,508.9	6,043.4	6,211.5
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities [Member] | Fixed maturities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	9,081.6	6,688.8	3,345.5
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities [Member] | Fixed maturities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	1,732.3	2,285.6	2,475.8
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities [Member] | Fixed maturities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	0	0	22.9
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities [Member] | Fixed maturities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	6,102.6	4,997.2	5,199.6
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities [Member] | Fixed maturities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	746.7	828.8	1,433
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities [Member] | Fixed maturities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	2,289.8	2,760.1	2,216.1
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities [Member] | Fixed maturities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	2,472.4	2,454.5	2,562.4
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities [Member] | Fixed maturities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	201.8	194.9	189.5
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities [Member] | Fixed maturities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	61.2	61.3	60.4
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities [Member] | Fixed maturities | Redeemable preferred stocks | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	10.7	10.1	30.6
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities [Member] | Fixed maturities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	129.9	123.5	98.5
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities [Member] | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	3,052.4	2,869.4	3,040.8
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	1,925.1	2,197.6	2,161.8
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	659.6	698.6	710.3
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	654.6	693.6	710.3
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	5	5	0
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	1,265.5	1,499	1,451.5
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	1,265.2	1,498.7	1,451.2
Carrying (Reported) Amount, Fair Value Disclosure | Equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments, Fair Value Disclosure	$ 0.3	$ 0.3	$ 0.3

Fair Value Summary of Quantitat

Fair Value Summary of Quantitative Information about Level 3 Fair Value Measurements (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	$ 29,251.6	$ 27,274.7	$ 24,303
Fair Value, Inputs, Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	5.3	5.3	0.3
Fair Value, Inputs, Level 3 | Third Party Pricing Exemption Securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	0.3	0.3	0.3
Fair Value, Inputs, Level 3 | Subtotal Level 3 Securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	5	5	0
Equity securities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	3,779.1	4,203.6	3,787.9
Equity securities | Nonredeemable preferred stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	745.9	803.8	809.8
Equity securities | Fair Value, Inputs, Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	5.3	5.3	0.3
Equity securities | Fair Value, Inputs, Level 3 | Nonredeemable preferred stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Investments, Fair Value Disclosure	$ 5	$ 5	$ 0
Valuation Technique	Internal price	Internal price
Fair Value Inputs, Original Issue Price	$ 3.9	$ 3.9

Fair Value Fair Value - Additio

Fair Value Fair Value - Additional Information (Detail) - USD ($)	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Fair Value Measurements [Line Items]
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	$ 0	$ 0	$ 0
Investments, Fair Value Disclosure	$ 29,251,600,000	$ 27,274,700,000	$ 24,303,000,000
Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	75.00%	66.00%	54.00%
Investments, Fair Value Disclosure	$ 14,639,400,000	$ 11,950,400,000	$ 8,684,300,000
Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	99.00%	98.00%	99.00%
Dealer Quoted Prices, Percent of FV Hierarchy Level	1.00%	2.00%	1.00%
Investments, Fair Value Disclosure	$ 14,606,900,000	$ 15,319,000,000	$ 15,618,400,000
Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	5,300,000	5,300,000	300,000
Available-for-sale Securities [Member]
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	25,472,500,000	23,071,100,000	20,515,100,000
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	11,527,600,000	8,470,300,000	5,575,400,000
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	13,944,900,000	14,600,800,000	14,939,700,000
Available-for-sale Securities [Member] | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	0	0	0
Available-for-sale Securities [Member] | Fixed maturities
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	22,420,100,000	20,201,700,000	17,474,300,000
Available-for-sale Securities [Member] | Fixed maturities | Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	8,958,100,000	6,645,900,000	3,348,700,000
Available-for-sale Securities [Member] | Fixed maturities | Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	13,462,000,000	13,555,800,000	14,125,600,000
Available-for-sale Securities [Member] | Fixed maturities | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	0	0	0
Equity securities
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	3,779,100,000	4,203,600,000	3,787,900,000
Equity securities | Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	3,111,800,000	3,480,100,000	3,108,900,000
Equity securities | Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	662,000,000	718,200,000	678,700,000
Equity securities | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	5,300,000	5,300,000	300,000
Equity securities | Nonredeemable preferred stocks
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	745,900,000	803,800,000	809,800,000
Equity securities | Nonredeemable preferred stocks | Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	78,900,000	80,600,000	131,100,000
Equity securities | Nonredeemable preferred stocks | Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	662,000,000	718,200,000	678,700,000
Equity securities | Nonredeemable preferred stocks | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	5,000,000	5,000,000	$ 0
Equity securities | Nonredeemable preferred stocks | Internal Pricing | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Investments, Fair Value Disclosure	$ 5,000,000	$ 5,000,000
Number of securities	1	1	0

