The Progressive Corporation (PGR) 10-Q 31 Jul 18 2018 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information	6 Months Ended
	Jun. 30, 2018shares
Document Document And Entity Information [Abstract]
Entity Registrant Name	PROGRESSIVE CORP/OH/
Entity Central Index Key	80,661
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Document Type	10-Q
Document Period End Date	Jun. 30, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q2
Amendment Flag	false
Trading Symbol	PGR
Entity Common Stock, Shares Outstanding	582,512,182

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) shares in Millions, $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues
Net premiums earned		$ 7,634.2	$ 6,313.3	$ 14,808.2	$ 12,340
Period over prior year period percent change in net premiums earned		21.00%		20.00%
Investment income		$ 192.1	138.8	$ 358.4	268
Period over prior year period percent change in investment income		38.00%		34.00%
Net realized gains (losses) on securities:
Gross Realized Gain (Loss), excluding Holding Period and OTTI		$ (9.6)	45.5	$ 97.4	97.6
Net Holding Period Realized Gain (Loss)		53.5	0.4	(101.7)	1.2
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net		(11.1)	(13.8)	(11.1)	(14.8)
Total net realized gains (losses) on securities		$ 32.8	32.1	$ (15.4)	84
Period Over Prior Year Period, Percent Increase (Decrease), Net Realized Gains (Losses) On Security Sales		(121.00%)		0.00%
Period Over Prior Year Period, Percent Increase (Decrease), Net Impairment Losses Recognized In Earnings		(20.00%)		(25.00%)
Period Over Prior Year Period, Percent Increase (Decrease), Total Net Realized Gains (Losses) On Securities		2.00%		(118.00%)
Fees and other revenues		$ 116	88.8	$ 219.8	174
Period over prior year period percent change in fees and other revenues		31.00%		26.00%
Service revenues		$ 42.9	32.7	$ 77.1	61.2
Period over prior year period percent change in service revenues		31.00%		26.00%
Other gains (losses)		$ 0	0	$ 0	0.2
Period Over Prior Year Period, Percent Increase (Decrease), Other Gains (Losses)				(100.00%)
Total revenues		$ 8,018	6,605.7	$ 15,448.1	12,927.4
Period over prior year period percent change in total revenues		21.00%		19.00%
Expenses
Losses and loss adjustment expenses		$ 5,375.3	4,614.9	$ 10,246.1	8,878.3
Period over prior year period percent change in losses and loss adjustment expenses		16.00%		15.00%
Policy acquisition costs		$ 630.8	514.2	$ 1,227	1,017.1
Period over prior year period percent change in policy acquisition costs		23.00%		21.00%
Other underwriting expenses		$ 1,046.9	845	$ 2,027.1	1,690.6
Period over prior year period percent change in other underwriting expenses		24.00%		20.00%
Investment expenses		$ 6.2	6.6	$ 12.2	12.2
Period over prior year period percent change in investment expenses		(6.00%)		0.00%
Service expenses		$ 37	27	$ 66.3	52.9
Period over prior year period percent change in service expenses		37.00%		25.00%
Interest expense		$ 41.7	43.4	$ 78.5	80.2
Period over prior year period percent change in interest expense		(4.00%)		(2.00%)
Total expenses		$ 7,137.9	6,051.1	$ 13,657.2	11,731.3
Period over prior year period percent change in total expenses		18.00%		16.00%
Net Income
Income before income taxes		$ 880.1	554.6	$ 1,790.9	1,196.1
Period over prior year period percent change in income before income taxes		59.00%		50.00%
Provision for income taxes		$ 178.9	181.9	$ 359.9	393.1
Period over prior year period percent change in provision for income taxes		(2.00%)		(8.00%)
Net income		$ 701.2	372.7	$ 1,431	803
Period over prior year period percent change in net income		88.00%		78.00%
Net (income) loss attributable to noncontrolling interest (NCI)		$ 3	(5.1)	$ (8.8)	(11.1)
Period over prior year period percent change in net (income) loss attributable to noncontrolling interest		(159.00%)		(21.00%)
Net income attributable to Progressive		$ 704.2	367.6	$ 1,422.2	791.9
Period over prior year period percent change in net income attributable to Progressive		92.00%		80.00%
Other Comprehensive Income (Loss)
Total net unrealized gains (losses) on securities		$ (50.1)	92.6	$ (204.6)	225
Period over prior year period percent change in total net unrealized gains (losses) on securities		(154.00%)		(191.00%)
Net unrealized losses on forecasted transactions		$ 0.2	(8)	$ 0.4	(5.7)
Period Over Prior Year Period, Percent Increase (Decrease), Unrealized Gain (Loss) On Forecasted Transactions		(103.00%)		(107.00%)
Foreign currency translation adjustment		$ 0	(0.1)	$ 0	0.2
Period over prior year period percent change in foreign currency translation adjustment		(100.00%)		(100.00%)
Other comprehensive income (loss)		$ (49.9)	84.5	$ (204.2)	219.5
Period over prior year period percent change in other comprehensive income		(159.00%)		(193.00%)
Other comprehensive (income) loss attributable to NCI		$ 0.6	(1.3)	$ 4.6	(2.2)
Period over prior year period change in Comprehensive (income) loss attributable to NCI		(146.00%)		(309.00%)
Comprehensive income attributable to Progressive		$ 654.9	450.8	$ 1,222.6	1,009.2
Period over prior year period percent change in comprehensive income		45.00%		21.00%
Computation of Per Share Earnings Available to Progressive Common Shareholders
Net income attributable to Progressive		$ 704.2	367.6	$ 1,422.2	791.9
Period over prior year period percent change in net income attributable to Progressive		92.00%		80.00%
Preferred Stock Dividends, Income Statement Impact		$ 6.7	0	$ 7.9	0
Net Income (Loss) Available to Common Stockholders, Basic		$ 697.5	$ 367.6	$ 1,414.3	$ 791.9
Period Over Prior Year Period, Percent Increase (Decrease), Net Income (Loss) Available to Common Stockholders		90.00%		79.00%
Average common shares outstanding - Basic		582	580.5	582	580.4
Period over prior year period percent change in average shares outstanding - basic		0.00%		0.00%
Net effect of dilutive stock-based compensation		3.8	3.3	3.6	3.1
Period over prior year period percent change in net effect of dilutive stock-based compensation		15.00%		16.00%
Total average equivalent common shares - Diluted		585.8	583.8	585.6	583.5
Period over prior year period percent change in total equivalent shares - diluted		0.00%		0.00%
Basic: Earnings per common share		$ 1.20	$ 0.63	$ 2.43	$ 1.36
Period over prior year period percent change in basic: earnings per share		89.00%		78.00%
Diluted: Earnings per common share		$ 1.19	0.63	$ 2.42	1.36
Period over prior year period percent change in diluted: earnings per share		89.00%		78.00%
Dividends declared per share	[1]	$ 0	$ 0	$ 0	$ 0
[1]	Progressive maintains an annual variable common share dividend program. See Note 9 – Dividends for further discussion.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Available-for-sale securities, at fair value:
Debt Securities, Available-for-sale, Fixed Maturities		$ 23,789.2	$ 20,201.7	$ 18,388.1
Short-term investments (amortized cost: $3,231.2, $3,729.7, and $2,869.4)		3,231.2	2,869.4	3,729.7
Fixed maturities (amortized cost: $24,061.8, $18,311.7, and $20,209.9)		27,020.4	23,071.1	22,117.8
Equity securities, at fair value:
Equity Securities FV NI Nonredeemable Preferred Stock		758.6	803.8	783.1
Equity Securities FV NI Common Equities		3,142.2	3,399.8	3,077.5
Equity Securities, FV-NI		3,900.8	4,203.6	3,860.6
Investments, Fair Value Disclosure		30,921.2	27,274.7	25,978.4
Cash and cash equivalents		154.8	265	161
Restricted Cash		2.9	10.3	0.8
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents		157.7	275.3	161.8
Accrued investment income		146.3	119.7	112.1
Premiums receivable, net of allowance for doubtful accounts of $210.5, $175.8, and $210.9		6,230.2	5,422.5	5,091.3
Reinsurance recoverables, including $117.6, $79.2, and $103.3 on paid losses and loss adjustment expenses		2,410.7	2,273.4	2,027.1
Prepaid reinsurance premiums		289.8	203.3	212.6
Deferred acquisition costs		895.7	780.5	727.2
Property and equipment, net of accumulated depreciation of $984.7, $894.7, and $940.6		1,116.4	1,119.6	1,152.1
Goodwill		452.7	452.7	452.7
Intangible assets, net of accumulated amortization of $211.7, $140.5, and $175.7		330.6	366.6	401.8
Other assets		412.2	412.9	372.7
Total assets		43,363.5	38,701.2	36,689.8
Liabilities
Unearned premiums		10,245.9	8,903.5	8,407.7
Loss and loss adjustment expense reserves		14,070.8	13,086.9	12,060.4
Net deferred income taxes		46	135	191.8
Accounts payable, accrued expenses, and other liabilities		3,922.3	3,481	3,153.2
Debt	[1]	3,859.5	3,306.3	3,383.4
Total liabilities		32,144.5	28,912.7	27,196.5
Redeemable noncontrolling interest (NCI)	[2]	218.2	503.7	501.8
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 215.0, 216.5, and 215.8)		582.5	581.7	581
Paid-in capital		1,425.9	1,389.2	1,351
Retained earnings		8,720.4	6,031.7	5,908.8
Accumulated other comprehensive income:
Net unrealized gains (losses) on securities		(210.9)	1,295	1,164.6
Net unrealized losses on forecasted transactions		(17.6)	(14.8)	(15.1)
Foreign currency translation adjustment		0	0	(0.9)
Accumulated other comprehensive (income) loss attributable to NCI		6.6	2	2.1
Total accumulated other comprehensive income attributable to Progressive		(221.9)	1,282.2	1,150.7
Total shareholders’ equity		11,000.8	9,284.8	8,991.5
Total liabilities, redeemable NCI, and shareholders’ equity		$ 43,363.5	$ 38,701.2	$ 36,689.8
[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	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Fixed maturities, amortized cost	$ 24,061.8	$ 20,209.9	$ 18,311.7
Nonredeemable preferred stocks, cost	677	698.6	666
Common equities, cost	1,314	1,499	1,472.3
Short-term investments, amortized cost	3,231.2	2,869.4	3,729.7
Premiums receivable, allowance for doubtful accounts	210.5	210.9	175.8
Reinsurance recoverables, paid losses and loss adjustment expenses	117.6	103.3	79.2
Intangible Assets, Accumulated Amortization	211.7	175.7	140.5
Property and equipment, accumulated depreciation	$ 984.7	$ 940.6	$ 894.7
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)	215,000,000	215,800,000	216,500,000
Serial Preferred Stock [Member]
Preferred Stock, Shares Authorized (shares)	20,000,000	20,000,000	20,000,000
Series B Preferred Stock [Member]
Preferred Stock, No Par Value	$ 0		$ 0
Preferred Stock, Liquidation Preference Per Share (USD per share)		$ 0	0
Series B Preferred Stock [Member] | Serial Preferred Stock [Member]
Preferred Stock, No Par Value	$ 0	$ 0	$ 0
Preferred Stock, Shares Authorized (shares)	500,000	0	0
Preferred Stock, Shares Issued (shares)	500,000	0	0
Preferred Stock, Shares Outstanding (shares)	500,000	0	0
Preferred Stock, Liquidation Preference Per Share (USD per share)	$ 1,000

Consolidated Statements of Chan

Consolidated Statements of Changes in Shareholders' Equity Statement - USD ($) $ in Millions		Total	Preferred Shares, No Par Value	Common Shares, $1.00 Par Value	Paid-in Capital	Retained Earnings	AOCI Attributable to Parent [Member]	Series B 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		$ 791.9				791.9
Treasury shares purchased				(0.7)	(1.5)	(21.9)
Stock Issued During Period, Value, Restricted Stock Award, Gross				1.8	(1.8)
Amortization of equity-based compensation					52.2
Reinvested dividends on restricted stock units					0.1	(0.1)
Adjustment to carrying amount of redeemable noncontrolling interest		1.4			(1.4)
Attributable to noncontrolling interest		(2.2)					(2.2)
Other comprehensive income							219.5
Other, net						(1.5)
Balance, End of period at Jun. 30, 2017		8,991.5	0	581	1,351	5,908.8	1,150.7
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification of Stranded Amounts in AOCI to Retained Earnings | Accounting Standards Update 2018-02 [Member]		4.3
Net income attributable to Progressive		367.6
Attributable to noncontrolling interest		(1.3)
Balance, End of period at Jun. 30, 2017		$ 8,991.5	0	581	1,351	5,908.8	1,150.7
Preferred Stock, No Par Value								$ 0
Preferred Stock, Shares Authorized (shares)									5,000,000
Preferred Stock, Shares Issued (shares)									0
Common Shares, par value (USD per share)		$ 1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-01 [Member]	[1]					0	0
Reclassification of Stranded Amounts in AOCI to Retained Earnings | Accounting Standards Update 2018-02 [Member]	[1]					0	0
Common Shares, par value (USD per share)		$ 1
Cumulative Effect of New Accounting Principle in Period of Adoption		$ 1,300.2
Reclassification of Stranded Amounts in AOCI to Retained Earnings		4.3
Balance, Beginning of period at Dec. 31, 2017		9,284.8	0	581.7	1,389.2	6,031.7	1,282.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Value, New Issues			493.9
Net income attributable to Progressive		1,422.2				1,422.2
Treasury shares purchased				(0.7)	(1.7)	(37)
Stock Issued During Period, Value, Restricted Stock Award, Gross				1.5	(1.5)
Amortization of equity-based compensation					37.5
Reinvested dividends on restricted stock units					(0.8)	0.8
Adjustment to carrying amount of redeemable noncontrolling interest		(3.2)			3.2
Attributable to noncontrolling interest		4.6					4.6
Other comprehensive income							(204.2)
Other, net						(1.8)
Balance, End of period at Jun. 30, 2018		11,000.8	493.9	582.5	1,425.9	8,720.4	(221.9)
Preferred Stock, Shares Issued (shares)								500,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to Progressive		704.2
Attributable to noncontrolling interest		0.6
Balance, End of period at Jun. 30, 2018		$ 11,000.8	$ 493.9	$ 582.5	$ 1,425.9	8,720.4	(221.9)
Preferred Stock, No Par Value								$ 0
Preferred Stock, Shares Authorized (shares)									5,000,000
Preferred Stock, Shares Issued (shares)									0
Common Shares, par value (USD per share)		$ 1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-01 [Member]	[1]					1,300.2	(1,300.2)
Reclassification of Stranded Amounts in AOCI to Retained Earnings | Accounting Standards Update 2018-02 [Member]	[1]					$ 4.3	$ (4.3)
[1]	See Note 14 – New Accounting Standards for further discussion.

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Statement of Cash Flows [Abstract]
Net income	$ 1,431	$ 803
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation	86.6	84
Amortization of intangible assets	36	31
Net amortization of fixed-income securities	23.2	44.1
Amortization of equity-based compensation	38.5	53.9
Net realized (gains) losses on securities	15.4	(84)
Net (gains) losses on disposition of property and equipment	1.7	3.2
Other (gains) losses	0	(0.2)
Changes in:
Premiums receivable	(807.7)	(582)
Reinsurance recoverables	(137.3)	(142.4)
Prepaid reinsurance premiums	(86.5)	(42.1)
Deferred acquisition costs	(115.2)	(76)
Income taxes	2	(64.3)
Unearned premiums	1,342.4	939
Loss and loss adjustment expense reserves	983.9	692.3
Accounts payable, accrued expenses, and other liabilities	700.4	430.2
Other, net	(31.1)	(67.1)
Net cash provided by operating activities	3,483.3	2,022.6
Purchases:
Fixed maturities	(10,780)	(6,803.3)
Equity securities	(136.4)	(89.5)
Sales:
Fixed maturities	3,916.1	2,359.9
Equity securities	460	133.3
Maturities, paydowns, calls, and other:
Fixed maturities	2,936.2	2,413.5
Equity securities	15	50
Net sales (purchases) of short-term investments	(343.8)	(146.8)
Net unsettled security transactions	367.9	259.3
Purchases of property and equipment	(102)	(73.2)
Sales of property and equipment	6.7	12.5
Payments to Acquire Additional Interest in Subsidiaries	(295.9)	0
Payments to Acquire Businesses, Net of Cash Acquired	0	(18.1)
Net cash used in investing activities	(3,956.2)	(1,902.4)
Cash Flows From Financing Activities
Net proceeds from issuance of Serial Preferred Shares, Series B	493.9	0
Net proceeds from debt issuance	589.5	841.1
Payments of debt	(37.1)	(12.5)
Redemption/reacquisition of subordinated debt	0	(594.4)
Dividends paid to common shareholders	(654.9)	(395.4)
Proceeds from exercise of equity options	3.3	0.5
Acquisition of treasury shares for restricted stock tax liabilities	(39)	(24.1)
Acquisition of treasury shares acquired in open market	(0.4)	0
Net cash provided by (used in) financing activities	355.3	(184.8)
Decrease in cash, cash equivalents, and restricted cash	(117.6)	(64.6)
Cash, cash equivalents, and restricted cash – January 1	275.3	226.4
Cash, cash equivalents, and restricted cash - March 31	$ 157.7	$ 161.8

Basis of Presentation

Basis of Presentation	6 Months Ended
	Jun. 30, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation — The accompanying consolidated financial statements include the accounts of The Progressive Corporation and ARX Holding Corp. (ARX), and their respective wholly owned insurance and non-insurance subsidiaries and affiliates in which Progressive or ARX has a controlling financial interest. The Progressive Corporation owned 86.7% of the outstanding capital stock of ARX at June 30, 2018 and 69.0% at June 30, 2017 and December 31, 2017 . The increase in Progressive’s ownership of ARX at June 30, 2018, is due to the minority ARX shareholders who “put” 204,527 of their ARX shares during the second quarter 2018, including exercised stock options, to Progressive pursuant to the stockholders’ agreement. All intercompany accounts and transactions are eliminated in consolidation. The consolidated financial statements reflect all normal recurring adjustments that, in the opinion of management, were necessary for a fair statement of the results for the interim periods presented. The results of operations for the period ended June 30, 2018 , are not necessarily indicative of the results expected for the full year. These consolidated financial statements and the notes thereto should be read in conjunction with Progressive’s audited financial statements and accompanying notes included in Exhibit 13 to our Annual Report on Form 10-K for the year ended December 31, 2017 ( “ 2017 Annual Report to Shareholders ” ). Other assets on the consolidated balance sheets include properties that are considered “ held for sale, ” if any. The fair value of these properties, less the estimated cost to sell them, was $17.4 million at June 30, 2018 and $5.3 million at December 31, 2017. No properties were held for sale at June 30, 2017.