Debt (Detail)

Debt (Detail) - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Debt Instrument [Line Items]
Fair Value		$ 4,048.3	$ 3,643.6	$ 3,319.4
Debt, Long-term and Short-term, Combined Amount	[1]	3,859.2	3,306.3	3,111.7
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Long-term Debt		498.9	498.8	498.5
Fair Value		512.1	520.7	527.9
2.45% Senior Notes due 2027
Debt Instrument [Line Items]
Long-term Debt		496.2	496.1	495.8
Fair Value		458.3	477.9	468.6
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Long-term Debt		296.2	296.1	296
Fair Value		370.4	382.3	386.3
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Long-term Debt		395.4	395.3	395.2
Fair Value		498.9	516.9	512.1
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Long-term Debt		346.5	346.5	346.5
Fair Value		360.2	388.7	366.7
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Long-term Debt		395.2	395.2	395.1
Fair Value		376.6	402.9	380
4.125% Senior Notes due 2047
Debt Instrument [Line Items]
Long-term Debt		841.3	841.2	0
Fair Value		858.6	917.1	0
4.20% Senior Notes due 2048
Debt Instrument [Line Items]
Long-term Debt		589.5	0	0
Fair Value		613.2	0	0
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Long-term Debt		0	0	563.5
Fair Value		0	0	556.7
Other debt instruments
Debt Instrument [Line Items]
Debt, Long-term and Short-term, Combined Amount		$ 0	$ 37.1	$ 121.1
[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	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Repayments of Debt				$ 37,100,000			$ 6,200,000
Repayment of debt				$ 0			$ 30,700,000
Discretionary Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 250,000,000
Debt term				30 days
Borrowings outstanding				$ 0				$ 0
Terminated Discretionary Line of Credit [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 100,000,000
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Debt, stated interest rate		3.75%		3.75%
2.45% Senior Notes due 2027
Debt Instrument [Line Items]
Debt, stated interest rate		2.45%		2.45%
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Debt, stated interest rate		6.625%		6.625%
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Debt, stated interest rate		6.25%		6.25%
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Debt, stated interest rate		4.35%		4.35%
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Debt, stated interest rate		3.70%		3.70%
4.125% Senior Notes due 2047
Debt Instrument [Line Items]
Debt, face amount	$ 850,000,000		$ 850,000,000
Debt, stated interest rate	4.125%	4.125%	4.125%	4.125%
Proceeds, net of underwriter's discounts and commissions			$ 841,100,000
Unrealized gain (loss) on interest rate cash flow hedges	$ (8,000,000)		$ (8,000,000)
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%
Proceeds, net of underwriter's discounts and commissions		$ 589,500,000
6.70% Fixed-to-Floating Rate Junior Subordinated Debentures due 2067
Debt Instrument [Line Items]
Debt, stated interest rate		6.70%		6.70%		6.70%
Repayment of debt						$ 563,700,000
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
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Valuation allowance	$ 0	$ 0	$ 0
Effective Income Tax Rate Reconciliation, Percent	19.90%	32.90%

Loss and Loss Adjustment Expe49

Loss and Loss Adjustment Expense Reserves (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	$ 13,329	$ 11,628.9	$ 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	2,137	1,850.4	2,170.1	1,801
Liability for Unpaid Claims and Claims Adjustment Expense, Net	11,192	9,778.5	$ 10,916.8	$ 9,567
Current Year Claims and Claims Adjustment Expense	4,815.2	4,152.2
Prior Year Claims and Claims Adjustment Expense	55.6	111.2
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	4,870.8	4,263.4
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	2,217.8	1,933.2
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	2,377.8	2,118.7
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	4,595.6	4,051.9
Commercial And Personal Auto Businesses [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		124
Accident years 2015 and prior
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	(6)
Accident year 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		100
Personal Lines | Personal auto businesses
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	35
Personal Lines | Agency Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	24	68
Personal Lines | Direct Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	11	$ 37
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	2
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 19

Supplemental Cash Flow Inform50

Supplemental Cash Flow Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Open reverse repurchase commitments	$ 0	$ 0	$ 0
Income Taxes Paid	0	700,000
Interest Paid	33,300,000	33,100,000
Cash and Cash Equivalents [Member] | ARX Holding Corp. [Member]
Open reverse repurchase commitments	$ 166,600,000	$ 140,000,000	$ 247,200,000

Segment Information Operating R

Segment Information Operating Results (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Segment Reporting Information [Line Items]
Revenues	$ 7,430.1	$ 6,321.7
Pretax Profit (Loss)	910.8	641.5
Amortization of intangible assets	18	15.5
Personal Lines | Agency | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	3,063.8	2,631.5
Pretax Profit (Loss)	409.1	249.7
Personal Lines | Direct | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	3,016.3	2,523.7
Pretax Profit (Loss)	298	175.3
Operating Segments | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	7,174	6,026.7
Pretax Profit (Loss)	830.6	500
Operating Segments | Personal Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	6,080.1	5,155.2
Pretax Profit (Loss)	$ 707.1	$ 425
Personal auto insurance percentage of the total personal lines segment net premiums earned	94.00%	93.00%
Operating Segments | Commercial Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	$ 808.6	$ 645.5
Pretax Profit (Loss)	94.8	67.4
Operating Segments | Property | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	285.3	226
Pretax Profit (Loss)	28.5	7.8
Amortization of intangible assets	18	15.5
Operating Segments | Other Indemnity | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	0	0
Pretax Profit (Loss)	0.2	(0.2)
Segment Reconciling Items | Fees and other revenues
Segment Reporting Information [Line Items]
Revenues	103.8	85.2
Segment Reconciling Items | Service businesses
Segment Reporting Information [Line Items]
Revenues	34.2	28.5
Pretax Profit (Loss)	4.9	2.6
Segment Reconciling Items | Investment income
Segment Reporting Information [Line Items]
Revenues	118.1	181.1
Pretax Profit (Loss)	112.1	175.5
Segment Reconciling Items | Other gains (losses)
Segment Reporting Information [Line Items]
Revenues	0	0.2
Pretax Profit (Loss)	0	0.2
Segment Reconciling Items | Interest expense
Segment Reporting Information [Line Items]
Pretax Profit (Loss)	$ (36.8)	$ (36.8)

Segment Information Underwritin

Segment Information Underwriting Margins and Combined Ratios for our Underwriting Operations (Detail) - Underwriting operations	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Personal Lines | Agency
Segment Reporting Information [Line Items]
Underwriting Margin	13.40%	9.50%
Combined Ratio	86.60%	90.50%
Personal Lines | Direct
Segment Reporting Information [Line Items]
Underwriting Margin	9.90%	6.90%
Combined Ratio	90.10%	93.10%
Operating Segments
Segment Reporting Information [Line Items]
Underwriting Margin	11.60%	8.30%
Combined Ratio	88.40%	91.70%
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Underwriting Margin	11.60%	8.20%
Combined Ratio	88.40%	91.80%
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Underwriting Margin	11.70%	10.40%
Combined Ratio	88.30%	89.60%
Operating Segments | Property
Segment Reporting Information [Line Items]
Underwriting Margin	10.00%	3.40%
Combined Ratio	90.00%	96.60%
Combined ratio amortization points	0.063	0.069