Investments

Investments	6 Months Ended
	Jun. 30, 2018
Investments Schedule [Abstract]
Investments	Investments — The following tables present the composition of our investment portfolio by major security type, consistent with our classification of how we manage, monitor, and measure the portfolio. Our securities are reported in our Consolidated Balance Sheets at fair value, with all of our equity securities having a readily determinable fair value. The changes in fair value for our fixed-maturity securities (other than hybrid securities) are reported as a component of accumulated other comprehensive income, net of deferred income taxes, in our Consolidated Balance Sheets. The net holding period gains (losses) reported below represent the inception-to-date changes in fair value. The changes in the net holding period gains (losses) between periods for the hybrid securities and, beginning in 2018 , equity securities are recorded as a component of net realized gains (losses) on securities in our Consolidated Statements of Comprehensive Income. Prior to 2018, the change in fair value of our equity securities was part of accumulated other comprehensive income (see Note 14 – New Accounting Standards for further discussion). ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value June 30, 2018 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 8,005.8 $ 0.9 $ (141.3 ) $ 0 $ 7,865.4 25.4 % State and local government obligations 1,678.9 4.3 (15.9 ) 0 1,667.3 5.4 Foreign government obligations 0 0 0 0 0 0 Corporate debt securities 7,422.4 4.2 (94.3 ) (2.0 ) 7,330.3 23.7 Residential mortgage-backed securities 819.6 8.4 (5.4 ) 0 822.6 2.7 Commercial mortgage-backed securities 2,725.5 2.9 (31.5 ) 0 2,696.9 8.7 Other asset-backed securities 3,189.2 2.0 (13.7 ) 0.1 3,177.6 10.3 Redeemable preferred stocks 220.4 14.1 (2.0 ) (3.4 ) 229.1 0.7 Total fixed maturities 24,061.8 36.8 (304.1 ) (5.3 ) 23,789.2 76.9 Short-term investments 3,231.2 0 0 0 3,231.2 10.5 Total available-for-sale securities 27,293.0 36.8 (304.1 ) (5.3 ) 27,020.4 87.4 Equity securities: Nonredeemable preferred stocks 677.0 0 0 81.6 758.6 2.4 Common equities 1,314.0 0 0 1,828.2 3,142.2 10.2 Total equity securities 1,991.0 0 0 1,909.8 3,900.8 12.6 Total portfolio 1,2 $ 29,284.0 $ 36.8 $ (304.1 ) $ 1,904.5 $ 30,921.2 100.0 % ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value June 30, 2017 Fixed maturities: U.S. government obligations $ 4,166.4 $ 4.4 $ (15.8 ) $ 0 $ 4,155.0 16.0 % State and local government obligations 2,473.3 32.5 (5.9 ) 0.2 2,500.1 9.7 Foreign government obligations 22.5 0 0 0 22.5 0.1 Corporate debt securities 4,991.6 29.4 (7.6 ) 0.4 5,013.8 19.3 Residential mortgage-backed securities 1,260.3 10.9 (5.9 ) 2.3 1,267.6 4.8 Commercial mortgage-backed securities 2,364.9 19.8 (9.7 ) 0 2,375.0 9.1 Other asset-backed securities 2,843.3 6.4 (2.5 ) 0.2 2,847.4 11.0 Redeemable preferred stocks 189.4 19.0 (1.7 ) 0 206.7 0.8 Total fixed maturities 18,311.7 122.4 (49.1 ) 3.1 18,388.1 70.8 Equity securities: Nonredeemable preferred stocks 666.0 123.6 (6.5 ) 0 783.1 3.0 Common equities 1,472.3 1,611.7 (6.5 ) 0 3,077.5 11.8 Short-term investments 3,729.7 0 0 0 3,729.7 14.4 Total portfolio 1,2 $ 24,179.7 $ 1,857.7 $ (62.1 ) $ 3.1 $ 25,978.4 100.0 % ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value December 31, 2017 Fixed maturities: U.S. government obligations $ 6,688.8 $ 1.1 $ (44.0 ) $ 0 $ 6,645.9 24.4 % State and local government obligations 2,285.6 20.7 (9.3 ) 0.1 2,297.1 8.4 Foreign government obligations 0 0 0 0 0 0 Corporate debt securities 4,997.2 14.8 (14.4 ) 0.1 4,997.7 18.3 Residential mortgage-backed securities 828.8 11.3 (3.4 ) 0 836.7 3.1 Commercial mortgage-backed securities 2,760.1 11.8 (13.3 ) 0 2,758.6 10.1 Other asset-backed securities 2,454.5 4.5 (4.5 ) 0.2 2,454.7 9.0 Redeemable preferred stocks 194.9 17.8 (1.5 ) (0.2 ) 211.0 0.8 Total fixed maturities 20,209.9 82.0 (90.4 ) 0.2 20,201.7 74.1 Equity securities: Nonredeemable preferred stocks 698.6 114.0 (8.8 ) 0 803.8 2.9 Common equities 1,499.0 1,901.0 (0.2 ) 0 3,399.8 12.5 Short-term investments 2,869.4 0 0 0 2,869.4 10.5 Total portfolio 1,2 $ 25,276.9 $ 2,097.0 $ (99.4 ) $ 0.2 $ 27,274.7 100.0 % 1 Our portfolio reflects the effect of unsettled security transactions; at June 30, 2018 and 2017 , we had $362.1 million and $287.1 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 June 30, 2018 and 2017 , and December 31, 2017 , included $1.7 billion , $1.1 billion , and $1.6 billion , respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company, net of any unsettled security transactions. Short-Term Investments Our short-term investments may include commercial paper and other investments that are expected to mature or are redeemable within one year. We did not have any open repurchase or reverse repurchase transactions in our short-term investment portfolio at June 30, 2018 and 2017 , or December 31, 2017 . To the extent we enter into repurchase or reverse repurchase transactions, and consistent with past practice, we would elect not to offset these transactions and would report them on a gross basis on our balance sheets despite the option to elect to offset these transactions as long as they were with the same counterparty and subject to an enforceable master netting arrangement. Hybrid Securities Included in our fixed-maturity securities are hybrid securities, which are reported at fair value: June 30, (millions) 2018 2017 December 31, 2017 Fixed maturities: State and local government obligations $ 3.6 $ 6.6 $ 6.1 Corporate debt securities 170.3 61.3 99.8 Residential mortgage-backed securities 0 189.7 0 Other asset-backed securities 5.5 7.7 6.7 Redeemable preferred stocks 67.3 0 30.3 Total hybrid securities $ 246.7 $ 265.3 $ 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 June 30, 2018 , was: (millions) Cost Fair Value Less than one year $ 3,805.2 $ 3,809.2 One to five years 15,190.7 14,999.1 Five to ten years 4,939.7 4,855.4 Ten years or greater 126.2 125.5 Total $ 24,061.8 $ 23,789.2 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 June 30, 2018 , we had $304.1 million of gross unrealized losses in our fixed-maturity securities. A review of our fixed-maturity securities indicated that the issuers were current with respect to their interest obligations and that there was no evidence of deterioration of the current cash flow projections that would indicate we would not receive the remaining principal at maturity. The following tables show the composition of gross unrealized losses by major security type and by the length of time that individual securities have been in a continuous unrealized loss position: Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses June 30, 2018 Fixed maturities: U.S. government obligations 70 $ 7,503.2 $ (141.3 ) 33 $ 4,543.5 $ (73.6 ) 37 $ 2,959.7 $ (67.7 ) State and local government obligations 355 1,050.9 (15.9 ) 219 631.9 (7.1 ) 136 419.0 (8.8 ) Corporate debt securities 391 6,153.5 (94.3 ) 322 5,439.3 (77.2 ) 69 714.2 (17.1 ) Residential mortgage-backed securities 227 380.3 (5.4 ) 52 188.6 (0.9 ) 175 191.7 (4.5 ) Commercial mortgage-backed securities 142 2,300.9 (31.5 ) 85 1,371.4 (14.4 ) 57 929.5 (17.1 ) Other asset-backed securities 219 2,479.7 (13.7 ) 148 2,055.1 (8.5 ) 71 424.6 (5.2 ) Redeemable preferred stocks 3 30.2 (2.0 ) 1 4.6 (0.2 ) 2 25.6 (1.8 ) Total fixed maturities 1,407 $ 19,898.7 $ (304.1 ) 860 $ 14,234.4 $ (181.9 ) 547 $ 5,664.3 $ (122.2 ) Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses June 30, 2017 Fixed maturities: U.S. government obligations 41 $ 2,781.8 $ (15.8 ) 39 $ 2,725.1 $ (15.1 ) 2 $ 56.7 $ (0.7 ) State and local government obligations 222 818.7 (5.9 ) 138 479.8 (3.3 ) 84 338.9 (2.6 ) Corporate debt securities 143 1,905.2 (7.6 ) 122 1,599.6 (5.3 ) 21 305.6 (2.3 ) Residential mortgage-backed securities 220 535.7 (5.9 ) 126 197.8 (1.1 ) 94 337.9 (4.8 ) Commercial mortgage-backed securities 74 905.6 (9.7 ) 52 679.5 (5.7 ) 22 226.1 (4.0 ) Other asset-backed securities 160 2,077.8 (2.5 ) 145 1,823.8 (1.9 ) 15 254.0 (0.6 ) Redeemable preferred stocks 1 10.8 (1.7 ) 0 0 0 1 10.8 (1.7 ) Total fixed maturities 861 9,035.6 (49.1 ) 622 7,505.6 (32.4 ) 239 1,530.0 (16.7 ) Equity securities: Nonredeemable preferred stocks 3 73.1 (6.5 ) 0 0 0 3 73.1 (6.5 ) Common equities 80 73.1 (6.5 ) 74 72.3 (6.5 ) 6 0.8 0 Total equity securities 83 146.2 (13.0 ) 74 72.3 (6.5 ) 9 73.9 (6.5 ) Total portfolio 944 $ 9,181.8 $ (62.1 ) 696 $ 7,577.9 $ (38.9 ) 248 $ 1,603.9 $ (23.2 ) Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses December 31, 2017 Fixed maturities: U.S. government obligations 58 $ 5,817.0 $ (44.0 ) 41 $ 4,869.3 $ (34.6 ) 17 $ 947.7 $ (9.4 ) State and local government obligations 358 1,200.3 (9.3 ) 230 737.6 (4.4 ) 128 462.7 (4.9 ) Corporate debt securities 222 2,979.4 (14.4 ) 171 2,072.9 (9.1 ) 51 906.5 (5.3 ) Residential mortgage-backed securities 201 300.9 (3.4 ) 30 75.1 (0.2 ) 171 225.8 (3.2 ) Commercial mortgage-backed securities 105 1,682.3 (13.3 ) 63 1,221.2 (5.9 ) 42 461.1 (7.4 ) Other asset-backed securities 197 1,837.3 (4.5 ) 134 1,377.8 (3.3 ) 63 459.5 (1.2 ) Redeemable preferred stocks 2 21.8 (1.5 ) 1 10.8 (0.1 ) 1 11.0 (1.4 ) Total fixed maturities 1,143 13,839.0 (90.4 ) 670 10,364.7 (57.6 ) 473 3,474.3 (32.8 ) Equity securities: Nonredeemable preferred stocks 4 127.8 (8.8 ) 1 56.5 (0.5 ) 3 71.3 (8.3 ) Common equities 19 13.4 (0.2 ) 18 13.4 (0.2 ) 1 0 0 Total equity securities 23 141.2 (9.0 ) 19 69.9 (0.7 ) 4 71.3 (8.3 ) Total portfolio 1,166 $ 13,980.2 $ (99.4 ) 689 $ 10,434.6 $ (58.3 ) 477 $ 3,545.6 $ (41.1 ) Since both June 30, 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 and all of the securities in the table above are current with respect to required principal and interest payments. Other-Than-Temporary Impairment (OTTI) The following table shows the total non-credit portion of the OTTI recorded in accumulated other comprehensive income, reflecting the original non-credit loss at the time the credit impairment was determined (i.e., unadjusted for valuation changes subsequent to the original write-down): June 30, December 31, (millions) 2018 2017 Fixed maturities: Residential mortgage-backed securities $ (19.7 ) $ (19.7 ) $ (19.7 ) Commercial mortgage-backed securities (0.3 ) (0.5 ) (0.3 ) Total fixed maturities $ (20.0 ) $ (20.2 ) $ (20.0 ) The following tables provide rollforwards of the amounts related to credit losses recognized in earnings for the periods ended June 30, 2018 and 2017 , for which a portion of the OTTI losses were also recognized in accumulated other comprehensive income at the time the credit impairments were determined and recognized: Three Months Ended June 30, 2018 Mortgage-Backed (millions) Residential Commercial Total Balance at March 31, 2018 $ 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 0.3 0 0.3 Balance at June 30, 2018 $ 0.3 $ 0.5 $ 0.8 Six Months Ended June 30, 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 0.3 0 0.3 Balance at June 30, 2018 $ 0.3 $ 0.5 $ 0.8 Three Months Ended June 30, 2017 Mortgage-Backed (millions) Residential Commercial Total Balance at March 31, 2017 $ 11.4 $ 0.1 $ 11.5 Reductions for securities sold/matured (11.4 ) 0 (11.4 ) Change in recoveries of future cash flows expected to be collected 1 0.2 0 0.2 Balance at June 30, 2017 $ 0.2 $ 0.1 $ 0.3 Six Months Ended June 30, 2017 Mortgage-Backed (millions) Residential Commercial Total Balance at December 31, 2016 $ 11.1 $ 0.4 $ 11.5 Reductions for securities sold/matured (10.9 ) (0.3 ) (11.2 ) Change in recoveries of future cash flows expected to be collected 1 0 0 0 Balance at June 30, 2017 $ 0.2 $ 0.1 $ 0.3 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 six months ended June 30, 2018 or 2017 . Realized Gains (Losses) The components of net realized gains (losses) for the three and six months ended June 30, were: Three Months Six Months (millions) 2018 2017 2018 2017 Gross realized gains on security sales Available-for-sale securities: U.S. government obligations $ 1.7 $ 4.5 $ 1.7 $ 4.9 State and local government obligations 0.6 2.6 9.2 3.1 Corporate and other debt securities 0.3 7.3 0.4 11.4 Residential mortgage-backed securities 0 20.9 0 21.0 Commercial mortgage-backed securities 0.3 1.2 2.0 2.4 Other asset-backed securities 0.1 0 0.1 0.3 Redeemable preferred stocks 3.2 0 4.3 0.3 Total available-for-sale securities 6.2 36.5 17.7 43.4 Equity securities: Nonredeemable preferred stocks 0 6.2 3.6 51.6 Common equities 18.5 9.9 138.4 17.3 Total equity securities 18.5 16.1 142.0 68.9 Subtotal gross realized gains on security sales 24.7 52.6 159.7 112.3 Gross realized losses on security sales Available-for-sale securities: U.S. government obligations (29.9 ) (0.4 ) (38.8 ) (3.6 ) State and local government obligations (0.6 ) 0 (1.9 ) (0.1 ) Corporate and other debt securities (1.0 ) (1.9 ) (4.1 ) (2.8 ) Residential mortgage-backed securities 0 (0.3 ) 0 (0.3 ) Commercial mortgage-backed securities 0 (0.7 ) (6.3 ) (3.1 ) Other asset-backed securities (0.9 ) 0 (1.0 ) 0 Total available-for-sale securities (32.4 ) (3.3 ) (52.1 ) (9.9 ) Equity securities: Nonredeemable preferred stocks (1.9 ) (4.6 ) (2.3 ) (5.8 ) Common equities 0 0 (7.9 ) (0.1 ) Total equity securities (1.9 ) (4.6 ) (10.2 ) (5.9 ) Subtotal gross realized losses on security sales (34.3 ) (7.9 ) (62.3 ) (15.8 ) Net realized gains (losses) on security sales Available-for-sale securities: U.S. government obligations (28.2 ) 4.1 (37.1 ) 1.3 State and local government obligations 0 2.6 7.3 3.0 Corporate and other debt securities (0.7 ) 5.4 (3.7 ) 8.6 Residential mortgage-backed securities 0 20.6 0 20.7 Commercial mortgage-backed securities 0.3 0.5 (4.3 ) (0.7 ) Other asset-backed securities (0.8 ) 0 (0.9 ) 0.3 Redeemable preferred stocks 3.2 0 4.3 0.3 Total available-for-sale securities (26.2 ) 33.2 (34.4 ) 33.5 Equity securities: Nonredeemable preferred stocks (1.9 ) 1.6 1.3 45.8 Common equities 18.5 9.9 130.5 17.2 Total equity securities 16.6 11.5 131.8 63.0 Litigation settlements and other gains (losses) 0 0.8 0 1.1 Subtotal net realized gains (losses) on security sales (9.6 ) 45.5 97.4 97.6 Net holding period gains (losses) Hybrid securities (2.3 ) 0.4 (5.5 ) 1.2 Equity securities 55.8 0 (96.2 ) 0 Subtotal net holding period gains (losses) 53.5 0.4 (101.7 ) 1.2 Other-than-temporary impairment losses Equity securities: Common equities 0 (2.6 ) 0 (3.6 ) Subtotal investment other-than-temporary impairment losses 0 (2.6 ) 0 (3.6 ) Other asset impairment (11.1 ) (11.2 ) (11.1 ) (11.2 ) Subtotal other-than-temporary impairment losses (11.1 ) (13.8 ) (11.1 ) (14.8 ) Total net realized gains (losses) on securities $ 32.8 $ 32.1 $ (15.4 ) $ 84.0 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, recoveries from litigation settlements related to investments, and write-downs for securities determined to be other-than-temporarily impaired. The other asset impairment relates to renewable energy investments, which are reflected in “other assets” on the balance sheet, under which the future pretax cash flows are expected to be less than the carrying value of the assets. The following table reflects our holding period realized gains (losses) on equity securities recognized for the three and six months ended June 30, 2018 for equity securities held at quarter end: Three Months Six Months (millions) 2018 2018 Total net gains (losses) recognized during the period on equity securities $ 72.4 $ 35.6 Less: Net gains (losses) recognized on equity securities sold during the period 16.6 131.8 Net holding period gains (losses) recognized during the period on equity securities held at period end $ 55.8 $ (96.2 ) Note: Comparative disclosure for the prior year period is not meaningful. Net Investment Income The components of net investment income for the three and six months ended June 30, were: Three Months Six Months (millions) 2018 2017 2018 2017 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 45.9 $ 17.0 $ 85.7 $ 29.7 State and local government obligations 9.1 13.1 19.1 26.4 Foreign government obligations 0 0.1 0 0.2 Corporate debt securities 51.8 31.3 88.0 60.9 Residential mortgage-backed securities 6.6 10.3 13.5 19.9 Commercial mortgage-backed securities 20.2 18.3 41.4 37.1 Other asset-backed securities 16.1 11.8 29.7 21.7 Redeemable preferred stocks 2.9 2.9 5.5 6.3 Total fixed maturities 152.6 104.8 282.9 202.2 Short-term investments 14.9 8.8 25.0 15.8 Total available-for-sale securities 167.5 113.6 307.9 218.0 Equity securities: Nonredeemable preferred stocks 10.4 10.6 21.3 22.1 Common equities 14.2 14.6 29.2 27.9 Total equity securities 24.6 25.2 50.5 50.0 Investment income 192.1 138.8 358.4 268.0 Investment expenses (6.2 ) (6.6 ) (12.2 ) (12.2 ) Net investment income $ 185.9 $ 132.2 $ 346.2 $ 255.8 The amount of investment income (interest and dividends) we recognize varies from year to year based on the average assets held during the year and the book yields of the securities in our portfolio. The increase in net investment income on a year-over-year basis for the three and six months ended June 30, 2018, was due to a combination of an increase in average assets and an increase in portfolio yields. The increase in average assets was due to strong underwriting growth and profitability, as well as the $600 million debt and $500 million preferred stock issuances in the first quarter of 2018. The increase in portfolio yields was a result of our decision to increase the portfolio duration from 2.3 years at the end of the second quarter 2017 to 2.6 years at the end of the second quarter 2018 and a general rise in interest rates. Trading Securities At June 30, 2018 and 2017 , and December 31, 2017 , we did not hold any trading securities and did not have any net realized gains (losses) on trading securities for the three and six months ended June 30, 2018 and 2017 . Derivative Instruments At June 30, 2018 and 2017 , and December 31, 2017 , we had no open derivative positions. During March 2017, we entered into a forecasted debt issuance hedge, against a possible rise in interest rates, in conjunction with the $850 million of 4.125% Senior Notes due 2047 issued in April 2017 . Upon issuance, we closed the hedge and recognized, as part of accumulated other comprehensive income, a pretax unrealized loss of $8.0 million in April 2017. See Note 4 – Debt for further discussion.

Fair Value

Fair Value	6 Months Ended
	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Fair Value	Fair Value — We have categorized our financial instruments, based on the degree of subjectivity inherent in the method by which they are valued, into a fair value hierarchy of three levels, as follows: • Level 1 : Inputs are unadjusted quoted prices in active markets for identical instruments at the measurement date (e.g., U.S. government obligations, which are continually priced on a daily basis, active exchange-traded equity securities, and certain short-term securities). • Level 2 : Inputs (other than quoted prices included within Level 1) that are observable for the instrument either directly or indirectly (e.g., certain corporate and municipal bonds and certain preferred stocks). This includes: (i) quoted prices for similar instruments in active markets, (ii) quoted prices for identical or similar instruments in markets that are not active, (iii) inputs other than quoted prices that are observable for the instruments, and (iv) inputs that are derived principally from or corroborated by observable market data by correlation or other means. • Level 3 : Inputs that are unobservable. Unobservable inputs reflect our subjective evaluation about the assumptions market participants would use in pricing the financial instrument (e.g., certain structured securities and privately held investments). Determining the fair value of the investment portfolio is the responsibility of management. As part of the responsibility, we evaluate whether a market is distressed or inactive in determining the fair value for our portfolio. We review certain market level inputs to evaluate whether sufficient activity, volume, and new issuances exist to create an active market. Based on this evaluation, we concluded that there was sufficient activity related to the sectors and securities for which we obtained valuations. The composition of the investment portfolio by major security type and our outstanding debt was: Fair Value (millions) Level 1 Level 2 Level 3 Total Cost June 30, 2018 Fixed maturities: U.S. government obligations $ 7,865.4 $ 0 $ 0 $ 7,865.4 $ 8,005.8 State and local government obligations 0 1,667.3 0 1,667.3 1,678.9 Foreign government obligations 0 0 0 0 0 Corporate debt securities 0 7,330.3 0 7,330.3 7,422.4 Subtotal 7,865.4 8,997.6 0 16,863.0 17,107.1 Asset-backed securities: Residential mortgage-backed 0 822.6 0 822.6 819.6 Commercial mortgage-backed 0 2,696.9 0 2,696.9 2,725.5 Other asset-backed 0 3,177.6 0 3,177.6 3,189.2 Subtotal asset-backed securities 0 6,697.1 0 6,697.1 6,734.3 Redeemable preferred stocks: Financials 0 67.7 0 67.7 65.3 Utilities 0 4.6 0 4.6 4.8 Industrials 10.1 146.7 0 156.8 150.3 Subtotal redeemable preferred stocks 10.1 219.0 0 229.1 220.4 Total fixed maturities 7,875.5 15,913.7 0 23,789.2 24,061.8 Short-term investments 2,954.2 277.0 0 3,231.2 3,231.2 Total available-for-sale securities 10,829.7 16,190.7 0 27,020.4 27,293.0 Equity securities: Nonredeemable preferred stocks: Financials 77.4 676.2 0 753.6 672.0 Industrials 0 0 5.0 5.0 5.0 Subtotal nonredeemable preferred stocks 77.4 676.2 5.0 758.6 677.0 Common equities: Common stocks 3,141.9 0 0 3,141.9 1,313.7 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 3,141.9 0 0.3 3,142.2 1,314.0 Total equity securities 3,219.3 676.2 5.3 3,900.8 1,991.0 Total portfolio $ 14,049.0 $ 16,866.9 $ 5.3 $ 30,921.2 $ 29,284.0 Debt $ 0 $ 3,959.0 $ 0 $ 3,959.0 $ 3,859.5 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost June 30, 2017 Fixed maturities: U.S. government obligations $ 4,155.0 $ 0 $ 0 $ 4,155.0 $ 4,166.4 State and local government obligations 0 2,500.1 0 2,500.1 2,473.3 Foreign government obligations 22.5 0 0 22.5 22.5 Corporate debt securities 0 5,013.8 0 5,013.8 4,991.6 Subtotal 4,177.5 7,513.9 0 11,691.4 11,653.8 Asset-backed securities: Residential mortgage-backed 0 1,267.6 0 1,267.6 1,260.3 Commercial mortgage-backed 0 2,375.0 0 2,375.0 2,364.9 Other asset-backed 0 2,847.4 0 2,847.4 2,843.3 Subtotal asset-backed securities 0 6,490.0 0 6,490.0 6,468.5 Redeemable preferred stocks: Financials 0 64.1 0 64.1 60.4 Utilities 0 32.2 0 32.2 30.5 Industrials 0 110.4 0 110.4 98.5 Subtotal redeemable preferred stocks 0 206.7 0 206.7 189.4 Total fixed maturities 4,177.5 14,210.6 0 18,388.1 18,311.7 Equity securities: Nonredeemable preferred stocks: Financials 85.3 697.8 0 783.1 666.0 Industrials 0 0 0 0 0 Subtotal nonredeemable preferred stocks 85.3 697.8 0 783.1 666.0 Common equities: Common stocks 3,077.2 0 0 3,077.2 1,472.0 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 3,077.2 0 0.3 3,077.5 1,472.3 Total fixed maturities and equity securities 7,340.0 14,908.4 0.3 22,248.7 20,450.0 Short-term investments 2,448.0 1,281.7 0 3,729.7 3,729.7 Total portfolio $ 9,788.0 $ 16,190.1 $ 0.3 $ 25,978.4 $ 24,179.7 Debt $ 0 $ 3,564.4 $ 114.8 $ 3,679.2 $ 3,383.4 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 June 30, 2018 , vendor-quoted prices represented 72% of our Level 1 classifications (excluding short-term investments), compared to 58% and 66% at June 30, 2017 and December 31, 2017 , respectively. The securities quoted by vendors in Level 1 primarily represent our holdings in U.S. Treasury Notes, which are frequently traded, and the quotes are considered similar to exchange-traded quotes. The balance of our Level 1 pricing comes from quotes obtained directly from trades made on active exchanges. At June 30, 2018 and 2017 , and December 31, 2017 , vendor-quoted prices comprised 98% , 99% , and 98% , respectively, of our Level 2 classifications (excluding short-term investments), while dealer-quoted prices represented 2% , 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 30 days or less to redemption, we believe that acquisition price is the best estimate of fair value. We also review data assumptions as supplied by our sources to determine if that data is relevant to current market conditions. In addition, we independently review each sector for transaction volumes, new issuances, and changes in spreads, as well as the overall movement of interest rates along the yield curve to determine if sufficient activity and liquidity exists to provide a credible source for our market valuations. During each valuation period, we create internal estimations of portfolio valuation (performance returns), based on current market-related activity (i.e., interest rate and credit spread movements and other credit-related factors) within each major sector of our portfolio. We compare our internally generated portfolio results with those generated based on quotes we receive externally and research material valuation differences. We compare our results to index returns for each major sector adjusting for duration and credit quality differences to better understand our portfolio’s results. Additionally, we review on a monthly basis our external sales transactions and compare the actual final market sales prices to previous market valuation prices. This review provides us further validation that our pricing sources are providing market level prices, since we are able to explain significant price changes (i.e., greater than 2%) as known events occur in the marketplace and affect a particular security’s price at sale. This analysis provides us with additional comfort regarding the source’s process, the quality of its review, and its willingness to improve its analysis based on feedback from clients. We believe this effort helps ensure that we are reporting the most representative fair values for our securities. Except as described below, our Level 3 securities are also priced externally; however, due to several factors (e.g., nature of the securities, level of activity, and lack of similar securities trading to obtain observable market level inputs), these valuations are more subjective in nature. Certain private equity investments and fixed-income investments included in the Level 3 category are valued using external pricing supplemented by internal review and analysis. After all the valuations are received and our review is complete, if the inputs used by vendors are determined to not contain sufficient observable market information, we will reclassify the affected security valuations to Level 3. At June 30, 2018 and 2017 , and December 31, 2017 , we did not have any securities in our fixed-maturity portfolio listed as Level 3. At June 30, 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 June 30, 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 and six months ended June 30, 2018 and 2017 . Due to the relative size of the Level 3 securities’ fair values compared to the total portfolio’s fair value, any changes in pricing methodology would not have a significant change in valuation that would materially impact net or comprehensive income.