Dividends Dividends (Details)

Dividends Dividends (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
	Feb. 28, 2018	Feb. 28, 2017	Dec. 31, 2017	Dec. 31, 2016
Payments of Ordinary Dividends, Common Stock	$ 654.9	$ 395.4
Dividends payable			$ 655.1	$ 395.4
Annual variable dividend [Member]
Common Stock, Dividends, Per Share, Cash Paid	$ 1.1247	$ 0.6808

Dividends - Additional Informat

Dividends - Additional Information (Detail)	1 Months Ended	3 Months Ended	60 Months Ended
	Mar. 31, 2023	Mar. 31, 2018USD ($)$ / sharesshares	Mar. 14, 2023	Dec. 31, 2017$ / sharesshares	Mar. 31, 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		1.99
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued (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)				0	0
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 (L55

Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
AOCI before Tax, Attributable to Parent, beginning balance	$ 1,977.8	$ 1,439.5	$ 1,439.5
AOCI before Tax, Attributable to Parent, after Cumulative Effect Adjustment			(28.2)
Investment securities, pre tax	(203.4)	254.5
Forecasted transactions, pre tax	0	4
Foreign currency translation adjustment, pre tax	0	0.5
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Noncontrolling Interest	5.1	(1.5)
Total other comprehensive income (loss) before reclassifications, pre tax	(198.3)	257.5
Total reclassification adjustment for amounts realized in net income, pre tax	(7.9)	51.1
Total other comprehensive income (loss), pre tax	(190.4)	206.4
AOCI before Tax, Attributable to Parent, ending balance	(218.6)	1,645.9	1,977.8
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
AOCI Tax, Attributable to Parent, beginning balance	(695.6)	(506.1)	(506.1)
AOCI Tax, Attributable to Parent, After Cumulative Effect Adjustment			(5.9)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Tax	42.8	(89.2)
Forecasted transactions, Total tax (provision) benefit	0	(1.4)
Foreign currency translation adjustment, Total tax (provision) benefit	0	(0.2)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Noncontrolling Interest	(1.1)	0.6
Other Comprehensive Income (Loss), Available-for-sale Securities, before Reclassification Adjustments, Tax	41.7	(90.2)
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	1.6	(17.9)
Total other comprehensive income (loss), Total tax (provision) benefit	40.1	(72.3)
AOCI Tax, Attributable to Parent, ending balance	46	(578.4)	(695.6)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	1,282.2	933.4	933.4
AOCI, Net of Tax, After Cumulative Effect Adjustment			(22.3)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	(160.6)	165.3
Forecasted transactions, after tax	0	2.6
Foreign currency translation adjustment, after tax	0	0.3
Other comprehensive (income) loss attributable to NCI	4	(0.9)	(2.3)
Total other comprehensive income (loss) before reclassifications, after tax	(156.6)	167.3
Total reclassification adjustment for amounts realized in net income, after tax	(6.3)	33.2
Other comprehensive income (loss)	(150.3)	134.1
After tax total accumulated other comprehensive income, ending balance	(172.6)	1,067.5	1,282.2
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, before Tax	0	1
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Tax	0	0.4
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0	0.6
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	(7.7)	51.6
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	1.6	(18.1)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	(6.1)	33.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	(0.2)	0.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	0	(0.2)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.2)	0.3
Net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	1,295	939.6	939.6
AOCI, Net of Tax, After Cumulative Effect Adjustment			(6.3)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	(160.6)	165.3
Forecasted transactions, after tax	0	0
Foreign currency translation adjustment, after tax	0	0
Other comprehensive (income) loss attributable to NCI	0	0
Total other comprehensive income (loss) before reclassifications, after tax	(160.6)	165.3
Total reclassification adjustment for amounts realized in net income, after tax	(6.1)	32.9
Other comprehensive income (loss)	(154.5)	132.4