Debt

Debt	6 Months Ended
	Jun. 30, 2018
Debt Disclosure [Abstract]
Debt	Debt — Debt at each of the balance sheet periods consisted of: June 30, 2018 June 30, 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 $ 506.4 $ 498.6 $ 528.0 $ 498.8 $ 520.7 2.45% Senior Notes due 2027 496.3 451.3 496.0 474.2 496.1 477.9 6 5/8% Senior Notes due 2029 296.2 364.4 296.0 389.0 296.1 382.3 6.25% Senior Notes due 2032 395.4 492.5 395.2 522.7 395.3 516.9 4.35% Senior Notes due 2044 346.6 356.7 346.5 376.5 346.5 388.7 3.70% Senior Notes due 2045 395.3 363.9 395.2 389.7 395.2 402.9 4.125% Senior Notes due 2047 841.3 831.5 841.1 884.3 841.2 917.1 4.20% Senior Notes due 2048 589.5 592.3 0 0 0 0 Other debt instruments 0 0 114.8 114.8 37.1 37.1 Total $ 3,859.5 $ 3,959.0 $ 3,383.4 $ 3,679.2 $ 3,306.3 $ 3,643.6 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 underwriters’ discounts, commissions, and other issuance costs, were approximately $589.5 million and $841.1 million , respectively. In addition, upon issuance of the 4.125% Senior Notes, we closed a forecasted debt issuance hedge, which was entered into to hedge against a possible rise in interest rates, and recognized an $8.0 million pretax unrealized loss as part of accumulated other comprehensive income (loss), which is being amortized as an adjustment to interest expense over the life of the 4.125% Senior Notes. Consistent with the other senior notes issued by Progressive, interest on the 4.20% Senior Notes and 4.125% Senior Notes is payable semiannually and both are redeemable, in whole or in part, at any time. The Progressive Corporation Line of Credit During the second quarter 2018, The Progressive Corporation entered into a new line of credit with PNC Bank, National Association (PNC) in the maximum principal amount of $250 million . The prior line of credit, entered into in April 2017, had expired. The line of credit is on the same terms and conditions as the previous line of credit. Subject to the terms and conditions of the line of credit documents, advances under the line of credit (if any) will bear interest at a variable rate equal to the higher of PNC’s Prime Rate or the sum of the Federal Funds Open Rate plus 50 basis points. Each advance must be repaid on the 30 th day after the advance or, if earlier, on April 30, 2019, the expiration date of the line of credit. Prepayments are permitted without penalty. The line of credit is uncommitted and, as such, all advances are subject to PNC’s discretion. We had no borrowings under either line of credit during the first six months of 2018 or throughout 2017.

Income Taxes

Income Taxes	6 Months Ended
	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes — Deferred tax assets and liabilities are recorded based on the difference between the financial statement and tax bases of assets and liabilities at the enacted tax rates. We review our deferred tax assets regularly for recoverability. At June 30, 2018 and 2017 , and December 31, 2017 , we determined that we did not need a valuation allowance on our gross deferred tax assets. Although realization of the deferred tax assets is not assured, management believes that it is more likely than not the deferred tax assets will be realized based on our expectation that we will be able to fully utilize the deductions that are ultimately recognized for tax purposes. For deferred taxes on items that are reported in accumulated other comprehensive income, our policy is to release the income tax effects related to these items on an aggregate portfolio approach. For this purpose, we consider our available-for-sale fixed maturity securities and hedges on forecasted transactions as separate portfolios. For the six months ended June 30, 2018 , there have been no material changes in our uncertain tax positions. The effective tax rate for the three and six months ended June 30, 2018 were 20.3% and 20.1% respectively, compared to 32.8% and 32.9% for the same periods in 2017. The current year effective tax rate is lower primarily due to the reduction in the U.S. corporate income tax rate from 35% to 21%, effective January 1, 2018. As of June 30, 2018, we have not fully completed our accounting for the tax effects of the enactment of the legislation commonly known as the Tax Cuts and Jobs Act of 2017 with regard to the deductibility of compensation expense for certain covered executives due to uncertainty surrounding the appropriate tax treatment of outstanding performance-based awards, and 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	6 Months Ended
	Jun. 30, 2018
Loss and Loss Adjustment Expenses Reserves [Abstract]
Loss and Loss Adjustment Expenses Reserves	Loss and Loss Adjustment Expense Reserves — Activity in the loss and loss adjustment expense reserves is summarized as follows: June 30, (millions) 2018 2017 Balance, Beginning of period $ 13,086.9 $ 11,368.0 Less reinsurance recoverables on unpaid losses 2,170.1 1,801.0 Net balance, Beginning of period 10,916.8 9,567.0 Incurred related to: Current year 10,164.6 8,804.1 Prior years 81.5 74.2 Total incurred 10,246.1 8,878.3 Paid related to: Current year 5,533.7 4,951.0 Prior years 3,851.5 3,381.8 Total paid 9,385.2 8,332.8 Net balance, End of period 11,777.7 10,112.5 Plus reinsurance recoverables on unpaid losses 2,293.1 1,947.9 Balance, End of period $ 14,070.8 $ 12,060.4 We experienced unfavorable reserve development of $81.5 million and $74.2 million for the first six months of 2018 and 2017, respectively, which is reflected as “Incurred related to prior years ” in the table above. Year-to-date June 30, 2018 • Approximately $72 million of the unfavorable prior year reserve development was attributable to accident years 2017 and 2016. • Our personal auto business incurred about $57 million of unfavorable loss and loss adjustment expense (LAE) reserve development, with the Agency and Direct auto businesses contributing about $36 million and $21 million , respectively, of unfavorable development. The unfavorable development was primarily due to an increase in reopened personal injury protection (PIP) claims. • Our Commercial Lines business experienced about $17 million of unfavorable development primarily due to late reported losses and higher LAE than anticipated. • Our Property business recognized unfavorable development of about $7 million , while our special lines products had minimal development during the first half of the year. Year-to-date June 30, 2017 • Approximately $56 million of the unfavorable prior year reserve development was attributable to accident year 2016 with the balance attributable to accident year 2015. • Our commercial and personal auto businesses incurred about $102 million of unfavorable loss and LAE reserve development for the first six months of 2017, partially offset by the favorable loss and LAE reserve development of about $28 million in our special lines and Property businesses. • Our Agency and Direct personal auto businesses incurred about $65 million and $29 million , respectively, of the total unfavorable reserve development, primarily due to an increase in costs related to property damage, more late reported bodily injury claims than anticipated, and higher LAE costs. • Our Property business experienced about $20 million in favorable development primarily due to the identification of prior year losses eligible to be ceded under our catastrophe bond reinsurance program.

Supplemental Cash Flow Informat

Supplemental Cash Flow Information	6 Months Ended
	Jun. 30, 2018
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information	Supplemental Cash Flow Information — Cash and cash equivalents include bank demand deposits and daily overnight reverse repurchase commitments of funds held in bank demand deposit accounts on ARX’s subsidiaries, which are primarily collateralized by U.S. Treasury notes. The amount of reverse repurchase commitments held by ARX’s subsidiaries at June 30, 2018 and 2017 , and December 31, 2017 , were $155.9 million , $130.2 million , and $247.2 million , respectively. Restricted cash on our consolidated balance sheets represents cash that is restricted to pay flood claims under the National Flood Insurance Program’s “Write Your Own” program, for which American Strategic Insurance and other subsidiaries of ARX (ASI) are administrators. We paid the following in the respective periods: Six Months Ended June 30, (millions) 2018 2017 Income taxes $ 358.9 $ 452.9 Interest 70.9 73.3 Income taxes paid for the six months ended June 30, 2017, was revised to correct the amount of tax payments disclosed for that period; there was no overall impact on the decrease in cash, cash equivalents, and restricted cash that was reported in our consolidated statement of cash flows for the six months ended June 30, 2017.

Segment Information

Segment Information	6 Months Ended
	Jun. 30, 2018
Segment Reporting [Abstract]
Segment Information	Segment Information — Our Personal Lines segment writes insurance for personal autos and recreational vehicles (our special lines products). Our Commercial Lines segment writes primary liability and physical damage insurance for automobiles and trucks owned and/or operated predominantly by small businesses in the business auto, for-hire transportation, contractor, for-hire specialty, tow, and for-hire livery markets. Our Property segment writes residential property insurance for homeowners, other property owners, and renters. Our other indemnity businesses include our run-off businesses. Our service businesses provide insurance-related services, including processing Commercial Automobile Insurance Procedures/Plans (CAIP) business and serving as an agent for homeowners, general liability, and workers’ compensation insurance, among other products, through our programs with ASI and unaffiliated insurance companies. All segment revenues are generated from external customers; all intercompany transactions, including those between Progressive and ASI, are eliminated in consolidation. Following are the operating results for the respective periods: Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 (millions) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Personal Lines Agency $ 3,225.7 $ 360.9 $ 2,752.5 $ 205.5 $ 6,289.5 $ 770.0 $ 5,384.0 $ 455.2 Direct 3,211.8 287.9 2,650.0 162.7 6,228.1 585.9 5,173.7 338.0 Total Personal Lines 1 6,437.5 648.8 5,402.5 368.2 12,517.6 1,355.9 10,557.7 793.2 Commercial Lines 884.3 100.3 671.7 56.2 1,692.9 195.1 1,317.2 123.6 Property 2 312.4 (51.9 ) 239.1 3.7 597.7 (23.4 ) 465.1 11.5 Other indemnity 0 0 0 (0.1 ) 0 0.2 0 (0.3 ) Total underwriting operations 7,634.2 697.2 6,313.3 428.0 14,808.2 1,527.8 12,340.0 928.0 Fees and other revenues 3 116.0 NA 88.8 NA 219.8 NA 174.0 NA Service businesses 42.9 5.9 32.7 5.7 77.1 10.8 61.2 8.3 Investments 4 224.9 218.7 170.9 164.3 343.0 330.8 352.0 339.8 Other gains (losses) 0 0 0 0 0 0 0.2 0.2 Interest expense NA (41.7 ) NA (43.4 ) NA (78.5 ) NA (80.2 ) Consolidated total $ 8,018.0 $ 880.1 $ 6,605.7 $ 554.6 $ 15,448.1 $ 1,790.9 $ 12,927.4 $ 1,196.1 NA = Not applicable 1 Personal auto insurance accounted for 94% of the total Personal Lines segment net premiums earned during the three and six months ended June 30, 2018 , and 93% for the same periods in 2017 ; insurance for our special lines products (e.g., motorcycles, ATVs, RVs, watercraft, and snowmobiles) accounted for the balance of the Personal Lines net premiums earned. 2 For the three and six months ended June 30, 2018 , pretax profit (loss) includes $18.0 million and $36.0 million , respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX and $15.5 million and $31.0 million for the same periods in 2017 . Although this expense is included in our Property segment, it is not reported in the consolidated results of ARX and, therefore, will not affect the value of net income (loss) attributable to noncontrolling interest. 3 Pretax profit (loss) for fees and other revenues is attributable to operating segments. 4 Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit is net of investment expense. Our management uses underwriting margin and combined ratio as primary measures of underwriting profitability. Underwriting profitability is calculated by subtracting losses and loss adjustment expenses, policy acquisition costs, and other underwriting expenses from the total of net premiums earned and fees and other revenues. The underwriting margin is the pretax underwriting profit (loss) expressed as a percentage of net premiums earned (i.e., revenues from underwriting operations). Combined ratio is the complement of the underwriting margin. Following are the underwriting margins/combined ratios for our underwriting operations for the respective periods: Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Under-writing Combined Ratio Under-writing Combined Ratio Under-writing Margin Combined Ratio Under-writing Margin Combined Ratio Personal Lines Agency 11.2 % 88.8 7.5 % 92.5 12.2 % 87.8 8.5 % 91.5 Direct 9.0 91.0 6.1 93.9 9.4 90.6 6.5 93.5 Total Personal Lines 10.1 89.9 6.8 93.2 10.8 89.2 7.5 92.5 Commercial Lines 11.3 88.7 8.4 91.6 11.5 88.5 9.4 90.6 Property 1 (16.6 ) 116.6 1.5 98.5 (3.9 ) 103.9 2.5 97.5 Total underwriting operations 9.1 90.9 6.8 93.2 10.3 89.7 7.5 92.5 1 Included in the three and six months ended June 30, 2018 is 5.8 points and 6.0 points, respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX and 6.5 points and 6.7 points, respectively, for the three and six months ended June 30, 2017 .

Dividends

Dividends	6 Months Ended
	Jun. 30, 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 (using the statutory tax rate) 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. 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 June 30, 2018 , the Gainshare factor was 1.91 . Since the final factor will be determined based on our results for the full year, the final factor may vary from the current factor. Our 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 of Common Share Dividends Dividend Type Declared Paid Per Share Accrued 1 Paid 1 Annual – Variable December 2017 February 2018 $ 1.1247 $ 655.1 $ 654.9 Annual – Variable December 2016 February 2017 0.6808 395.4 395.4 1 Variance between accrued and paid, if any, reflects the difference between the number of estimated and actual shares outstanding as of the record date. Preferred Share Dividends In the first quarter 2018, we issued 500,000 Series B Fixed-to-Floating Rate Cumulative Perpetual Serial Preferred Shares, without par value (the “Series B Preferred Shares”), with a liquidation preference of $1,000 per share (the “stated amount”). Holders of the Series B Preferred Shares will be entitled to receive cumulative cash dividends semi-annually in March and September, if and when declared by the Board of Directors. Until March 15, 2023 (the “fixed-rate period”), the annual dividend rate is fixed at 5.375% of the stated amount per share. Beginning March 15, 2023, the annual dividend rate switches to a floating rate equal to the three-month LIBOR rate plus a spread of 2.539% applied to the stated amount per share. After the fixed-rate period and up until redemption of the Series B Preferred Shares, the dividends would be payable quarterly, if and when declared by the Board of Directors. The Series B Preferred Shares are perpetual and have no stated maturity date. After the fixed-rate period, we may redeem the Series B Preferred Shares at the stated amount plus all accrued and unpaid dividends.

Other Comprehensive Income (Los

Other Comprehensive Income (Loss)	6 Months Ended
	Jun. 30, 2018
Equity [Abstract]
Other Comprehensive Income (Loss)	Other Comprehensive Income (Loss) — The components of other comprehensive income (loss), including reclassification adjustments by income statement line item, were as follows: Components of Changes in Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income 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 March 31, 2018 $ (218.6 ) $ 46.0 $ (172.6 ) $ (160.8 ) $ (17.8 ) $ 0 $ 6.0 Other comprehensive income (loss) before reclassifications: Investment securities (90.4 ) 18.8 (71.6 ) (71.6 ) 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) 0.7 (0.1 ) 0.6 0 0 0 0.6 Total other comprehensive income (loss) before reclassifications (89.7 ) 18.7 (71.0 ) (71.6 ) 0 0 0.6 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 (27.2 ) 5.7 (21.5 ) (21.5 ) 0 0 0 Interest expense (0.3 ) 0.1 (0.2 ) 0 (0.2 ) 0 0 Total reclassification adjustment for amounts realized in net income (27.5 ) 5.8 (21.7 ) (21.5 ) (0.2 ) 0 0 Total other comprehensive income (loss) (62.2 ) 12.9 (49.3 ) (50.1 ) 0.2 0 0.6 Balance at June 30, 2018 $ (280.8 ) $ 58.9 $ (221.9 ) $ (210.9 ) $ (17.6 ) $ 0 $ 6.6 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 (293.8 ) 61.6 (232.2 ) (232.2 ) 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) 5.8 (1.2 ) 4.6 0 0 0 4.6 Total other comprehensive income (loss) before reclassifications (288.0 ) 60.4 (227.6 ) (232.2 ) 0 0 4.6 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 (34.9 ) 7.3 (27.6 ) (27.6 ) 0 0 0 Interest expense (0.5 ) 0.1 (0.4 ) 0 (0.4 ) 0 0 Total reclassification adjustment for amounts realized in net income (35.4 ) 7.4 (28.0 ) (27.6 ) (0.4 ) 0 0 Total other comprehensive income (loss) (252.6 ) 53.0 (199.6 ) (204.6 ) 0.4 0 4.6 Balance at June 30, 2018 $ (280.8 ) $ 58.9 $ (221.9 ) $ (210.9 ) $ (17.6 ) $ 0 $ 6.6 1 Reflects the fair value changes on equity securities as of December 31, 2017 , which are reported as realized gains (losses) under the new accounting guidance. See Note 14 – New Accounting Standards for additional information. 2 Reflects the effect of the change in the U.S. federal tax rate on our available-for-sale fixed maturity securities and our hedges on forecasted transactions as of December 31, 2017 . See Note 14 – New Accounting Standards for additional information. 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 March 31, 2017 $ 1,645.9 $ (578.4 ) $ 1,067.5 $ 1,072.0 $ (7.1 ) $ (0.8 ) $ 3.4 Other comprehensive income (loss) before reclassifications: Investment securities 177.2 (62.1 ) 115.1 115.1 0 0 0 Forecasted transactions (12.0 ) 4.2 (7.8 ) 0 (7.8 ) 0 0 Foreign currency translation adjustment (0.2 ) 0.1 (0.1 ) 0 0 (0.1 ) 0 Loss attributable to noncontrolling interest (NCI) (1.9 ) 0.6 (1.3 ) 0 0 0 (1.3 ) Total other comprehensive income (loss) before reclassifications 163.1 (57.2 ) 105.9 115.1 (7.8 ) (0.1 ) (1.3 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (2.6 ) 0.9 (1.7 ) (1.7 ) 0 0 0 Net realized gains (losses) on securities 37.2 (13.0 ) 24.2 24.2 0 0 0 Interest expense 0.3 (0.1 ) 0.2 0 0.2 0 0 Total reclassification adjustment for amounts realized in net income 34.9 (12.2 ) 22.7 22.5 0.2 0 0 Total other comprehensive income (loss) 128.2 (45.0 ) 83.2 92.6 (8.0 ) (0.1 ) (1.3 ) Balance at June 30, 2017 $ 1,774.1 $ (623.4 ) $ 1,150.7 $ 1,164.6 $ (15.1 ) $ (0.9 ) $ 2.1 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 431.7 (151.3 ) 280.4 280.4 0 0 0 Forecasted transactions (8.0 ) 2.8 (5.2 ) 0 (5.2 ) 0 0 Foreign currency translation adjustment 0.3 (0.1 ) 0.2 0 0 0.2 0 Loss attributable to noncontrolling interest (NCI) (3.4 ) 1.2 (2.2 ) 0 0 0 (2.2 ) Total other comprehensive income (loss) before reclassifications 420.6 (147.4 ) 273.2 280.4 (5.2 ) 0.2 (2.2 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (3.6 ) 1.3 (2.3 ) (2.3 ) 0 0 0 Net realized gains (losses) on securities 88.8 (31.1 ) 57.7 57.7 0 0 0 Interest expense 0.8 (0.3 ) 0.5 0 0.5 0 0 Total reclassification adjustment for amounts realized in net income 86.0 (30.1 ) 55.9 55.4 0.5 0 0 Total other comprehensive income (loss) 334.6 (117.3 ) 217.3 225.0 (5.7 ) 0.2 (2.2 ) Balance at June 30, 2017 $ 1,774.1 $ (623.4 ) $ 1,150.7 $ 1,164.6 $ (15.1 ) $ (0.9 ) $ 2.1 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	6 Months Ended
	Jun. 30, 2018
Commitments and Contingencies Disclosure [Abstract]
Litigation	Litigation — The Progressive Corporation and/or its insurance subsidiaries are named as defendants in various lawsuits arising out of claims made under insurance policies written by our insurance subsidiaries in the ordinary course of business. We consider all legal actions relating to such claims in establishing our loss and loss adjustment expense reserves. In addition, The Progressive Corporation and/or its subsidiaries are named as defendants in a number of class action or individual lawsuits that challenge certain of the operations of the subsidiaries. These cases include those alleging damages as a result of our subsidiaries’ practices in evaluating or paying medical or injury claims or benefits, including, but not limited to, personal injury protection, medical payments, and bodily injury benefits; the utilization, content, or appearance of policy documents; labor rates paid to auto body repair shops; wage and hour issues; and cases challenging other aspects of our subsidiaries’ claims, marketing, or sales practices, or other business operations. Other insurance companies face many of these same issues. The nature and volume of litigation to which The Progressive Corporation is subject is similar to that which was disclosed in Note 12 – Litigation in our 2017 Annual Report to Shareholders. We plan to contest the pending lawsuits vigorously, but may pursue settlement negotiations in some cases, if appropriate. The outcomes of pending cases are uncertain at this time. We establish accruals for these lawsuits when it is probable that a loss has been or will be incurred and we can reasonably estimate potential loss exposure, which may include a range of loss. As to lawsuits for which the loss is considered neither probable or estimable, or is considered probable but not estimable, we do not establish an accrual. Nevertheless, we continue to evaluate this pending litigation to determine if any losses not deemed probable and estimable become so, at which point we would establish an accrual at our best estimate of the loss or range of loss. With respect to our pending lawsuits that are not related to claims under insurance policies, the accruals that we have established, if any, were not material at June 30, 2018 or 2017, and there were no material settlements during the first six months of 2018 or 2017. For most of these lawsuits, we do not consider any losses to be both probable and estimable, and we are unable to estimate a meaningful range of loss, if any, at this time, due to the factors discussed in Note 12 – Litigation in our 2017 Annual Report to Shareholders. In the event that any one or more of these lawsuits results in a substantial judgment against or settlement by us, or if our accruals (if any) prove to be inadequate by a significant amount, the resulting liability could have a material adverse effect on our consolidated financial condition, cash flows, and/or results of operations. For a further discussion on our pending litigation and related reserving policies, see Note 12 – Litigation in our 2017 Annual Report to Shareholders.

Redeemable Noncontrolling Inter

Redeemable Noncontrolling Interest	6 Months Ended
	Jun. 30, 2018
Temporary Equity Disclosure [Abstract]
Redeemable Noncontrolling Interest	Redeemable Noncontrolling Interest — In connection with the April 2015 acquisition of a controlling interest in ARX, The Progressive Corporation entered into a stockholders’ agreement with the other ARX stockholders. As part of the stockholders’ agreement, the minority ARX stockholders had the right to “put” a portion of their ARX shares to Progressive in 2018, and have the right to put all of their remaining shares to Progressive in 2021. During the second quarter 2018, the minority ARX stockholders put 204,527 shares, including 5,483 shares that were issued upon the exercise of outstanding stock options, bringing Progressive’s total ownership interest in ARX to 86.7% . Progressive acquired these additional shares, in a cash transaction, for a total cost of $295.9 million . If ARX stockholders do not put all of their shares to Progressive in 2021, Progressive has the ability to “call ” all of the outstanding shares shortly thereafter and to bring its ownership stake to 100% in 2021. See Note 15 – Redeemable Noncontrolling Interest in our 2017 Annual Report to Shareholders for a discussion of the purchase price for shares to be purchased by Progressive pursuant to these put or call rights. 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 June 30, 2018 , ARX employees held options to purchase 16,067 ARX shares. These options and any shares issued upon exercise are subject to the stockholders’ agreement, including the “put ” and “call ” rights described above. Until the options are exercised, the underlying obligation of approximately $23.2 million is not recorded as part of redeemable NCI. See Note 9 – Employee Benefit Plans in our 2017 Annual Report to Shareholders for a discussion of our employee stock options. The changes in the components of redeemable NCI during the six months ended June 30, 2018 and 2017 , and the year ended December 31, 2017 , were: (millions) June 30, 2018 June 30, 2017 December 31, 2017 Balance, Beginning of period $ 503.7 $ 483.7 $ 483.7 Net income attributable to NCI 8.8 11.1 5.9 Other comprehensive income (loss) attributable to NCI (4.6 ) 2.2 2.3 Exercise of employee stock options 9.4 3.4 3.4 Purchase of ARX shares (295.9 ) 0 0 Change in redemption value of NCI (3.2 ) 1.4 8.4 Balance, End of period $ 218.2 $ 501.8 $ 503.7

Goodwill and Intangible Assets

Goodwill and Intangible Assets	6 Months Ended
	Jun. 30, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	Goodwill and Intangible Assets Goodwill During the six months ended June 30, 2018 , there were no changes to the carrying amount of goodwill. Goodwill recorded at June 30, 2018 and 2017, and December 31, 2017 was $452.7 million . No accumulated goodwill impairment losses exist. Intangible Assets The following table is a summary of the net carrying amount of other intangible assets as of June 30, 2018 and 2017 , and December 31, 2017 : (millions) June 30, 2018 June 30, 2017 December 31, 2017 Intangible assets subject to amortization $ 318.2 $ 389.4 $ 354.2 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 330.6 $ 401.8 $ 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) June 30, 2018 June 30, 2017 December 31, 2017 Category Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Policies in force $ 256.2 $ 119.0 $ 137.2 $ 256.2 $ 82.4 $ 173.8 $ 256.2 $ 100.7 $ 155.5 Agency relationships 159.2 37.0 122.2 159.2 25.6 133.6 159.2 31.3 127.9 Software rights 79.1 34.7 44.4 79.1 24.1 55.0 79.1 29.4 49.7 Trade name 34.8 20.4 14.4 34.8 7.8 27.0 34.8 13.7 21.1 Total $ 529.3 $ 211.1 $ 318.2 $ 529.3 $ 139.9 $ 389.4 $ 529.3 $ 175.1 $ 354.2 Amortization expense was $18.0 million and $36.0 million for the three and six months ended June 30, 2018 , respectively, compared to $15.5 million and $31.0 million during the same periods last year. During the third quarter 2017, we revised our estimate of the economic useful life of our trade name intangible asset from an original life of 10 years to a remaining life of 2 years . The decrease in the useful life represents the estimated length of time that it is expected to take to transition the branding of our Property products from the ASI trade name to the Progressive brand. As of June 30, 2018 , the remaining average life of all of our intangible assets is 4.4 years .

New Accounting Standards

New Accounting Standards	6 Months Ended
	Jun. 30, 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 June 30, 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 fair value of goodwill in measuring an impairment loss. Upon adoption, the measurement of a goodwill impairment will represent the excess of the reporting unit’s carrying value over fair value, limited to the carrying value of goodwill. This ASU is effective for goodwill impairment tests in fiscal years beginning after December 15, 2019 (2020 for calendar-year companies), with early adoption permitted. We do not expect this standard to have a material impact on our financial position or results of operations. In June 2016, the FASB issued an ASU intended to improve the timing, and enhance the accounting and disclosure, of credit losses on financial assets. Additionally, this update will modify the existing accounting guidance related to the impairment evaluation for available-for-sale debt securities and will result in the creation of an allowance for credit losses as a contra asset account. The ASU will require cumulative-effect changes to retained earnings in the period of adoption, if any occur, and will also require prospective changes on previously recorded impairments. This ASU is effective for fiscal years (including interim periods within those fiscal years) beginning after December 15, 2019 (2020 for calendar-year companies), with early adoption permissible (including interim periods within that fiscal year) beginning after December 15, 2018 (2019 for calendar-year companies). While the ASU creates additional accounting complexities related to the recognition of the impairment losses, and subsequent recoveries, through an allowance for credit losses account, we do not expect the ASU to have a material impact on our current method of evaluating securities for credit losses or the timing or recognition of the amounts of the impairment losses. Adopted On January 1, 2018, we adopted the ASU intended to improve the recognition and measurement of financial instruments. Under this update, the changes in fair value of equity securities are recognized as a component of net income. Upon adoption, we recorded a cumulative-effect adjustment of $1.3 billion , which is net of taxes. The cumulative-effect adjustment represents the amount of after-tax net unrealized gains on equity securities that was recorded as part of accumulated other comprehensive income at December 31, 2017. The adoption of this ASU had no impact on comprehensive income. Consistent with our historical presentation, cash flows on equity securities will be reflected as investing activities in the Consolidated Statements of Cash Flows. In the first quarter 2018, we adopted the ASU related to the reclassification of certain tax effects from accumulated other comprehensive income. This update provided companies with the option to reclassify disproportionate tax effects in accumulated other comprehensive income caused by the legislation commonly known as the Tax Cuts and Jobs Act of 2017 to retained earnings. We opted to make the reclassification, which resulted in a decrease to accumulated other comprehensive income and an offsetting increase to retained earnings of $4.3 million . This reclass was solely due to the effect of the change in the U.S. federal tax rate on our available-for-sale fixed-maturity securities and our hedges on forecasted transactions. There were no disproportionate tax effects related to our equity securities subsequent to adopting the ASU related to classification and measurement discussed above.

Reclassification (Notes)

Reclassification (Notes)	6 Months Ended
	Jun. 30, 2018
Accounting Policies [Abstract]
Reclassifications [Text Block]	Reclassification — For the three and six months ended June 30, 2017, we reclassified the change in the net holding period gains (losses) on hybrid securities held at June 30, 2017 to “net holding period gains (losses) on securities” out of “net realized gains (losses) on security sales” to conform with the current-year Consolidated Statements of Comprehensive Income presentation. There was no overall impact on total net realized gains (losses) on securities. We also reclassified the classification and presentation of restricted cash in our Consolidated Statements of Cash Flows in accordance with the accounting guidance we adopted for the year ended December 31, 2017 relating to this item; there was no overall impact on the increase in cash, cash equivalents, and restricted cash as a result of this reclassification. In addition, on our Consolidated Balance Sheets for December 31, 2017, we reclassified our “dividends payable” into “accounts payable, accrued expenses, and other liabilities” to be consistent with the current period presentation.

Investments (Tables)

Investments (Tables)	6 Months Ended
	Jun. 30, 2018
Investments Schedule [Abstract]
Schedule of Debt and Equity Securities Reconciliation [Table Text Block]	The following tables present the composition of our investment portfolio by major security type, consistent with our classification of how we manage, monitor, and measure the portfolio. Our securities are reported in our Consolidated Balance Sheets at fair value, with all of our equity securities having a readily determinable fair value. The changes in fair value for our fixed-maturity securities (other than hybrid securities) are reported as a component of accumulated other comprehensive income, net of deferred income taxes, in our Consolidated Balance Sheets. The net holding period gains (losses) reported below represent the inception-to-date changes in fair value. The changes in the net holding period gains (losses) between periods for the hybrid securities and, beginning in 2018 , equity securities are recorded as a component of net realized gains (losses) on securities in our Consolidated Statements of Comprehensive Income. Prior to 2018, the change in fair value of our equity securities was part of accumulated other comprehensive income (see Note 14 – New Accounting Standards for further discussion). ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value June 30, 2018 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 8,005.8 $ 0.9 $ (141.3 ) $ 0 $ 7,865.4 25.4 % State and local government obligations 1,678.9 4.3 (15.9 ) 0 1,667.3 5.4 Foreign government obligations 0 0 0 0 0 0 Corporate debt securities 7,422.4 4.2 (94.3 ) (2.0 ) 7,330.3 23.7 Residential mortgage-backed securities 819.6 8.4 (5.4 ) 0 822.6 2.7 Commercial mortgage-backed securities 2,725.5 2.9 (31.5 ) 0 2,696.9 8.7 Other asset-backed securities 3,189.2 2.0 (13.7 ) 0.1 3,177.6 10.3 Redeemable preferred stocks 220.4 14.1 (2.0 ) (3.4 ) 229.1 0.7 Total fixed maturities 24,061.8 36.8 (304.1 ) (5.3 ) 23,789.2 76.9 Short-term investments 3,231.2 0 0 0 3,231.2 10.5 Total available-for-sale securities 27,293.0 36.8 (304.1 ) (5.3 ) 27,020.4 87.4 Equity securities: Nonredeemable preferred stocks 677.0 0 0 81.6 758.6 2.4 Common equities 1,314.0 0 0 1,828.2 3,142.2 10.2 Total equity securities 1,991.0 0 0 1,909.8 3,900.8 12.6 Total portfolio 1,2 $ 29,284.0 $ 36.8 $ (304.1 ) $ 1,904.5 $ 30,921.2 100.0 % ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value June 30, 2017 Fixed maturities: U.S. government obligations $ 4,166.4 $ 4.4 $ (15.8 ) $ 0 $ 4,155.0 16.0 % State and local government obligations 2,473.3 32.5 (5.9 ) 0.2 2,500.1 9.7 Foreign government obligations 22.5 0 0 0 22.5 0.1 Corporate debt securities 4,991.6 29.4 (7.6 ) 0.4 5,013.8 19.3 Residential mortgage-backed securities 1,260.3 10.9 (5.9 ) 2.3 1,267.6 4.8 Commercial mortgage-backed securities 2,364.9 19.8 (9.7 ) 0 2,375.0 9.1 Other asset-backed securities 2,843.3 6.4 (2.5 ) 0.2 2,847.4 11.0 Redeemable preferred stocks 189.4 19.0 (1.7 ) 0 206.7 0.8 Total fixed maturities 18,311.7 122.4 (49.1 ) 3.1 18,388.1 70.8 Equity securities: Nonredeemable preferred stocks 666.0 123.6 (6.5 ) 0 783.1 3.0 Common equities 1,472.3 1,611.7 (6.5 ) 0 3,077.5 11.8 Short-term investments 3,729.7 0 0 0 3,729.7 14.4 Total portfolio 1,2 $ 24,179.7 $ 1,857.7 $ (62.1 ) $ 3.1 $ 25,978.4 100.0 % ($ in millions) Cost Gross Unrealized Gains Gross Losses Net (Losses) Fair Value % of Value December 31, 2017 Fixed maturities: U.S. government obligations $ 6,688.8 $ 1.1 $ (44.0 ) $ 0 $ 6,645.9 24.4 % State and local government obligations 2,285.6 20.7 (9.3 ) 0.1 2,297.1 8.4 Foreign government obligations 0 0 0 0 0 0 Corporate debt securities 4,997.2 14.8 (14.4 ) 0.1 4,997.7 18.3 Residential mortgage-backed securities 828.8 11.3 (3.4 ) 0 836.7 3.1 Commercial mortgage-backed securities 2,760.1 11.8 (13.3 ) 0 2,758.6 10.1 Other asset-backed securities 2,454.5 4.5 (4.5 ) 0.2 2,454.7 9.0 Redeemable preferred stocks 194.9 17.8 (1.5 ) (0.2 ) 211.0 0.8 Total fixed maturities 20,209.9 82.0 (90.4 ) 0.2 20,201.7 74.1 Equity securities: Nonredeemable preferred stocks 698.6 114.0 (8.8 ) 0 803.8 2.9 Common equities 1,499.0 1,901.0 (0.2 ) 0 3,399.8 12.5 Short-term investments 2,869.4 0 0 0 2,869.4 10.5 Total portfolio 1,2 $ 25,276.9 $ 2,097.0 $ (99.4 ) $ 0.2 $ 27,274.7 100.0 % 1 Our portfolio reflects the effect of unsettled security transactions; at June 30, 2018 and 2017 , we had $362.1 million and $287.1 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 June 30, 2018 and 2017 , and December 31, 2017 , included $1.7 billion , $1.1 billion , and $1.6 billion , respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company, net of any unsettled security transactions.
Hybrid Instruments Table [Table Text Block]	Included in our fixed-maturity securities are hybrid securities, which are reported at fair value: June 30, (millions) 2018 2017 December 31, 2017 Fixed maturities: State and local government obligations $ 3.6 $ 6.6 $ 6.1 Corporate debt securities 170.3 61.3 99.8 Residential mortgage-backed securities 0 189.7 0 Other asset-backed securities 5.5 7.7 6.7 Redeemable preferred stocks 67.3 0 30.3 Total hybrid securities $ 246.7 $ 265.3 $ 142.9
Investments Classified by Contractual Maturity Date [Table Text Block]	The composition of fixed maturities by maturity at June 30, 2018 , was: (millions) Cost Fair Value Less than one year $ 3,805.2 $ 3,809.2 One to five years 15,190.7 14,999.1 Five to ten years 4,939.7 4,855.4 Ten years or greater 126.2 125.5 Total $ 24,061.8 $ 23,789.2
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 June 30, 2018 Fixed maturities: U.S. government obligations 70 $ 7,503.2 $ (141.3 ) 33 $ 4,543.5 $ (73.6 ) 37 $ 2,959.7 $ (67.7 ) State and local government obligations 355 1,050.9 (15.9 ) 219 631.9 (7.1 ) 136 419.0 (8.8 ) Corporate debt securities 391 6,153.5 (94.3 ) 322 5,439.3 (77.2 ) 69 714.2 (17.1 ) Residential mortgage-backed securities 227 380.3 (5.4 ) 52 188.6 (0.9 ) 175 191.7 (4.5 ) Commercial mortgage-backed securities 142 2,300.9 (31.5 ) 85 1,371.4 (14.4 ) 57 929.5 (17.1 ) Other asset-backed securities 219 2,479.7 (13.7 ) 148 2,055.1 (8.5 ) 71 424.6 (5.2 ) Redeemable preferred stocks 3 30.2 (2.0 ) 1 4.6 (0.2 ) 2 25.6 (1.8 ) Total fixed maturities 1,407 $ 19,898.7 $ (304.1 ) 860 $ 14,234.4 $ (181.9 ) 547 $ 5,664.3 $ (122.2 ) Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses June 30, 2017 Fixed maturities: U.S. government obligations 41 $ 2,781.8 $ (15.8 ) 39 $ 2,725.1 $ (15.1 ) 2 $ 56.7 $ (0.7 ) State and local government obligations 222 818.7 (5.9 ) 138 479.8 (3.3 ) 84 338.9 (2.6 ) Corporate debt securities 143 1,905.2 (7.6 ) 122 1,599.6 (5.3 ) 21 305.6 (2.3 ) Residential mortgage-backed securities 220 535.7 (5.9 ) 126 197.8 (1.1 ) 94 337.9 (4.8 ) Commercial mortgage-backed securities 74 905.6 (9.7 ) 52 679.5 (5.7 ) 22 226.1 (4.0 ) Other asset-backed securities 160 2,077.8 (2.5 ) 145 1,823.8 (1.9 ) 15 254.0 (0.6 ) Redeemable preferred stocks 1 10.8 (1.7 ) 0 0 0 1 10.8 (1.7 ) Total fixed maturities 861 9,035.6 (49.1 ) 622 7,505.6 (32.4 ) 239 1,530.0 (16.7 ) Equity securities: Nonredeemable preferred stocks 3 73.1 (6.5 ) 0 0 0 3 73.1 (6.5 ) Common equities 80 73.1 (6.5 ) 74 72.3 (6.5 ) 6 0.8 0 Total equity securities 83 146.2 (13.0 ) 74 72.3 (6.5 ) 9 73.9 (6.5 ) Total portfolio 944 $ 9,181.8 $ (62.1 ) 696 $ 7,577.9 $ (38.9 ) 248 $ 1,603.9 $ (23.2 ) Total No. of Sec. Total Gross Unrealized Losses Less than 12 Months 12 Months or Greater ($ in millions) No. of Sec. Fair Unrealized Losses No. of Sec. Fair Unrealized Losses December 31, 2017 Fixed maturities: U.S. government obligations 58 $ 5,817.0 $ (44.0 ) 41 $ 4,869.3 $ (34.6 ) 17 $ 947.7 $ (9.4 ) State and local government obligations 358 1,200.3 (9.3 ) 230 737.6 (4.4 ) 128 462.7 (4.9 ) Corporate debt securities 222 2,979.4 (14.4 ) 171 2,072.9 (9.1 ) 51 906.5 (5.3 ) Residential mortgage-backed securities 201 300.9 (3.4 ) 30 75.1 (0.2 ) 171 225.8 (3.2 ) Commercial mortgage-backed securities 105 1,682.3 (13.3 ) 63 1,221.2 (5.9 ) 42 461.1 (7.4 ) Other asset-backed securities 197 1,837.3 (4.5 ) 134 1,377.8 (3.3 ) 63 459.5 (1.2 ) Redeemable preferred stocks 2 21.8 (1.5 ) 1 10.8 (0.1 ) 1 11.0 (1.4 ) Total fixed maturities 1,143 13,839.0 (90.4 ) 670 10,364.7 (57.6 ) 473 3,474.3 (32.8 ) Equity securities: Nonredeemable preferred stocks 4 127.8 (8.8 ) 1 56.5 (0.5 ) 3 71.3 (8.3 ) Common equities 19 13.4 (0.2 ) 18 13.4 (0.2 ) 1 0 0 Total equity securities 23 141.2 (9.0 ) 19 69.9 (0.7 ) 4 71.3 (8.3 ) Total portfolio 1,166 $ 13,980.2 $ (99.4 ) 689 $ 10,434.6 $ (58.3 ) 477 $ 3,545.6 $ (41.1 )
Non Credit Portion Of Other Than Temporary Impairment Recorded In Accumulated Other Comprehensive Income Table [Table Text Block]	The following table shows the total non-credit portion of the OTTI recorded in accumulated other comprehensive income, reflecting the original non-credit loss at the time the credit impairment was determined (i.e., unadjusted for valuation changes subsequent to the original write-down): June 30, December 31, (millions) 2018 2017 Fixed maturities: Residential mortgage-backed securities $ (19.7 ) $ (19.7 ) $ (19.7 ) Commercial mortgage-backed securities (0.3 ) (0.5 ) (0.3 ) Total fixed maturities $ (20.0 ) $ (20.2 ) $ (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 June 30, 2018 and 2017 , for which a portion of the OTTI losses were also recognized in accumulated other comprehensive income at the time the credit impairments were determined and recognized: Three Months Ended June 30, 2018 Mortgage-Backed (millions) Residential Commercial Total Balance at March 31, 2018 $ 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 0.3 0 0.3 Balance at June 30, 2018 $ 0.3 $ 0.5 $ 0.8 Six Months Ended June 30, 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 0.3 0 0.3 Balance at June 30, 2018 $ 0.3 $ 0.5 $ 0.8 Three Months Ended June 30, 2017 Mortgage-Backed (millions) Residential Commercial Total Balance at March 31, 2017 $ 11.4 $ 0.1 $ 11.5 Reductions for securities sold/matured (11.4 ) 0 (11.4 ) Change in recoveries of future cash flows expected to be collected 1 0.2 0 0.2 Balance at June 30, 2017 $ 0.2 $ 0.1 $ 0.3 Six Months Ended June 30, 2017 Mortgage-Backed (millions) Residential Commercial Total Balance at December 31, 2016 $ 11.1 $ 0.4 $ 11.5 Reductions for securities sold/matured (10.9 ) (0.3 ) (11.2 ) Change in recoveries of future cash flows expected to be collected 1 0 0 0 Balance at June 30, 2017 $ 0.2 $ 0.1 $ 0.3 1 Reflects the current period change in the expected recovery of prior impairments that will be accreted into income over the remaining life of the security.
Realized Gain (Loss) on Investments [Table Text Block]	The components of net realized gains (losses) for the three and six months ended June 30, were: Three Months Six Months (millions) 2018 2017 2018 2017 Gross realized gains on security sales Available-for-sale securities: U.S. government obligations $ 1.7 $ 4.5 $ 1.7 $ 4.9 State and local government obligations 0.6 2.6 9.2 3.1 Corporate and other debt securities 0.3 7.3 0.4 11.4 Residential mortgage-backed securities 0 20.9 0 21.0 Commercial mortgage-backed securities 0.3 1.2 2.0 2.4 Other asset-backed securities 0.1 0 0.1 0.3 Redeemable preferred stocks 3.2 0 4.3 0.3 Total available-for-sale securities 6.2 36.5 17.7 43.4 Equity securities: Nonredeemable preferred stocks 0 6.2 3.6 51.6 Common equities 18.5 9.9 138.4 17.3 Total equity securities 18.5 16.1 142.0 68.9 Subtotal gross realized gains on security sales 24.7 52.6 159.7 112.3 Gross realized losses on security sales Available-for-sale securities: U.S. government obligations (29.9 ) (0.4 ) (38.8 ) (3.6 ) State and local government obligations (0.6 ) 0 (1.9 ) (0.1 ) Corporate and other debt securities (1.0 ) (1.9 ) (4.1 ) (2.8 ) Residential mortgage-backed securities 0 (0.3 ) 0 (0.3 ) Commercial mortgage-backed securities 0 (0.7 ) (6.3 ) (3.1 ) Other asset-backed securities (0.9 ) 0 (1.0 ) 0 Total available-for-sale securities (32.4 ) (3.3 ) (52.1 ) (9.9 ) Equity securities: Nonredeemable preferred stocks (1.9 ) (4.6 ) (2.3 ) (5.8 ) Common equities 0 0 (7.9 ) (0.1 ) Total equity securities (1.9 ) (4.6 ) (10.2 ) (5.9 ) Subtotal gross realized losses on security sales (34.3 ) (7.9 ) (62.3 ) (15.8 ) Net realized gains (losses) on security sales Available-for-sale securities: U.S. government obligations (28.2 ) 4.1 (37.1 ) 1.3 State and local government obligations 0 2.6 7.3 3.0 Corporate and other debt securities (0.7 ) 5.4 (3.7 ) 8.6 Residential mortgage-backed securities 0 20.6 0 20.7 Commercial mortgage-backed securities 0.3 0.5 (4.3 ) (0.7 ) Other asset-backed securities (0.8 ) 0 (0.9 ) 0.3 Redeemable preferred stocks 3.2 0 4.3 0.3 Total available-for-sale securities (26.2 ) 33.2 (34.4 ) 33.5 Equity securities: Nonredeemable preferred stocks (1.9 ) 1.6 1.3 45.8 Common equities 18.5 9.9 130.5 17.2 Total equity securities 16.6 11.5 131.8 63.0 Litigation settlements and other gains (losses) 0 0.8 0 1.1 Subtotal net realized gains (losses) on security sales (9.6 ) 45.5 97.4 97.6 Net holding period gains (losses) Hybrid securities (2.3 ) 0.4 (5.5 ) 1.2 Equity securities 55.8 0 (96.2 ) 0 Subtotal net holding period gains (losses) 53.5 0.4 (101.7 ) 1.2 Other-than-temporary impairment losses Equity securities: Common equities 0 (2.6 ) 0 (3.6 ) Subtotal investment other-than-temporary impairment losses 0 (2.6 ) 0 (3.6 ) Other asset impairment (11.1 ) (11.2 ) (11.1 ) (11.2 ) Subtotal other-than-temporary impairment losses (11.1 ) (13.8 ) (11.1 ) (14.8 ) Total net realized gains (losses) on securities $ 32.8 $ 32.1 $ (15.4 ) $ 84.0
Components of Equity Realized Gains (Losses) [Table Text Block]	The following table reflects our holding period realized gains (losses) on equity securities recognized for the three and six months ended June 30, 2018 for equity securities held at quarter end: Three Months Six Months (millions) 2018 2018 Total net gains (losses) recognized during the period on equity securities $ 72.4 $ 35.6 Less: Net gains (losses) recognized on equity securities sold during the period 16.6 131.8 Net holding period gains (losses) recognized during the period on equity securities held at period end $ 55.8 $ (96.2 ) Note: Comparative disclosure for the prior year period is not meaningful.
Components of Net Investment Income	The components of net investment income for the three and six months ended June 30, were: Three Months Six Months (millions) 2018 2017 2018 2017 Available-for-sale securities: Fixed maturities: U.S. government obligations $ 45.9 $ 17.0 $ 85.7 $ 29.7 State and local government obligations 9.1 13.1 19.1 26.4 Foreign government obligations 0 0.1 0 0.2 Corporate debt securities 51.8 31.3 88.0 60.9 Residential mortgage-backed securities 6.6 10.3 13.5 19.9 Commercial mortgage-backed securities 20.2 18.3 41.4 37.1 Other asset-backed securities 16.1 11.8 29.7 21.7 Redeemable preferred stocks 2.9 2.9 5.5 6.3 Total fixed maturities 152.6 104.8 282.9 202.2 Short-term investments 14.9 8.8 25.0 15.8 Total available-for-sale securities 167.5 113.6 307.9 218.0 Equity securities: Nonredeemable preferred stocks 10.4 10.6 21.3 22.1 Common equities 14.2 14.6 29.2 27.9 Total equity securities 24.6 25.2 50.5 50.0 Investment income 192.1 138.8 358.4 268.0 Investment expenses (6.2 ) (6.6 ) (12.2 ) (12.2 ) Net investment income $ 185.9 $ 132.2 $ 346.2 $ 255.8