After tax total accumulated other comprehensive income, ending balance	(160.8)	1,072	1,295
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0	0.6
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	(6.1)	33.5
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0	0
Net unrealized gains on forecasted transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(14.8)	(9.4)	(9.4)
AOCI, Net of Tax, After Cumulative Effect Adjustment			(18)
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	0	0
Forecasted transactions, after tax	0	2.6
Foreign currency translation adjustment, after tax	0	0
Other comprehensive (income) loss attributable to NCI	0	0
Total other comprehensive income (loss) before reclassifications, after tax	0	2.6
Total reclassification adjustment for amounts realized in net income, after tax	(0.2)	0.3
Other comprehensive income (loss)	0.2	2.3
After tax total accumulated other comprehensive income, ending balance	(17.8)	(7.1)	(14.8)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.2)	0.3
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	0	(1.1)	(1.1)
AOCI, Net of Tax, After Cumulative Effect Adjustment			0
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	0	0
Forecasted transactions, after tax	0	0
Foreign currency translation adjustment, after tax	0	0.3
Other comprehensive (income) loss attributable to NCI	0	0
Total other comprehensive income (loss) before reclassifications, after tax	0	0.3
Total reclassification adjustment for amounts realized in net income, after tax	0	0
Other comprehensive income (loss)	0	0.3
After tax total accumulated other comprehensive income, ending balance	0	(0.8)	0
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0	0
Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	2	4.3	4.3
AOCI, Net of Tax, After Cumulative Effect Adjustment			2
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	0	0
Forecasted transactions, after tax	0	0
Foreign currency translation adjustment, after tax	0	0
Other comprehensive (income) loss attributable to NCI	4	(0.9)
Total other comprehensive income (loss) before reclassifications, after tax	4	(0.9)
Total reclassification adjustment for amounts realized in net income, after tax	0	0
Other comprehensive income (loss)	4	(0.9)
After tax total accumulated other comprehensive income, ending balance	6	3.4	$ 2
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0	$ 0
Accounting Standards Update 2016-01 [Member]
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Pretax Cumulative Effect of New Accounting Principal in Period of Adoption	(2,006)
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
Tax, Cumulative Effect of New Accounting Principal in Period of Adoption	705.8
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption	(1,300.2)
Accounting Standards Update 2016-01 [Member] | Net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption	(1,300.2)
Accounting Standards Update 2016-01 [Member] | Net unrealized gains on forecasted transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Accounting Standards Update 2016-01 [Member] | Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Accounting Standards Update 2016-01 [Member] | Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Accounting Standards Update 2018-02 [Member]
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
Pretax Cumulative Effect of New Accounting Principal in Period of Adoption	0
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
Tax, Cumulative Effect of New Accounting Principal in Period of Adoption	(4.3)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption	(4.3)
Accounting Standards Update 2018-02 [Member] | Net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption	(1.1)
Accounting Standards Update 2018-02 [Member] | Net unrealized gains on forecasted transactions
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption	(3.2)
Accounting Standards Update 2018-02 [Member] | Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption	0
Accounting Standards Update 2018-02 [Member] | Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption	$ 0

Other Comprehensive Income (L56

Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement, Additional Information (Detail) $ in Millions	Mar. 31, 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 Int57