Fair Value (Tables)

Fair Value (Tables)	6 Months Ended
	Jun. 30, 2018
Fair Value Disclosures [Abstract]
Composition of Investment Portfolio by Major Security Type	The composition of the investment portfolio by major security type and our outstanding debt was: Fair Value (millions) Level 1 Level 2 Level 3 Total Cost June 30, 2018 Fixed maturities: U.S. government obligations $ 7,865.4 $ 0 $ 0 $ 7,865.4 $ 8,005.8 State and local government obligations 0 1,667.3 0 1,667.3 1,678.9 Foreign government obligations 0 0 0 0 0 Corporate debt securities 0 7,330.3 0 7,330.3 7,422.4 Subtotal 7,865.4 8,997.6 0 16,863.0 17,107.1 Asset-backed securities: Residential mortgage-backed 0 822.6 0 822.6 819.6 Commercial mortgage-backed 0 2,696.9 0 2,696.9 2,725.5 Other asset-backed 0 3,177.6 0 3,177.6 3,189.2 Subtotal asset-backed securities 0 6,697.1 0 6,697.1 6,734.3 Redeemable preferred stocks: Financials 0 67.7 0 67.7 65.3 Utilities 0 4.6 0 4.6 4.8 Industrials 10.1 146.7 0 156.8 150.3 Subtotal redeemable preferred stocks 10.1 219.0 0 229.1 220.4 Total fixed maturities 7,875.5 15,913.7 0 23,789.2 24,061.8 Short-term investments 2,954.2 277.0 0 3,231.2 3,231.2 Total available-for-sale securities 10,829.7 16,190.7 0 27,020.4 27,293.0 Equity securities: Nonredeemable preferred stocks: Financials 77.4 676.2 0 753.6 672.0 Industrials 0 0 5.0 5.0 5.0 Subtotal nonredeemable preferred stocks 77.4 676.2 5.0 758.6 677.0 Common equities: Common stocks 3,141.9 0 0 3,141.9 1,313.7 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 3,141.9 0 0.3 3,142.2 1,314.0 Total equity securities 3,219.3 676.2 5.3 3,900.8 1,991.0 Total portfolio $ 14,049.0 $ 16,866.9 $ 5.3 $ 30,921.2 $ 29,284.0 Debt $ 0 $ 3,959.0 $ 0 $ 3,959.0 $ 3,859.5 Fair Value (millions) Level 1 Level 2 Level 3 Total Cost June 30, 2017 Fixed maturities: U.S. government obligations $ 4,155.0 $ 0 $ 0 $ 4,155.0 $ 4,166.4 State and local government obligations 0 2,500.1 0 2,500.1 2,473.3 Foreign government obligations 22.5 0 0 22.5 22.5 Corporate debt securities 0 5,013.8 0 5,013.8 4,991.6 Subtotal 4,177.5 7,513.9 0 11,691.4 11,653.8 Asset-backed securities: Residential mortgage-backed 0 1,267.6 0 1,267.6 1,260.3 Commercial mortgage-backed 0 2,375.0 0 2,375.0 2,364.9 Other asset-backed 0 2,847.4 0 2,847.4 2,843.3 Subtotal asset-backed securities 0 6,490.0 0 6,490.0 6,468.5 Redeemable preferred stocks: Financials 0 64.1 0 64.1 60.4 Utilities 0 32.2 0 32.2 30.5 Industrials 0 110.4 0 110.4 98.5 Subtotal redeemable preferred stocks 0 206.7 0 206.7 189.4 Total fixed maturities 4,177.5 14,210.6 0 18,388.1 18,311.7 Equity securities: Nonredeemable preferred stocks: Financials 85.3 697.8 0 783.1 666.0 Industrials 0 0 0 0 0 Subtotal nonredeemable preferred stocks 85.3 697.8 0 783.1 666.0 Common equities: Common stocks 3,077.2 0 0 3,077.2 1,472.0 Other risk investments 0 0 0.3 0.3 0.3 Subtotal common equities 3,077.2 0 0.3 3,077.5 1,472.3 Total fixed maturities and equity securities 7,340.0 14,908.4 0.3 22,248.7 20,450.0 Short-term investments 2,448.0 1,281.7 0 3,729.7 3,729.7 Total portfolio $ 9,788.0 $ 16,190.1 $ 0.3 $ 25,978.4 $ 24,179.7 Debt $ 0 $ 3,564.4 $ 114.8 $ 3,679.2 $ 3,383.4 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

Debt (Tables)

Debt (Tables)	6 Months Ended
	Jun. 30, 2018
Debt Table	Debt at each of the balance sheet periods consisted of: June 30, 2018 June 30, 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 $ 506.4 $ 498.6 $ 528.0 $ 498.8 $ 520.7 2.45% Senior Notes due 2027 496.3 451.3 496.0 474.2 496.1 477.9 6 5/8% Senior Notes due 2029 296.2 364.4 296.0 389.0 296.1 382.3 6.25% Senior Notes due 2032 395.4 492.5 395.2 522.7 395.3 516.9 4.35% Senior Notes due 2044 346.6 356.7 346.5 376.5 346.5 388.7 3.70% Senior Notes due 2045 395.3 363.9 395.2 389.7 395.2 402.9 4.125% Senior Notes due 2047 841.3 831.5 841.1 884.3 841.2 917.1 4.20% Senior Notes due 2048 589.5 592.3 0 0 0 0 Other debt instruments 0 0 114.8 114.8 37.1 37.1 Total $ 3,859.5 $ 3,959.0 $ 3,383.4 $ 3,679.2 $ 3,306.3 $ 3,643.6

Loss and Loss Adjustment Expe25

Loss and Loss Adjustment Expense Reserves (Tables)	6 Months Ended
	Jun. 30, 2018
Loss and Loss Adjustment Expenses Reserves [Abstract]
Schedule of Liability for Unpaid Claims and Claims Adjustment Expense [Table Text Block]	Activity in the loss and loss adjustment expense reserves is summarized as follows: June 30, (millions) 2018 2017 Balance, Beginning of period $ 13,086.9 $ 11,368.0 Less reinsurance recoverables on unpaid losses 2,170.1 1,801.0 Net balance, Beginning of period 10,916.8 9,567.0 Incurred related to: Current year 10,164.6 8,804.1 Prior years 81.5 74.2 Total incurred 10,246.1 8,878.3 Paid related to: Current year 5,533.7 4,951.0 Prior years 3,851.5 3,381.8 Total paid 9,385.2 8,332.8 Net balance, End of period 11,777.7 10,112.5 Plus reinsurance recoverables on unpaid losses 2,293.1 1,947.9 Balance, End of period $ 14,070.8 $ 12,060.4

Supplemental Cash Flow Inform26

Supplemental Cash Flow Information (Tables)	6 Months Ended
	Jun. 30, 2018
Supplemental Cash Flow Elements [Abstract]
Supplemental Cash Flow Information Table	We paid the following in the respective periods: Six Months Ended June 30, (millions) 2018 2017 Income taxes $ 358.9 $ 452.9 Interest 70.9 73.3

Segment Information (Tables)

Segment Information (Tables)	6 Months Ended
	Jun. 30, 2018
Segment Reporting [Abstract]
Reconciliation of Revenue and operating Income from Segments to Consolidated	Following are the operating results for the respective periods: Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 (millions) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Revenues Pretax Profit (Loss) Personal Lines Agency $ 3,225.7 $ 360.9 $ 2,752.5 $ 205.5 $ 6,289.5 $ 770.0 $ 5,384.0 $ 455.2 Direct 3,211.8 287.9 2,650.0 162.7 6,228.1 585.9 5,173.7 338.0 Total Personal Lines 1 6,437.5 648.8 5,402.5 368.2 12,517.6 1,355.9 10,557.7 793.2 Commercial Lines 884.3 100.3 671.7 56.2 1,692.9 195.1 1,317.2 123.6 Property 2 312.4 (51.9 ) 239.1 3.7 597.7 (23.4 ) 465.1 11.5 Other indemnity 0 0 0 (0.1 ) 0 0.2 0 (0.3 ) Total underwriting operations 7,634.2 697.2 6,313.3 428.0 14,808.2 1,527.8 12,340.0 928.0 Fees and other revenues 3 116.0 NA 88.8 NA 219.8 NA 174.0 NA Service businesses 42.9 5.9 32.7 5.7 77.1 10.8 61.2 8.3 Investments 4 224.9 218.7 170.9 164.3 343.0 330.8 352.0 339.8 Other gains (losses) 0 0 0 0 0 0 0.2 0.2 Interest expense NA (41.7 ) NA (43.4 ) NA (78.5 ) NA (80.2 ) Consolidated total $ 8,018.0 $ 880.1 $ 6,605.7 $ 554.6 $ 15,448.1 $ 1,790.9 $ 12,927.4 $ 1,196.1 NA = Not applicable 1 Personal auto insurance accounted for 94% of the total Personal Lines segment net premiums earned during the three and six months ended June 30, 2018 , and 93% for the same periods in 2017 ; insurance for our special lines products (e.g., motorcycles, ATVs, RVs, watercraft, and snowmobiles) accounted for the balance of the Personal Lines net premiums earned. 2 For the three and six months ended June 30, 2018 , pretax profit (loss) includes $18.0 million and $36.0 million , respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX and $15.5 million and $31.0 million for the same periods in 2017 . Although this expense is included in our Property segment, it is not reported in the consolidated results of ARX and, therefore, will not affect the value of net income (loss) attributable to noncontrolling interest. 3 Pretax profit (loss) for fees and other revenues is attributable to operating segments. 4 Revenues represent recurring investment income and total net realized gains (losses) on securities; pretax profit is net of investment expense.
Underwriting Margins and Combined Ratios for our Underwriting Operations	Following are the underwriting margins/combined ratios for our underwriting operations for the respective periods: Three Months Ended June 30, Six Months Ended June 30, 2018 2017 2018 2017 Under-writing Combined Ratio Under-writing Combined Ratio Under-writing Margin Combined Ratio Under-writing Margin Combined Ratio Personal Lines Agency 11.2 % 88.8 7.5 % 92.5 12.2 % 87.8 8.5 % 91.5 Direct 9.0 91.0 6.1 93.9 9.4 90.6 6.5 93.5 Total Personal Lines 10.1 89.9 6.8 93.2 10.8 89.2 7.5 92.5 Commercial Lines 11.3 88.7 8.4 91.6 11.5 88.5 9.4 90.6 Property 1 (16.6 ) 116.6 1.5 98.5 (3.9 ) 103.9 2.5 97.5 Total underwriting operations 9.1 90.9 6.8 93.2 10.3 89.7 7.5 92.5 1 Included in the three and six months ended June 30, 2018 is 5.8 points and 6.0 points, respectively, of amortization expense predominately associated with the acquisition of a controlling interest in ARX and 6.5 points and 6.7 points, respectively, for the three and six months ended June 30, 2017 .

Dividends Dividends (Tables)

Dividends Dividends (Tables)	6 Months Ended
	Jun. 30, 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 of Common Share Dividends Dividend Type Declared Paid Per Share Accrued 1 Paid 1 Annual – Variable December 2017 February 2018 $ 1.1247 $ 655.1 $ 654.9 Annual – Variable December 2016 February 2017 0.6808 395.4 395.4 1 Variance between accrued and paid, if any, reflects the difference between the number of estimated and actual shares outstanding as of the record date.

Other Comprehensive Income (L29

Other Comprehensive Income (Loss) (Tables)	6 Months Ended
	Jun. 30, 2018
Equity [Abstract]
Components of Other Comprehensive Income (Loss)	The components of other comprehensive income (loss), including reclassification adjustments by income statement line item, were as follows: Components of Changes in Accumulated Other Comprehensive Income (after tax) (millions) Pretax total accumulated other comprehensive income 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 March 31, 2018 $ (218.6 ) $ 46.0 $ (172.6 ) $ (160.8 ) $ (17.8 ) $ 0 $ 6.0 Other comprehensive income (loss) before reclassifications: Investment securities (90.4 ) 18.8 (71.6 ) (71.6 ) 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) 0.7 (0.1 ) 0.6 0 0 0 0.6 Total other comprehensive income (loss) before reclassifications (89.7 ) 18.7 (71.0 ) (71.6 ) 0 0 0.6 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 (27.2 ) 5.7 (21.5 ) (21.5 ) 0 0 0 Interest expense (0.3 ) 0.1 (0.2 ) 0 (0.2 ) 0 0 Total reclassification adjustment for amounts realized in net income (27.5 ) 5.8 (21.7 ) (21.5 ) (0.2 ) 0 0 Total other comprehensive income (loss) (62.2 ) 12.9 (49.3 ) (50.1 ) 0.2 0 0.6 Balance at June 30, 2018 $ (280.8 ) $ 58.9 $ (221.9 ) $ (210.9 ) $ (17.6 ) $ 0 $ 6.6 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 (293.8 ) 61.6 (232.2 ) (232.2 ) 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) 5.8 (1.2 ) 4.6 0 0 0 4.6 Total other comprehensive income (loss) before reclassifications (288.0 ) 60.4 (227.6 ) (232.2 ) 0 0 4.6 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 (34.9 ) 7.3 (27.6 ) (27.6 ) 0 0 0 Interest expense (0.5 ) 0.1 (0.4 ) 0 (0.4 ) 0 0 Total reclassification adjustment for amounts realized in net income (35.4 ) 7.4 (28.0 ) (27.6 ) (0.4 ) 0 0 Total other comprehensive income (loss) (252.6 ) 53.0 (199.6 ) (204.6 ) 0.4 0 4.6 Balance at June 30, 2018 $ (280.8 ) $ 58.9 $ (221.9 ) $ (210.9 ) $ (17.6 ) $ 0 $ 6.6 1 Reflects the fair value changes on equity securities as of December 31, 2017 , which are reported as realized gains (losses) under the new accounting guidance. See Note 14 – New Accounting Standards for additional information. 2 Reflects the effect of the change in the U.S. federal tax rate on our available-for-sale fixed maturity securities and our hedges on forecasted transactions as of December 31, 2017 . See Note 14 – New Accounting Standards for additional information. 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 March 31, 2017 $ 1,645.9 $ (578.4 ) $ 1,067.5 $ 1,072.0 $ (7.1 ) $ (0.8 ) $ 3.4 Other comprehensive income (loss) before reclassifications: Investment securities 177.2 (62.1 ) 115.1 115.1 0 0 0 Forecasted transactions (12.0 ) 4.2 (7.8 ) 0 (7.8 ) 0 0 Foreign currency translation adjustment (0.2 ) 0.1 (0.1 ) 0 0 (0.1 ) 0 Loss attributable to noncontrolling interest (NCI) (1.9 ) 0.6 (1.3 ) 0 0 0 (1.3 ) Total other comprehensive income (loss) before reclassifications 163.1 (57.2 ) 105.9 115.1 (7.8 ) (0.1 ) (1.3 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (2.6 ) 0.9 (1.7 ) (1.7 ) 0 0 0 Net realized gains (losses) on securities 37.2 (13.0 ) 24.2 24.2 0 0 0 Interest expense 0.3 (0.1 ) 0.2 0 0.2 0 0 Total reclassification adjustment for amounts realized in net income 34.9 (12.2 ) 22.7 22.5 0.2 0 0 Total other comprehensive income (loss) 128.2 (45.0 ) 83.2 92.6 (8.0 ) (0.1 ) (1.3 ) Balance at June 30, 2017 $ 1,774.1 $ (623.4 ) $ 1,150.7 $ 1,164.6 $ (15.1 ) $ (0.9 ) $ 2.1 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 431.7 (151.3 ) 280.4 280.4 0 0 0 Forecasted transactions (8.0 ) 2.8 (5.2 ) 0 (5.2 ) 0 0 Foreign currency translation adjustment 0.3 (0.1 ) 0.2 0 0 0.2 0 Loss attributable to noncontrolling interest (NCI) (3.4 ) 1.2 (2.2 ) 0 0 0 (2.2 ) Total other comprehensive income (loss) before reclassifications 420.6 (147.4 ) 273.2 280.4 (5.2 ) 0.2 (2.2 ) Less: Reclassification adjustment for amounts realized in net income by income statement line item: Net impairment losses recognized in earnings (3.6 ) 1.3 (2.3 ) (2.3 ) 0 0 0 Net realized gains (losses) on securities 88.8 (31.1 ) 57.7 57.7 0 0 0 Interest expense 0.8 (0.3 ) 0.5 0 0.5 0 0 Total reclassification adjustment for amounts realized in net income 86.0 (30.1 ) 55.9 55.4 0.5 0 0 Total other comprehensive income (loss) 334.6 (117.3 ) 217.3 225.0 (5.7 ) 0.2 (2.2 ) Balance at June 30, 2017 $ 1,774.1 $ (623.4 ) $ 1,150.7 $ 1,164.6 $ (15.1 ) $ (0.9 ) $ 2.1

Redeemable Noncontrolling Int30

Redeemable Noncontrolling Interest (Tables)	6 Months Ended
	Jun. 30, 2018
Temporary Equity Disclosure [Abstract]
Redeemable Noncontrolling Interest	The changes in the components of redeemable NCI during the six months ended June 30, 2018 and 2017 , and the year ended December 31, 2017 , were: (millions) June 30, 2018 June 30, 2017 December 31, 2017 Balance, Beginning of period $ 503.7 $ 483.7 $ 483.7 Net income attributable to NCI 8.8 11.1 5.9 Other comprehensive income (loss) attributable to NCI (4.6 ) 2.2 2.3 Exercise of employee stock options 9.4 3.4 3.4 Purchase of ARX shares (295.9 ) 0 0 Change in redemption value of NCI (3.2 ) 1.4 8.4 Balance, End of period $ 218.2 $ 501.8 $ 503.7

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	6 Months Ended
	Jun. 30, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Net Carrying Amount of Other Intangible Assets	The following table is a summary of the net carrying amount of other intangible assets as of June 30, 2018 and 2017 , and December 31, 2017 : (millions) June 30, 2018 June 30, 2017 December 31, 2017 Intangible assets subject to amortization $ 318.2 $ 389.4 $ 354.2 Indefinite-lived intangible assets 1 12.4 12.4 12.4 Total $ 330.6 $ 401.8 $ 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) June 30, 2018 June 30, 2017 December 31, 2017 Category Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Gross Carrying Amount Accumulated Amortization Net Carrying Amount Policies in force $ 256.2 $ 119.0 $ 137.2 $ 256.2 $ 82.4 $ 173.8 $ 256.2 $ 100.7 $ 155.5 Agency relationships 159.2 37.0 122.2 159.2 25.6 133.6 159.2 31.3 127.9 Software rights 79.1 34.7 44.4 79.1 24.1 55.0 79.1 29.4 49.7 Trade name 34.8 20.4 14.4 34.8 7.8 27.0 34.8 13.7 21.1 Total $ 529.3 $ 211.1 $ 318.2 $ 529.3 $ 139.9 $ 389.4 $ 529.3 $ 175.1 $ 354.2

Basis of Presentation Basis of

Basis of Presentation Basis of Presentation-Additional Information (Details)	Jun. 30, 2018USD ($)shares	Dec. 31, 2017USD ($)	Jun. 30, 2017USD ($)
Business Acquisition [Line Items]
Assets Held-for-sale, Property, Plant and Equipment | $	$ 17,400,000	$ 5,300,000	$ 0
ARX Holding Corp. [Member]
Business Acquisition [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage	86.70%	69.00%	69.00%
Number of Shares Put Pursuant to Stockholders' Agreement | shares	204,527

Investments Investment Portfoli

Investments Investment Portfolio by Major Security Type (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Fixed maturities:
Fair Value	$ 27,020.4	$ 23,071.1	$ 22,117.8
Total available-for-sale securities
Cost	27,293
Gross Unrealized Gains	36.8
Gross Unrealized Losses	304.1
Net Holding Period Gains (Losses)	(5.3)
Fair Value	$ 27,020.4
% of Total Fair Value	87.40%
Equity securities:
Cost	$ 1,991
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Net Holding Period Gains (Losses)	1,909.8
Fair Value	$ 3,900.8	4,203.6	3,860.6
% of Total Fair Value	12.60%
Total portfolio
Cost	$ 29,284
Gross Unrealized Gains	36.8
Gross Unrealized Losses	(304.1)
Net Holding Period Gains (Losses)	1,904.5
Fair Value	$ 30,921.2	27,274.7	25,978.4
% of Total Fair Value	100.00%
Available for Sale Securities
Cost		25,276.9	24,179.7
Gross Unrealized Gains		2,097	1,857.7
Gross Unrealized Losses		99.4	62.1
Net Holding Period Gains (Losses)		0.2	3.1
Fair Value		$ 27,274.7	$ 25,978.4
% of Total Fair Value		100.00%	100.00%
Unsettled security transactions	$ 362.1		$ 287.1
Unsettled security transactions		$ 5.8
Total fair value of portfolio	1,700	1,600	1,100
Nonredeemable preferred stocks
Equity securities:
Cost	677
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Net Holding Period Gains (Losses)	81.6
Fair Value	$ 758.6
% of Total Fair Value	2.40%
Available for Sale Securities
Cost		698.6	666
Gross Unrealized Gains		114	123.6
Gross Unrealized Losses		8.8	6.5
Net Holding Period Gains (Losses)		0	0
Fair Value		$ 803.8	$ 783.1
% of Total Fair Value		2.90%	3.00%
Common equities
Equity securities:
Cost	$ 1,314
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Net Holding Period Gains (Losses)	1,828.2
Fair Value	$ 3,142.2
% of Total Fair Value	10.20%
Available for Sale Securities
Cost		$ 1,499	$ 1,472.3
Gross Unrealized Gains		1,901	1,611.7
Gross Unrealized Losses		0.2	6.5
Net Holding Period Gains (Losses)		0	0
Fair Value		$ 3,399.8	$ 3,077.5
% of Total Fair Value		12.50%	11.80%
Total fixed maturities
Fixed maturities:
Cost	$ 24,061.8
Gross Unrealized Gains	36.8
Gross Unrealized Losses	(304.1)
Net Holding Period Gains (Losses)	(5.3)
Fair Value	$ 23,789.2
% of Total Fair Value	76.90%
Available for Sale Securities
Cost		$ 20,209.9	$ 18,311.7
Gross Unrealized Gains		82	122.4
Gross Unrealized Losses		90.4	49.1
Net Holding Period Gains (Losses)		0.2	3.1
Fair Value		$ 20,201.7	$ 18,388.1
% of Total Fair Value		74.10%	70.80%
Total fixed maturities | U.S. government obligations
Fixed maturities:
Cost	$ 8,005.8
Gross Unrealized Gains	0.9
Gross Unrealized Losses	(141.3)
Net Holding Period Gains (Losses)	0
Fair Value	$ 7,865.4
% of Total Fair Value	25.40%
Available for Sale Securities
Cost		$ 6,688.8	$ 4,166.4
Gross Unrealized Gains		1.1	4.4
Gross Unrealized Losses		44	15.8
Net Holding Period Gains (Losses)		0	0
Fair Value		$ 6,645.9	$ 4,155
% of Total Fair Value		24.40%	16.00%
Total fixed maturities | State and local government obligations
Fixed maturities:
Cost	$ 1,678.9
Gross Unrealized Gains	4.3
Gross Unrealized Losses	(15.9)
Net Holding Period Gains (Losses)	0
Fair Value	$ 1,667.3
% of Total Fair Value	5.40%
Available for Sale Securities
Cost		$ 2,285.6	$ 2,473.3
Gross Unrealized Gains		20.7	32.5
Gross Unrealized Losses		9.3	5.9
Net Holding Period Gains (Losses)		0.1	0.2
Fair Value		$ 2,297.1	$ 2,500.1
% of Total Fair Value		8.40%	9.70%
Total fixed maturities | Foreign government obligations
Fixed maturities:
Cost	$ 0
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Net Holding Period Gains (Losses)	0
Fair Value	$ 0
% of Total Fair Value	0.00%
Available for Sale Securities
Cost		$ 0	$ 22.5
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Net Holding Period Gains (Losses)		0	0
Fair Value		$ 0	$ 22.5
% of Total Fair Value		0.00%	0.10%
Total fixed maturities | Corporate debt securities
Fixed maturities:
Cost	$ 7,422.4
Gross Unrealized Gains	4.2
Gross Unrealized Losses	(94.3)
Net Holding Period Gains (Losses)	(2)
Fair Value	$ 7,330.3
% of Total Fair Value	23.70%
Available for Sale Securities
Cost		$ 4,997.2	$ 4,991.6
Gross Unrealized Gains		14.8	29.4
Gross Unrealized Losses		14.4	7.6
Net Holding Period Gains (Losses)		0.1	0.4
Fair Value		$ 4,997.7	$ 5,013.8
% of Total Fair Value		18.30%	19.30%
Total fixed maturities | Residential mortgage-backed securities
Fixed maturities:
Cost	$ 819.6
Gross Unrealized Gains	8.4
Gross Unrealized Losses	(5.4)
Net Holding Period Gains (Losses)	0
Fair Value	$ 822.6
% of Total Fair Value	2.70%
Available for Sale Securities
Cost		$ 828.8	$ 1,260.3
Gross Unrealized Gains		11.3	10.9
Gross Unrealized Losses		3.4	5.9
Net Holding Period Gains (Losses)		0	2.3
Fair Value		$ 836.7	$ 1,267.6
% of Total Fair Value		3.10%	4.80%
Total fixed maturities | Commercial mortgage-backed securities
Fixed maturities:
Cost	$ 2,725.5
Gross Unrealized Gains	2.9
Gross Unrealized Losses	(31.5)
Net Holding Period Gains (Losses)	0
Fair Value	$ 2,696.9
% of Total Fair Value	8.70%
Available for Sale Securities
Cost		$ 2,760.1	$ 2,364.9
Gross Unrealized Gains		11.8	19.8
Gross Unrealized Losses		13.3	9.7
Net Holding Period Gains (Losses)		0	0
Fair Value		$ 2,758.6	$ 2,375
% of Total Fair Value		10.10%	9.10%
Total fixed maturities | Other asset-backed securities
Fixed maturities:
Cost	$ 3,189.2
Gross Unrealized Gains	2
Gross Unrealized Losses	(13.7)
Net Holding Period Gains (Losses)	0.1
Fair Value	$ 3,177.6
% of Total Fair Value	10.30%
Available for Sale Securities
Cost		$ 2,454.5	$ 2,843.3
Gross Unrealized Gains		4.5	6.4
Gross Unrealized Losses		4.5	2.5
Net Holding Period Gains (Losses)		0.2	0.2
Fair Value		$ 2,454.7	$ 2,847.4
% of Total Fair Value		9.00%	11.00%
Total fixed maturities | Redeemable preferred stocks
Fixed maturities:
Cost	$ 220.4
Gross Unrealized Gains	14.1
Gross Unrealized Losses	(2)
Net Holding Period Gains (Losses)	(3.4)
Fair Value	$ 229.1
% of Total Fair Value	0.70%
Available for Sale Securities
Cost		$ 194.9	$ 189.4
Gross Unrealized Gains		17.8	19
Gross Unrealized Losses		1.5	1.7
Net Holding Period Gains (Losses)		(0.2)	0
Fair Value		$ 211	$ 206.7
% of Total Fair Value		0.80%	0.80%
Short-term investments
Short-term investments
Cost	$ 3,231.2
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Net Holding Period Gains (Losses)	0
Fair Value	$ 3,231.2
% of Total Fair Value	10.50%
Available for Sale Securities
Cost		$ 2,869.4	$ 3,729.7
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Net Holding Period Gains (Losses)		0	0
Fair Value		$ 2,869.4	$ 3,729.7
% of Total Fair Value		10.50%	14.40%

Investments Hybrid Securities (

Investments Hybrid Securities (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Hybrid Instruments [Line Items]
Hybrid securities	$ 246.7	$ 142.9	$ 265.3
State and local government obligations
Hybrid Instruments [Line Items]
Hybrid securities	3.6	6.1	6.6
Corporate debt securities
Hybrid Instruments [Line Items]
Hybrid securities	170.3	99.8	61.3
Residential mortgage-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	0	0	189.7
Other asset-backed securities
Hybrid Instruments [Line Items]
Hybrid securities	5.5	6.7	7.7
Redeemable preferred stocks
Hybrid Instruments [Line Items]
Hybrid securities	$ 67.3	$ 30.3	$ 0

Investments Composition of Fixe

Investments Composition of Fixed Maturities by Maturity (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Schedule of Debt Securities by Contractual Maturity [Line Items]
Fixed maturities (amortized cost: $24,061.8, $18,311.7, and $20,209.9)	$ 27,020.4	$ 23,071.1	$ 22,117.8
Total fixed maturities
Schedule of Debt Securities by Contractual Maturity [Line Items]
Less than one year	3,805.2
One to five years	15,190.7
Five to ten years	4,939.7
Ten years or greater	126.2
Cost	24,061.8
Less than one year	3,809.2
One to five years	14,999.1
Five to ten years	4,855.4
Ten years or greater	125.5
Fixed maturities (amortized cost: $24,061.8, $18,311.7, and $20,209.9)	$ 23,789.2

Investments Gross Unrealized lo

Investments Gross Unrealized losses by Major Security (Detail) $ in Millions	Jun. 30, 2018USD ($)security	Dec. 31, 2017USD ($)security	Jun. 30, 2017USD ($)security
No. of Sec.
Total | security	1,407
Less than 12 Months | security	860
12 Months or Greater | security	547
Total | security		1,166	944
Less than 12 Months | security		689	696
12 Months or Greater | security		477	248
Fair Value
Total	$ 19,898.7
Less than 12 Months	14,234.4
12 Months or Greater	5,664.3
Total		$ 13,980.2	$ 9,181.8
Less than 12 Months		10,434.6	7,577.9
12 Months or Greater		3,545.6	1,603.9
Unrealized Losses
Total	304.1
Less than 12 Months	181.9
12 Months or Greater	$ 122.2
Total		99.4	62.1
Less than 12 Months		58.3	38.9
12 Months or Greater		$ 41.1	$ 23.2
U.S. government obligations
No. of Sec.
Total | security	70
Less than 12 Months | security	33
12 Months or Greater | security	37
Total | security		58	41
Less than 12 Months | security		41	39
12 Months or Greater | security		17	2
Fair Value
Total	$ 7,503.2
Less than 12 Months	4,543.5
12 Months or Greater	2,959.7
Total		$ 5,817	$ 2,781.8
Less than 12 Months		4,869.3	2,725.1
12 Months or Greater		947.7	56.7
Unrealized Losses
Total	141.3
Less than 12 Months	73.6
12 Months or Greater	$ 67.7
Total		44	15.8
Less than 12 Months		34.6	15.1
12 Months or Greater		$ 9.4	$ 0.7
State and local government obligations
No. of Sec.
Total | security	355
Less than 12 Months | security	219
12 Months or Greater | security	136
Total | security		358	222
Less than 12 Months | security		230	138
12 Months or Greater | security		128	84
Fair Value
Total	$ 1,050.9
Less than 12 Months	631.9
12 Months or Greater	419
Total		$ 1,200.3	$ 818.7
Less than 12 Months		737.6	479.8
12 Months or Greater		462.7	338.9
Unrealized Losses
Total	15.9
Less than 12 Months	7.1
12 Months or Greater	$ 8.8
Total		9.3	5.9
Less than 12 Months		4.4	3.3
12 Months or Greater		$ 4.9	$ 2.6
Corporate debt securities
No. of Sec.
Total | security	391
Less than 12 Months | security	322
12 Months or Greater | security	69
Total | security		222	143
Less than 12 Months | security		171	122
12 Months or Greater | security		51	21
Fair Value
Total	$ 6,153.5
Less than 12 Months	5,439.3
12 Months or Greater	714.2
Total		$ 2,979.4	$ 1,905.2
Less than 12 Months		2,072.9	1,599.6
12 Months or Greater		906.5	305.6
Unrealized Losses
Total	94.3
Less than 12 Months	77.2
12 Months or Greater	$ 17.1
Total		14.4	7.6
Less than 12 Months		9.1	5.3
12 Months or Greater		$ 5.3	$ 2.3
Residential mortgage-backed securities
No. of Sec.
Total | security	227
Less than 12 Months | security	52
12 Months or Greater | security	175
Total | security		201	220
Less than 12 Months | security		30	126
12 Months or Greater | security		171	94
Fair Value
Total	$ 380.3
Less than 12 Months	188.6
12 Months or Greater	191.7
Total		$ 300.9	$ 535.7
Less than 12 Months		75.1	197.8
12 Months or Greater		225.8	337.9
Unrealized Losses
Total	5.4
Less than 12 Months	0.9
12 Months or Greater	$ 4.5
Total		3.4	5.9
Less than 12 Months		0.2	1.1
12 Months or Greater		$ 3.2	$ 4.8
Commercial mortgage-backed securities
No. of Sec.
Total | security	142
Less than 12 Months | security	85
12 Months or Greater | security	57
Total | security		105	74
Less than 12 Months | security		63	52
12 Months or Greater | security		42	22
Fair Value
Total	$ 2,300.9
Less than 12 Months	1,371.4
12 Months or Greater	929.5
Total		$ 1,682.3	$ 905.6
Less than 12 Months		1,221.2	679.5
12 Months or Greater		461.1	226.1
Unrealized Losses
Total	31.5
Less than 12 Months	14.4
12 Months or Greater	$ 17.1
Total		13.3	9.7
Less than 12 Months		5.9	5.7
12 Months or Greater		$ 7.4	$ 4
Other asset-backed securities
No. of Sec.
Total | security	219
Less than 12 Months | security	148
12 Months or Greater | security	71
Total | security		197	160
Less than 12 Months | security		134	145
12 Months or Greater | security		63	15
Fair Value
Total	$ 2,479.7
Less than 12 Months	2,055.1
12 Months or Greater	424.6
Total		$ 1,837.3	$ 2,077.8
Less than 12 Months		1,377.8	1,823.8
12 Months or Greater		459.5	254
Unrealized Losses
Total	13.7
Less than 12 Months	8.5
12 Months or Greater	$ 5.2
Total		4.5	2.5
Less than 12 Months		3.3	1.9
12 Months or Greater		$ 1.2	$ 0.6
Redeemable preferred stocks
No. of Sec.
Total | security	3
Less than 12 Months | security	1
12 Months or Greater | security	2
Total | security		2	1
Less than 12 Months | security		1	0
12 Months or Greater | security		1	1
Fair Value
Total	$ 30.2
Less than 12 Months	4.6
12 Months or Greater	25.6
Total		$ 21.8	$ 10.8
Less than 12 Months		10.8	0
12 Months or Greater		11	10.8
Unrealized Losses
Total	2
Less than 12 Months	0.2
12 Months or Greater	$ 1.8
Total		1.5	1.7
Less than 12 Months		0.1	0
12 Months or Greater		$ 1.4	$ 1.7
Total fixed maturities
No. of Sec.
Total | security		1,143	861
Less than 12 Months | security		670	622
12 Months or Greater | security		473	239
Fair Value
Total		$ 13,839	$ 9,035.6
Less than 12 Months		10,364.7	7,505.6
12 Months or Greater		3,474.3	1,530
Unrealized Losses
Total		90.4	49.1
Less than 12 Months		57.6	32.4
12 Months or Greater		$ 32.8	$ 16.7
Nonredeemable preferred stocks
No. of Sec.
Total | security		4	3
Less than 12 Months | security		1	0
12 Months or Greater | security		3	3
Fair Value
Total		$ 127.8	$ 73.1
Less than 12 Months		56.5	0
12 Months or Greater		71.3	73.1
Unrealized Losses
Total		8.8	6.5
Less than 12 Months		0.5	0
12 Months or Greater		$ 8.3	$ 6.5
Common equities
No. of Sec.
Total | security		19	80
Less than 12 Months | security		18	74
12 Months or Greater | security		1	6
Fair Value
Total		$ 13.4	$ 73.1
Less than 12 Months		13.4	72.3
12 Months or Greater		0	0.8
Unrealized Losses
Total		0.2	6.5
Less than 12 Months		0.2	6.5
12 Months or Greater		$ 0	$ 0
Total equity securities
No. of Sec.
Total | security		23	83
Less than 12 Months | security		19	74
12 Months or Greater | security		4	9
Fair Value
Total		$ 141.2	$ 146.2
Less than 12 Months		69.9	72.3
12 Months or Greater		71.3	73.9
Unrealized Losses
Total		9	13
Less than 12 Months		0.7	6.5
12 Months or Greater		$ 8.3	$ 6.5