Redeemable Noncontrolling Interest (Details) $ in Millions		1 Months Ended		3 Months Ended			12 Months Ended
		Apr. 30, 2018USD ($)shares		Mar. 31, 2018USD ($)shares		Mar. 31, 2017USD ($)	Dec. 31, 2017USD ($)	Apr. 01, 2021installment	Apr. 01, 2018sharesinstallment	Apr. 01, 2015installment
Redeemable Noncontrolling Interest [Line Items]
Number of Installments Related to Stockholders Agreement | installment										2
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance, Beginning of period		$ 514.2	[1]	$ 503.7	[1]	$ 483.7	$ 483.7
Net income attributable to NCI				11.8		6	5.9
Other comprehensive income (loss) attributable to NCI				(4)		0.9	2.3
Exercise of employee stock options				1.4		3.4	3.4
Change in redemption value of NCI				1.3		0.2	8.4
Balance, End of period	[1]			$ 514.2		$ 494.2	$ 503.7
ARX Holding Corp. [Member]
Redeemable Noncontrolling Interest [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage				68.90%		69.00%	69.00%
Shares held in Employee Stock Option Plan, Suspense Shares | shares				21,550
Employee Stock Ownership Plan (ESOP), Deferred Shares, Fair Value				$ 31.6
Scenario, Forecast [Member]
Redeemable Noncontrolling Interest [Line Items]
Number of Installments Related to Stockholders Agreement | installment								1	1
Scenario, Forecast [Member] | ARX Holding Corp. [Member]
Redeemable Noncontrolling Interest [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage								100.00%	80.00%
Subsequent Event [Member] | ARX Holding Corp. [Member]
Redeemable Noncontrolling Interest [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage									86.70%
Number of Shares Put Pursuant to Stockholders' Agreement | shares									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
[1]	See Note 12 – Redeemable Noncontrolling Interest for further discussion.

Goodwill and Intangible Asset58

Goodwill and Intangible Assets Goodwill (Details) - USD ($)	3 Months Ended		6 Months Ended	9 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jun. 30, 2017	Mar. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill, Acquired During Period	$ 0
Goodwill	452,700,000	$ 449,400,000		$ 452,700,000	$ 452,700,000
Goodwill, Impaired, Accumulated Impairment Loss	0			$ 0
Amortization of intangible assets	$ 18,000,000	$ 15,500,000
Weighted Average [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Remaining Amortization Period	4 years 7 months 25 days
Trade name [Member]
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life			10 years	2 years

Goodwill and Intangible Asset59

Goodwill and Intangible Assets - Net Carrying Amount of Other Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Schedule of Other Intangible Assets [Line Items]
Indefinite lived intangible assets	$ 12.4	$ 12.4	$ 12.4
Total	348.6	366.6	417.3
Accumulated amortization	193.7	175.7	125
Licensing Agreements [Member]
Schedule of Other Intangible Assets [Line Items]
Accumulated amortization	$ 0.6	$ 0.6	$ 0.6

Goodwill and Intangible Asset60

Goodwill and Intangible Assets - Finite-lived Intangible Assets (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	$ 193.7	$ 175.7	$ 125
Policies in force [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	256.2	256.2	256.2
Accumulated Amortization	109.8	100.7	73.2
Net Carrying Amount	146.4	155.5	183
Agency relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	159.2	159.2	159.2
Accumulated Amortization	34.2	31.3	22.7
Net Carrying Amount	125	127.9	136.5
Software rights [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	79.1	79.1	79.1
Accumulated Amortization	32.1	29.4	21.5
Net Carrying Amount	47	49.7	57.6
Trade name [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	34.8	34.8	34.8
Accumulated Amortization	17	13.7	7
Net Carrying Amount	17.8	21.1	27.8
Finite-Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	529.3	529.3	529.3
Accumulated Amortization	193.1	175.1	124.4
Net Carrying Amount	$ 336.2	$ 354.2	$ 404.9

New Accounting Standards Effect

New Accounting Standards Effect of Standard Change (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Item Effected [Line Items]
Liabilities		$ 30,493.5	$ 25,793.4		$ 28,912.7
Assets		41,330.9	34,810.5		$ 38,701.2
Provision (benefit) for income taxes		181	211.2
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		140
Assets		140
Accounting Standards Update 2018-02 [Member]
Item Effected [Line Items]
Reclassification of Stranded Amounts in AOCI to Retained Earnings		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
Retained Earnings | Accounting Standards Update 2018-02 [Member]
Item Effected [Line Items]
Reclassification of Stranded Amounts in AOCI to Retained Earnings	[1]	$ 4.3	$ 0
[1]	See Note 14 – New Accounting Standards for further discussion.