Investments Total Non-Credit Po

Investments Total Non-Credit Portion of Other-Than-Temporary Impairment Recorded in Accumulated Other Comprehensive Income, Reflecting Original Non-Credit Loss at Time Credit Impairment (Detail) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Other Than Temporary Impairment Non Credit Losses Recognized In Accumulated Other Comprehensive Income [Line Items]
Non-credit portion of other-than-temporary impairments recognized in accumulated other comprehensive income	$ (20)	$ (20)	$ (20.2)
Residential mortgage-backed securities
Other Than Temporary Impairment Non Credit Losses Recognized In Accumulated Other Comprehensive Income [Line Items]
Non-credit portion of other-than-temporary impairments recognized in accumulated other comprehensive income	(19.7)	(19.7)	(19.7)
Commercial mortgage-backed securities
Other Than Temporary Impairment Non Credit Losses Recognized In Accumulated Other Comprehensive Income [Line Items]
Non-credit portion of other-than-temporary impairments recognized in accumulated other comprehensive income	$ (0.3)	$ (0.3)	$ (0.5)

Investments OTTI Credit Losses

Investments OTTI Credit Losses Recognized in Earnings (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	$ 0.5	$ 11.5	$ 0.5	$ 11.5
Reductions for securities sold/matured	0	(11.4)	0	(11.2)
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0.3	0.2	0.3	0
Ending balance	0.8	0.3	0.8	0.3
Residential mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	0	11.4	0	11.1
Reductions for securities sold/matured	0	(11.4)	0	(10.9)
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0.3	0.2	0.3	0
Ending balance	0.3	0.2	0.3	0.2
Commercial mortgage-backed securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]
Beginning balance	0.5	0.1	0.5	0.4
Reductions for securities sold/matured	0	0	0	(0.3)
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows	0	0	0	0
Ending balance	$ 0.5	$ 0.1	$ 0.5	$ 0.1

Investments Components of Net R

Investments Components of Net Realized Gains (Losses) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Gain (Loss) on Securities [Line Items]
Gross realized gains on security sales		$ 52.6		$ 112.3
Gross realized gains on security sales	$ 24.7		$ 159.7
Gross realized losses on security sales		(7.9)		(15.8)
Gross realized losses on security sales	34.3		62.3
Net realized gains (losses) on security sales, equity securities	16.6		131.8
Litigation settlements and other gains (losses)	0	0.8	0	1.1
Subtotal net realized gains (losses) on security sales	(9.6)	45.5	97.4	97.6
Net holding period gains (losses), hybrid securities	(2.3)	0.4	(5.5)	1.2
Net holding period gains (losses), equity securities	55.8		(96.2)
Subtotal net holding period gains (losses)	53.5	0.4	(101.7)	1.2
Total net realized gains (losses) on securities, equity securities	0	(2.6)	0	(3.6)
Other asset impairment	(11.1)	(13.8)	(11.1)	(14.8)
Total net realized gains (losses) on securities	32.8	32.1	(15.4)	84
U.S. government obligations
Gain (Loss) on Securities [Line Items]
Gross realized gains on security sales, available for sale securities	1.7		1.7
Gross realized gains on security sales		4.5		4.9
Gross realized losses on security sales, available for sale securities	(29.9)		(38.8)
Gross realized losses on security sales		(0.4)		(3.6)
Net realized gains (losses) on security sales, available for sale securities	(28.2)		(37.1)
Net realized gains (losses) on security sales		4.1		1.3
State and local government obligations
Gain (Loss) on Securities [Line Items]
Gross realized gains on security sales, available for sale securities	0.6		9.2
Gross realized gains on security sales		2.6		3.1
Gross realized losses on security sales, available for sale securities	(0.6)		(1.9)
Gross realized losses on security sales		0		(0.1)
Net realized gains (losses) on security sales, available for sale securities	0		7.3
Net realized gains (losses) on security sales		2.6		3
Corporate debt securities
Gain (Loss) on Securities [Line Items]
Gross realized gains on security sales, available for sale securities	0.3		0.4
Gross realized gains on security sales		7.3		11.4
Gross realized losses on security sales, available for sale securities	(1)		(4.1)
Gross realized losses on security sales		(1.9)		(2.8)
Net realized gains (losses) on security sales, available for sale securities	(0.7)		(3.7)
Net realized gains (losses) on security sales		5.4		8.6
Residential mortgage-backed securities
Gain (Loss) on Securities [Line Items]
Gross realized gains on security sales, available for sale securities	0		0
Gross realized gains on security sales		20.9		21
Gross realized losses on security sales, available for sale securities	0		0
Gross realized losses on security sales		(0.3)		(0.3)
Net realized gains (losses) on security sales, available for sale securities	0		0
Net realized gains (losses) on security sales		20.6		20.7
Commercial mortgage-backed securities
Gain (Loss) on Securities [Line Items]
Gross realized gains on security sales, available for sale securities	0.3		2
Gross realized gains on security sales		1.2		2.4
Gross realized losses on security sales, available for sale securities	0		(6.3)
Gross realized losses on security sales		(0.7)		(3.1)
Net realized gains (losses) on security sales, available for sale securities	0.3		(4.3)
Net realized gains (losses) on security sales		0.5		(0.7)
Other asset-backed securities
Gain (Loss) on Securities [Line Items]
Gross realized gains on security sales, available for sale securities	0.1		0.1
Gross realized gains on security sales		0		0.3
Gross realized losses on security sales, available for sale securities	(0.9)		(1)
Gross realized losses on security sales		0		0
Net realized gains (losses) on security sales, available for sale securities	(0.8)		(0.9)
Net realized gains (losses) on security sales		0		0.3
Redeemable preferred stocks
Gain (Loss) on Securities [Line Items]
Gross realized gains on security sales, available for sale securities	3.2		4.3
Gross realized gains on security sales		0		0.3
Net realized gains (losses) on security sales, available for sale securities	3.2		4.3
Net realized gains (losses) on security sales		0		0.3
Total fixed maturities
Gain (Loss) on Securities [Line Items]
Gross realized gains on security sales, equity securities	18.5		142
Gross realized gains on security sales		16.1		68.9
Gross realized losses on security sales, available for sale securities	(32.4)		(52.1)
Gross realized losses on security sales		(3.3)		(9.9)
Net realized gains (losses) on security sales, available for sale securities	(26.2)		(34.4)
Net realized gains (losses) on security sales		33.2		33.5
Nonredeemable preferred stocks
Gain (Loss) on Securities [Line Items]
Gross realized gains on security sales, equity securities	0		3.6
Gross realized gains on security sales		6.2		51.6
Gross realized losses on security sales, equity securities	(1.9)		(2.3)
Gross realized losses on security sales		(4.6)		(5.8)
Net realized gains (losses) on security sales, equity securities	(1.9)		1.3
Net realized gains (losses) on security sales		1.6		45.8
Common equities
Gain (Loss) on Securities [Line Items]
Gross realized gains on security sales, equity securities	18.5		138.4
Gross realized gains on security sales		9.9		17.3
Gross realized losses on security sales, equity securities	0		(7.9)
Gross realized losses on security sales		0		(0.1)
Net realized gains (losses) on security sales, equity securities	18.5		130.5
Net realized gains (losses) on security sales		9.9		17.2
Total net realized gains (losses) on securities, equity securities	0	(2.6)	0	(3.6)
Total equity securities
Gain (Loss) on Securities [Line Items]
Gross realized gains on security sales, available for sale securities	6.2		17.7
Gross realized gains on security sales		36.5		43.4
Gross realized losses on security sales, equity securities	(1.9)		(10.2)
Gross realized losses on security sales		(4.6)		(5.9)
Net realized gains (losses) on security sales, equity securities	16.6		131.8
Net realized gains (losses) on security sales		11.5		63
Other Assets [Member]
Gain (Loss) on Securities [Line Items]
Other asset impairment	$ (11.1)	$ (11.2)	$ (11.1)	$ (11.2)

Investments Components of Equit

Investments Components of Equity Realized Gains (Losses) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Components of Equity Realized Gains (Losses) [Line Items]
Realized Investment Gains (Losses)	$ 72.4	$ 35.6
Net realized gains (losses) on security sales, equity securities	16.6	131.8
Equity Securities, FV-NI, Unrealized Gain (Loss)	$ 55.8	$ (96.2)

Investments Components of Net I

Investments Components of Net Investment Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net Investment Income [Line Items]
Gross investment income	$ 192.1	$ 138.8	$ 358.4	$ 268
Investment expenses	(6.2)	(6.6)	(12.2)	(12.2)
Net Investment Income	185.9	132.2	346.2	255.8
Total equity securities
Net Investment Income [Line Items]
Gross investment income	24.6	25.2	50.5	50
Total equity securities | Nonredeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	10.4	10.6	21.3	22.1
Total equity securities | Common equities
Net Investment Income [Line Items]
Gross investment income	14.2	14.6	29.2	27.9
Available-for-sale Securities
Net Investment Income [Line Items]
Gross investment income	167.5	113.6	307.9	218
Short-term investments
Net Investment Income [Line Items]
Gross investment income	14.9	8.8	25	15.8
Total fixed maturities
Net Investment Income [Line Items]
Gross investment income	152.6	104.8	282.9	202.2
Total fixed maturities | U.S. government obligations
Net Investment Income [Line Items]
Gross investment income	45.9	17	85.7	29.7
Total fixed maturities | State and local government obligations
Net Investment Income [Line Items]
Gross investment income	9.1	13.1	19.1	26.4
Total fixed maturities | Foreign government obligations
Net Investment Income [Line Items]
Gross investment income	0	0.1	0	0.2
Total fixed maturities | Corporate debt securities
Net Investment Income [Line Items]
Gross investment income	51.8	31.3	88	60.9
Total fixed maturities | Residential mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	6.6	10.3	13.5	19.9
Total fixed maturities | Commercial mortgage-backed securities
Net Investment Income [Line Items]
Gross investment income	20.2	18.3	41.4	37.1
Total fixed maturities | Other asset-backed securities
Net Investment Income [Line Items]
Gross investment income	16.1	11.8	29.7	21.7
Total fixed maturities | Redeemable preferred stocks
Net Investment Income [Line Items]
Gross investment income	$ 2.9	$ 2.9	$ 5.5	$ 6.3

Investments - Additional Inform

Investments - Additional Information (Detail) - USD ($)	Apr. 03, 2017	Apr. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Mar. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Securities Sold under Agreements to Repurchase			$ 0	$ 0	$ 0	$ 0		$ 0
Open reverse repurchase commitments			0	0	0	0		0
Gross unrealized losses			304,100,000		304,100,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment					$ 0	$ 0
Investment Portfolio Duration					2 years 7 months 10 days	2 years 3 months 20 days
Trading securities			0	0	$ 0	$ 0		0
Debt and Equity Securities, Gain (Loss)			0	0	0	0
Derivative Assets (Liabilities), at Fair Value, Net			$ 0	$ 0	$ 0	$ 0		$ 0
4.20% Senior Notes due 2048
Schedule of Investments [Line Items]
Debt, face amount							$ 600,000,000
Debt, stated interest rate			4.20%		4.20%		4.20%
4.125% Senior Notes due 2047
Schedule of Investments [Line Items]
Debt, face amount	$ 850,000,000	$ 850,000,000
Debt, stated interest rate	4.125%	4.125%	4.125%		4.125%
Unrealized gain (loss) on interest rate cash flow hedges	$ (8,000,000)	$ (8,000,000)
Series B Preferred Stock [Member]
Schedule of Investments [Line Items]
Preferred Stock, Value, Issued							$ 500,000,000

Fair Value Composition of Inves

Fair Value Composition of Investment Portfolio by Major Security Type (Detail) - USD ($)	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 30,921,200,000	$ 27,274,700,000	$ 25,978,400,000
Fair Value	3,959,000,000	3,643,600,000	3,679,200,000
Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	14,049,000,000	11,950,400,000	9,788,000,000
Fair Value	0	0	0
Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	16,866,900,000	15,319,000,000	16,190,100,000
Fair Value	3,959,000,000	3,606,500,000	3,564,400,000
Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,300,000	5,300,000	300,000
Fair Value	0	37,100,000	114,800,000
Total Fixed Maturities And Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		24,405,300,000	22,248,700,000
Total Fixed Maturities And Equity Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		10,126,000,000	7,340,000,000
Total Fixed Maturities And Equity Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		14,274,000,000	14,908,400,000
Total Fixed Maturities And Equity Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		5,300,000	300,000
Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	27,020,400,000
Available-for-sale Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	10,829,700,000
Available-for-sale Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	16,190,700,000
Available-for-sale Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0
Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,900,800,000
Total equity securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,219,300,000
Total equity securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	676,200,000
Total equity securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,300,000
Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	758,600,000	803,800,000	783,100,000
Total equity securities | Nonredeemable preferred stocks | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	77,400,000	80,600,000	85,300,000
Total equity securities | Nonredeemable preferred stocks | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	676,200,000	718,200,000	697,800,000
Total equity securities | Nonredeemable preferred stocks | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,000,000	5,000,000	0
Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	753,600,000	798,800,000	783,100,000
Total equity securities | Nonredeemable preferred stocks | Financials | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	77,400,000	80,600,000	85,300,000
Total equity securities | Nonredeemable preferred stocks | Financials | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	676,200,000	718,200,000	697,800,000
Total equity securities | Nonredeemable preferred stocks | Financials | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,000,000	5,000,000	0
Total equity securities | Nonredeemable preferred stocks | Industrials | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total equity securities | Nonredeemable preferred stocks | Industrials | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total equity securities | Nonredeemable preferred stocks | Industrials | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,000,000	5,000,000	0
Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,142,200,000	3,399,800,000	3,077,500,000
Total equity securities | Common equities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,141,900,000	3,399,500,000	3,077,200,000
Total equity securities | Common equities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total equity securities | Common equities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	300,000	300,000	300,000
Total equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,141,900,000	3,399,500,000	3,077,200,000
Total equity securities | Common equities | Common equities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,141,900,000	3,399,500,000	3,077,200,000
Total equity securities | Common equities | Common equities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total equity securities | Common equities | Common equities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	300,000	300,000	300,000
Total equity securities | Common equities | Other risk investments | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total equity securities | Common equities | Other risk investments | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total equity securities | Common equities | Other risk investments | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	300,000	300,000	300,000
Carrying (Reported) Amount, Fair Value Disclosure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	29,284,000,000	25,276,900,000	24,179,700,000
Long-term Debt	3,859,500,000	3,306,300,000	3,383,400,000
Carrying (Reported) Amount, Fair Value Disclosure | Total Fixed Maturities And Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value		22,407,500,000	20,450,000,000
Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	27,293,000,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,991,000,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	677,000,000	698,600,000	666,000,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	672,000,000	693,600,000	666,000,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Nonredeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	5,000,000	5,000,000	0
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,314,000,000	1,499,000,000	1,472,300,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities | Common equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,313,700,000	1,498,700,000	1,472,000,000
Carrying (Reported) Amount, Fair Value Disclosure | Total equity securities | Common equities | Other risk investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	300,000	300,000	300,000
Total fixed maturities | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	23,789,200,000	20,201,700,000	18,388,100,000
Total fixed maturities | Available-for-sale Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,875,500,000	6,645,900,000	4,177,500,000
Total fixed maturities | Available-for-sale Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	15,913,700,000	13,555,800,000	14,210,600,000
Total fixed maturities | Available-for-sale Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	16,863,000,000	13,940,700,000	11,691,400,000
Total fixed maturities | Available-for-sale Securities | Debt Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,865,400,000	6,645,900,000	4,177,500,000
Total fixed maturities | Available-for-sale Securities | Debt Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	8,997,600,000	7,294,800,000	7,513,900,000
Total fixed maturities | Available-for-sale Securities | Debt Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,697,100,000	6,050,000,000	6,490,000,000
Total fixed maturities | Available-for-sale Securities | Asset-backed Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Asset-backed Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,697,100,000	6,050,000,000	6,490,000,000
Total fixed maturities | Available-for-sale Securities | Asset-backed Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,865,400,000	6,645,900,000	4,155,000,000
Total fixed maturities | Available-for-sale Securities | U.S. government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,865,400,000	6,645,900,000	4,155,000,000
Total fixed maturities | Available-for-sale Securities | U.S. government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | U.S. government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,667,300,000	2,297,100,000	2,500,100,000
Total fixed maturities | Available-for-sale Securities | State and local government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | State and local government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,667,300,000	2,297,100,000	2,500,100,000
Total fixed maturities | Available-for-sale Securities | State and local government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	22,500,000
Total fixed maturities | Available-for-sale Securities | Foreign government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	22,500,000
Total fixed maturities | Available-for-sale Securities | Foreign government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Foreign government obligations | Debt Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,330,300,000	4,997,700,000	5,013,800,000
Total fixed maturities | Available-for-sale Securities | Corporate debt securities | Debt Securities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Corporate debt securities | Debt Securities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,330,300,000	4,997,700,000	5,013,800,000
Total fixed maturities | Available-for-sale Securities | Corporate debt securities | Debt Securities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	822,600,000	836,700,000	1,267,600,000
Total fixed maturities | Available-for-sale Securities | Residential mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Residential mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	822,600,000	836,700,000	1,267,600,000
Total fixed maturities | Available-for-sale Securities | Residential mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,696,900,000	2,758,600,000	2,375,000,000
Total fixed maturities | Available-for-sale Securities | Commercial mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Commercial mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,696,900,000	2,758,600,000	2,375,000,000
Total fixed maturities | Available-for-sale Securities | Commercial mortgage-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,177,600,000	2,454,700,000	2,847,400,000
Total fixed maturities | Available-for-sale Securities | Other asset-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Other asset-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,177,600,000	2,454,700,000	2,847,400,000
Total fixed maturities | Available-for-sale Securities | Other asset-backed securities | Asset-backed Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	229,100,000	211,000,000	206,700,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	10,100,000	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	219,000,000	211,000,000	206,700,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	67,700,000	64,100,000	64,100,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Financials | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Financials | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	67,700,000	64,100,000	64,100,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Financials | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Public Utility, Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	4,600,000	11,400,000	32,200,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Public Utility, Equities [Member] | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Public Utility, Equities [Member] | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	4,600,000	11,400,000	32,200,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Public Utility, Equities [Member] | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	156,800,000	135,500,000	110,400,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Industrials | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	10,100,000	0	0
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Industrials | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	146,700,000	135,500,000	110,400,000
Total fixed maturities | Available-for-sale Securities | Redeemable preferred stocks | Industrials | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	24,061,800,000	20,209,900,000	18,311,700,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	17,107,100,000	13,971,600,000	11,653,800,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	6,734,300,000	6,043,400,000	6,468,500,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | U.S. government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	8,005,800,000	6,688,800,000	4,166,400,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | State and local government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	1,678,900,000	2,285,600,000	2,473,300,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Foreign government obligations | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	22,500,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Corporate debt securities | Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	7,422,400,000	4,997,200,000	4,991,600,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Residential mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	819,600,000	828,800,000	1,260,300,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Commercial mortgage-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,725,500,000	2,760,100,000	2,364,900,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Other asset-backed securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,189,200,000	2,454,500,000	2,843,300,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Redeemable preferred stocks
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	220,400,000	194,900,000	189,400,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Redeemable preferred stocks | Financials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	65,300,000	61,300,000	60,400,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Redeemable preferred stocks | Public Utility, Equities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	4,800,000	10,100,000	30,500,000
Total fixed maturities | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities | Redeemable preferred stocks | Industrials
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	150,300,000	123,500,000	98,500,000
Short-term investments | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	3,231,200,000	2,869,400,000	3,729,700,000
Short-term investments | Available-for-sale Securities | Fair Value, Inputs, Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	2,954,200,000	1,824,400,000	2,448,000,000
Short-term investments | Available-for-sale Securities | Fair Value, Inputs, Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	277,000,000	1,045,000,000	1,281,700,000
Short-term investments | Available-for-sale Securities | Fair Value, Inputs, Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	0	0	0
Short-term investments | Carrying (Reported) Amount, Fair Value Disclosure | Available-for-sale Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value	$ 3,231,200,000	$ 2,869,400,000	$ 3,729,700,000

Fair Value Fair Value - Additio

Fair Value Fair Value - Additional Information (Detail)	Jun. 30, 2018USD ($)security	Dec. 31, 2017USD ($)security	Jun. 30, 2017USD ($)
Fair Value Measurements [Line Items]
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	$ 0	$ 0	$ 0
Fair Value	$ 30,921,200,000	$ 27,274,700,000	$ 25,978,400,000
Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	72.00%	66.00%	58.00%
Fair Value	$ 14,049,000,000	$ 11,950,400,000	$ 9,788,000,000
Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Vendor Quoted Prices, Percent of FV Hierarchy Level	98.00%	98.00%	99.00%
Dealer Quoted Prices, Percent of FV Hierarchy Level	2.00%	2.00%	1.00%
Fair Value	$ 16,866,900,000	$ 15,319,000,000	$ 16,190,100,000
Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Fair Value	5,300,000	5,300,000	$ 300,000
Available-for-sale Securities
Fair Value Measurements [Line Items]
Fair Value	27,020,400,000
Available-for-sale Securities | Fair Value, Inputs, Level 1
Fair Value Measurements [Line Items]
Fair Value	10,829,700,000
Available-for-sale Securities | Fair Value, Inputs, Level 2
Fair Value Measurements [Line Items]
Fair Value	16,190,700,000
Available-for-sale Securities | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Fair Value	0
Nonredeemable preferred stocks | Internal Pricing | Fair Value, Inputs, Level 3
Fair Value Measurements [Line Items]
Fair Value	$ 5,000,000	$ 5,000,000
Number of securities	1	1	0

Debt (Detail)

Debt (Detail) - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Debt Instrument [Line Items]
Fair Value		$ 3,959	$ 3,643.6	$ 3,679.2
Debt, Long-term and Short-term, Combined Amount	[1]	3,859.5	3,306.3	3,383.4
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Long-term Debt		498.9	498.8	498.6
Fair Value		506.4	520.7	528
2.45% Senior Notes due 2027
Debt Instrument [Line Items]
Long-term Debt		496.3	496.1	496
Fair Value		451.3	477.9	474.2
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Long-term Debt		296.2	296.1	296
Fair Value		364.4	382.3	389
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Long-term Debt		395.4	395.3	395.2
Fair Value		492.5	516.9	522.7
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Long-term Debt		346.6	346.5	346.5
Fair Value		356.7	388.7	376.5
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Long-term Debt		395.3	395.2	395.2
Fair Value		363.9	402.9	389.7
4.125% Senior Notes due 2047
Debt Instrument [Line Items]
Long-term Debt		841.3	841.2	841.1
Fair Value		831.5	917.1	884.3
4.20% Senior Notes due 2048
Debt Instrument [Line Items]
Long-term Debt		589.5	0	0
Fair Value		592.3	0	0
Other debt instruments
Debt Instrument [Line Items]
Debt, Long-term and Short-term, Combined Amount		$ 0	$ 37.1	$ 114.8
[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, 2018	Jun. 30, 2017	Dec. 31, 2017
Debt Instrument [Line Items]
Repayments of Debt						$ 37,100,000	$ 12,500,000
Repayment of debt						0	$ 594,400,000
Discretionary Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity						$ 250,000,000
Debt term						30 days
Borrowings outstanding						$ 0		$ 0
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Debt, stated interest rate						3.75%
2.45% Senior Notes due 2027
Debt Instrument [Line Items]
Debt, stated interest rate						2.45%
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Debt, stated interest rate						6.625%
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Debt, stated interest rate						6.25%
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Debt, stated interest rate						4.35%
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Debt, stated interest rate						3.70%
4.125% Senior Notes due 2047
Debt Instrument [Line Items]
Debt, face amount	$ 850,000,000		$ 850,000,000
Debt, stated interest rate	4.125%		4.125%			4.125%
Proceeds, net of underwriter's discounts and commissions			$ 841,100,000
Unrealized gain (loss) on interest rate cash flow hedges	$ (8,000,000)		$ (8,000,000)
4.20% Senior Notes due 2048
Debt Instrument [Line Items]
Debt, face amount		$ 600,000,000		$ 600,000,000
Debt, stated interest rate		4.20%		4.20%		4.20%
Proceeds, net of underwriter's discounts and commissions		$ 589,500,000
Federal Funds Open Rate | Discretionary Line of Credit
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate						0.50%
Notes Payable, Other Payables | Other debt instruments
Debt Instrument [Line Items]
Repayments of Debt				$ 37,100,000
Trust Preferred Debt Securities [Member] | Other debt instruments
Debt Instrument [Line Items]
Repayments of Debt					$ 65,200,000

Income Taxes Additional Informa

Income Taxes Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Valuation allowance	$ 0	$ 0	$ 0	$ 0	$ 0
Effective Income Tax Rate Reconciliation, Percent	20.30%	32.80%	20.10%	32.90%

Loss and Loss Adjustment Expe48

Loss and Loss Adjustment Expense Reserves (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Loss and loss adjustment expense reserves	$ 14,070.8	$ 12,060.4	$ 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	2,293.1	1,947.9	2,170.1	1,801
Liability for Unpaid Claims and Claims Adjustment Expense, Net	11,777.7	10,112.5	$ 10,916.8	$ 9,567
Current Year Claims and Claims Adjustment Expense	10,164.6	8,804.1
Prior Year Claims and Claims Adjustment Expense	81.5	74.2
Liability for Unpaid Claims and Claims Adjustment Expense, Incurred Claims	10,246.1	8,878.3
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Current Year	5,533.7	4,951
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	3,851.5	3,381.8
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid	9,385.2	8,332.8
Commercial And Personal Auto Businesses [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		102
Special Lines and Property Businesses [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		(28)
Accident years 2017 and 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	72
Accident year 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense		56
Personal Lines | Personal auto businesses
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	57
Personal Lines | Agency Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	36	65
Personal Lines | Direct Auto [Member]
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	21	29
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	7	$ (20)
Commercial Lines
Liability for Claims and Claims Adjustment Expense [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 17

Supplemental Cash Flow Inform49

Supplemental Cash Flow Information (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Open reverse repurchase commitments	$ 0	$ 0	$ 0
Income Taxes Paid	358,900,000	452,900,000
Interest Paid, Including Capitalized Interest, Operating and Investing Activities	70,900,000	73,300,000
Cash and Cash Equivalents [Member] | ARX Holding Corp. [Member]
Open reverse repurchase commitments	$ 155,900,000	$ 130,200,000	$ 247,200,000

Segment Information Operating R

Segment Information Operating Results (Detail) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018USD ($)	Jun. 30, 2017USD ($)	Jun. 30, 2018USD ($)	Jun. 30, 2017USD ($)
Segment Reporting Information [Line Items]
Revenues	$ 8,018	$ 6,605.7	$ 15,448.1	$ 12,927.4
Pretax Profit (Loss)	880.1	554.6	1,790.9	1,196.1
Amortization of intangible assets	18	15.5	36	31
Personal Lines | Agency | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	3,225.7	2,752.5	6,289.5	5,384
Pretax Profit (Loss)	360.9	205.5	770	455.2
Personal Lines | Direct | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	3,211.8	2,650	6,228.1	5,173.7
Pretax Profit (Loss)	287.9	162.7	585.9	338
Operating Segments | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	7,634.2	6,313.3	14,808.2	12,340
Pretax Profit (Loss)	697.2	428	1,527.8	928
Operating Segments | Personal Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	6,437.5	5,402.5	12,517.6	10,557.7
Pretax Profit (Loss)	$ 648.8	$ 368.2	$ 1,355.9	$ 793.2
Personal auto insurance percentage of the total personal lines segment net premiums earned	94.00%	93.00%	94.00%	93.00%
Operating Segments | Commercial Lines | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	$ 884.3	$ 671.7	$ 1,692.9	$ 1,317.2
Pretax Profit (Loss)	$ 100.3	$ 56.2	$ 195.1	$ 123.6
Operating Segments | Property | Underwriting operations
Segment Reporting Information [Line Items]
Combined ratio amortization points	0.058	0.065	0.060	0.067
Revenues	$ 312.4	$ 239.1	$ 597.7	$ 465.1
Pretax Profit (Loss)	(51.9)	3.7	(23.4)	11.5
Amortization of intangible assets	18	15.5	36	31
Operating Segments | Other Indemnity | Underwriting operations
Segment Reporting Information [Line Items]
Revenues	0	0	0	0
Pretax Profit (Loss)	0	(0.1)	0.2	(0.3)
Segment Reconciling Items | Fees and other revenues
Segment Reporting Information [Line Items]
Revenues	116	88.8	219.8	174
Segment Reconciling Items | Service businesses
Segment Reporting Information [Line Items]
Revenues	42.9	32.7	77.1	61.2
Pretax Profit (Loss)	5.9	5.7	10.8	8.3
Segment Reconciling Items | Investment income
Segment Reporting Information [Line Items]
Revenues	224.9	170.9	343	352
Pretax Profit (Loss)	218.7	164.3	330.8	339.8
Segment Reconciling Items | Other gains (losses)
Segment Reporting Information [Line Items]
Revenues	0	0	0	0.2
Pretax Profit (Loss)	0	0	0	0.2
Segment Reconciling Items | Interest expense
Segment Reporting Information [Line Items]
Pretax Profit (Loss)	$ (41.7)	$ (43.4)	$ (78.5)	$ (80.2)

Segment Information Underwritin

Segment Information Underwriting Margins and Combined Ratios for our Underwriting Operations (Detail) - Underwriting operations	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Personal Lines | Agency
Segment Reporting Information [Line Items]
Underwriting Margin	11.20%	7.50%	12.20%	8.50%
Combined Ratio	88.80%	92.50%	87.80%	91.50%
Personal Lines | Direct
Segment Reporting Information [Line Items]
Underwriting Margin	9.00%	6.10%	9.40%	6.50%
Combined Ratio	91.00%	93.90%	90.60%	93.50%
Operating Segments
Segment Reporting Information [Line Items]
Underwriting Margin	9.10%	6.80%	10.30%	7.50%
Combined Ratio	90.90%	93.20%	89.70%	92.50%
Operating Segments | Personal Lines
Segment Reporting Information [Line Items]
Underwriting Margin	10.10%	6.80%	10.80%	7.50%
Combined Ratio	89.90%	93.20%	89.20%	92.50%
Operating Segments | Commercial Lines
Segment Reporting Information [Line Items]
Underwriting Margin	11.30%	8.40%	11.50%	9.40%
Combined Ratio	88.70%	91.60%	88.50%	90.60%
Operating Segments | Property
Segment Reporting Information [Line Items]
Underwriting Margin	(16.60%)	1.50%	(3.90%)	2.50%
Combined Ratio	116.60%	98.50%	103.90%	97.50%
Combined ratio amortization points	0.058	0.065	0.060	0.067

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	6 Months Ended	60 Months Ended
	Mar. 31, 2023	Jun. 30, 2018USD ($)$ / sharesshares	Mar. 14, 2023	Mar. 31, 2018$ / sharesshares	Dec. 31, 2017$ / sharesshares	Jun. 30, 2017$ / sharesshares
Class of Stock [Line Items]
Target percentage used to determine annual variable dividend		33.33%			33.33%
Gainshare factor used to determine annual variable dividend range, minimum		0
Minimum Payments of Ordinary Dividends, Common Stock | $		$ 0
Gainshare factor used to determine annual variable dividend range, maximum		2
Gainshare factor used to determine annual variable dividend		1.91
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued (shares) | shares				500,000
Preferred Stock, Liquidation Preference Per Share (USD per share) | $ / shares				$ 1,000	$ 0	$ 0
Series B Preferred Stock [Member] | Serial Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Shares Issued (shares) | shares		500,000			0	0
Preferred Stock, Liquidation Preference Per Share (USD per share) | $ / shares		$ 1,000
Scenario, Forecast [Member] | Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage			5.375%
London Interbank Offered Rate (LIBOR) [Member] | Scenario, Forecast [Member] | Series B Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Basis Spread on Variable Rate	2.539%

Other Comprehensive Income (L54

Other Comprehensive Income (Loss) Components of Other Comprehensive Income (Loss) Including Reclassification Adjustments by Income Statement (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss), before Tax [Roll Forward]
AOCI before Tax, Attributable to Parent, beginning balance	$ (218.6)	$ 1,645.9	$ 1,977.8	$ 1,439.5	$ 1,439.5
Pretax Cumulative Effect of New Accounting Principal in Period of Adoption					(2,006)
Pretax Reclassification of Stranded Amounts in AOCI to Retained Earnings					0
AOCI before Tax, Attributable to Parent, after Cumulative Effect Adjustment					(28.2)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, before Tax	(90.4)	177.2	(293.8)	431.7
Other Comprehensive Income (Loss), Unrealized Gain (Loss) on Derivatives Arising During Period, before Tax	0	(12)	0	(8)
Other Comprehensive Income (Loss), Foreign Currency Transaction and Translation Adjustment, before Tax	0	(0.2)	0	0.3
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Noncontrolling Interest	0.7	(1.9)	5.8	(3.4)
OCI, before Reclassifications, before Tax, Attributable to Parent	(89.7)	163.1	(288)	420.6
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, before Tax	0	(2.6)	0	(3.6)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, before Tax	(27.2)	37.2	(34.9)	88.8
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, before Tax	(0.3)	0.3	(0.5)	0.8
Total reclassification adjustment for amounts realized in net income, pre tax	(27.5)	34.9	(35.4)	86
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	(62.2)	128.2	(252.6)	334.6
AOCI before Tax, Attributable to Parent, ending balance	(280.8)	1,774.1	(280.8)	1,774.1	1,977.8
Accumulated Other Comprehensive Income (Loss), Tax [Roll Forward]
AOCI Tax, Attributable to Parent, beginning balance	46	(578.4)	(695.6)	(506.1)	(506.1)
Tax, Cumulative Effect of New Accounting Principal in Period of Adoption					705.8
Tax, Reclassification of Stranded Amounts in AOCI to Retained Earnings					(4.3)
AOCI Tax, Attributable to Parent, After Cumulative Effect Adjustment					(5.9)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, Tax	18.8	(62.1)	61.6	(151.3)
Forecasted transactions, Total tax (provision) benefit	0	4.2	0	2.8
Other Comprehensive Income (Loss), Foreign Current Translation Adjustment, Tax	0	0.1	0	(0.1)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Noncontrolling Interest	(0.1)	0.6	(1.2)	1.2
Other Comprehensive Income (Loss), Available-for-sale Securities, before Reclassification Adjustments, Tax	18.7	(57.2)	60.4	(147.4)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Tax	0	0.9	0	1.3
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Tax	5.7	(13)	7.3	(31.1)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Tax	0.1	(0.1)	0.1	(0.3)
Total reclassification adjustment for amounts realized in net income, Total tax (provision) benefit	5.8	(12.2)	7.4	(30.1)
Other Comprehensive Income (Loss), Tax, Portion Attributable to Parent	12.9	(45)	53	(117.3)
AOCI Tax, Attributable to Parent, ending balance	58.9	(623.4)	58.9	(623.4)	(695.6)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(172.6)	1,067.5	1,282.2	933.4	933.4
Cumulative Effect of New Accounting Principle in Period of Adoption					(1,300.2)
Reclassification of Stranded Amounts in AOCI to Retained Earnings					(4.3)
AOCI, Net of Tax, After Cumulative Effect Adjustment					(22.3)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	(71.6)	115.1	(232.2)	280.4
Forecasted transactions, after tax	0	(7.8)	0	(5.2)
Foreign currency translation adjustment	0	(0.1)	0	0.2
Other comprehensive (income) loss attributable to NCI	0.6	(1.3)	4.6	(2.2)	(2.3)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(71)	105.9	(227.6)	273.2
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0	(1.7)	0	(2.3)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	(21.5)	24.2	(27.6)	57.7
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.2)	0.2	(0.4)	0.5
Total reclassification adjustment for amounts realized in net income, after tax	(21.7)	22.7	(28)	55.9
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(49.3)	83.2	(199.6)	217.3
After tax total accumulated other comprehensive income, ending balance	(221.9)	1,150.7	(221.9)	1,150.7	1,282.2
Net unrealized gains (losses) on securities
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(160.8)	1,072	1,295	939.6	939.6
Cumulative Effect of New Accounting Principle in Period of Adoption					(1,300.2)
Reclassification of Stranded Amounts in AOCI to Retained Earnings					(1.1)
AOCI, Net of Tax, After Cumulative Effect Adjustment					(6.3)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	(71.6)	115.1	(232.2)	280.4
Forecasted transactions, after tax	0	0	0	0
Foreign currency translation adjustment	0	0	0	0
Other comprehensive (income) loss attributable to NCI	0	0	0	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	(71.6)	115.1	(232.2)	280.4
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0	(1.7)	0	(2.3)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	(21.5)	24.2	(27.6)	57.7
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0	0	0	0
Total reclassification adjustment for amounts realized in net income, after tax	(21.5)	22.5	(27.6)	55.4
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	(50.1)	92.6	(204.6)	225
After tax total accumulated other comprehensive income, ending balance	(210.9)	1,164.6	(210.9)	1,164.6	1,295
Accumulated Gain (Loss), Net, Cash Flow Hedge, Parent [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	(17.8)	(7.1)	(14.8)	(9.4)	(9.4)
Cumulative Effect of New Accounting Principle in Period of Adoption					0
Reclassification of Stranded Amounts in AOCI to Retained Earnings					(3.2)
AOCI, Net of Tax, After Cumulative Effect Adjustment					(18)
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	0	0	0	0
Forecasted transactions, after tax	0	(7.8)	0	(5.2)
Foreign currency translation adjustment	0	0	0	0
Other comprehensive (income) loss attributable to NCI	0	0	0	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	(7.8)	0	(5.2)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0	0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.2)	0.2	(0.4)	0.5
Total reclassification adjustment for amounts realized in net income, after tax	(0.2)	0.2	(0.4)	0.5
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.2	(8)	0.4	(5.7)
After tax total accumulated other comprehensive income, ending balance	(17.6)	(15.1)	(17.6)	(15.1)	(14.8)
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	0	(0.8)	0	(1.1)	(1.1)
Cumulative Effect of New Accounting Principle in Period of Adoption					0
Reclassification of Stranded Amounts in AOCI to Retained Earnings					0
AOCI, Net of Tax, After Cumulative Effect Adjustment					0
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	0	0	0	0
Forecasted transactions, after tax	0	0	0	0
Foreign currency translation adjustment	0	(0.1)	0	0.2
Other comprehensive (income) loss attributable to NCI	0	0	0	0
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0	(0.1)	0	0.2
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0	0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0	0	0	0
Total reclassification adjustment for amounts realized in net income, after tax	0	0	0	0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0	(0.1)	0	0.2
After tax total accumulated other comprehensive income, ending balance	0	(0.9)	0	(0.9)	0
Noncontrolling Interest [Member]
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
After tax total accumulated other comprehensive income, beginning balance	6	3.4	2	4.3	4.3
Cumulative Effect of New Accounting Principle in Period of Adoption					0
Reclassification of Stranded Amounts in AOCI to Retained Earnings					0
AOCI, Net of Tax, After Cumulative Effect Adjustment					2
Other Comprehensive Income (Loss), Securities, Available-for-Sale, Unrealized Holding Gain (Loss) Arising During Period, after Tax	0	0	0	0
Forecasted transactions, after tax	0	0	0	0
Foreign currency translation adjustment	0	0	0	0
Other comprehensive (income) loss attributable to NCI	0.6	(1.3)	4.6	(2.2)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax	0.6	(1.3)	4.6	(2.2)
Other Comprehensive (Income) Loss, Reclassification Adjustment from AOCI for Write-down of Securities, Net of Tax	0	0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Sale of Securities, Net of Tax	0	0	0	0
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0	0	0	0
Total reclassification adjustment for amounts realized in net income, after tax	0	0	0	0
Other Comprehensive Income (Loss), Net of Tax, Portion Attributable to Parent	0.6	(1.3)	4.6	(2.2)
After tax total accumulated other comprehensive income, ending balance	$ 6.6	$ 2.1	$ 6.6	$ 2.1	$ 2

Other Comprehensive Income (L55

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

Redeemable Noncontrolling Int56

Redeemable Noncontrolling Interest (Details) $ in Millions		3 Months Ended			6 Months Ended			12 Months Ended
		Jun. 30, 2018USD ($)shares	Dec. 31, 2017USD ($)	Jun. 30, 2017USD ($)	Jun. 30, 2018USD ($)shares		Jun. 30, 2017USD ($)	Dec. 31, 2017USD ($)	Apr. 01, 2021
Increase (Decrease) in Temporary Equity [Roll Forward]
Balance, Beginning of period					$ 503.7	[1]	$ 483.7	$ 483.7
Net income attributable to NCI		$ (3)		$ 5.1	8.8		11.1	5.9
Other comprehensive income (loss) attributable to NCI		(0.6)		1.3	(4.6)		2.2	2.3
Exercise of employee stock options					9.4		3.4	3.4
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests		(295.9)	$ 0	0
Change in redemption value of NCI					(3.2)		1.4	8.4
Balance, End of period	[1]	$ 218.2	$ 503.7	$ 501.8	$ 218.2		$ 501.8	$ 503.7
ARX Holding Corp. [Member]
Redeemable Noncontrolling Interest [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage		86.70%	69.00%	69.00%	86.70%		69.00%	69.00%
Shares held in Employee Stock Option Plan, Suspense Shares | shares		16,067			16,067
Employee Stock Ownership Plan (ESOP), Deferred Shares, Fair Value		$ 23.2			$ 23.2
Number of Shares Put Pursuant to Stockholders' Agreement | shares		204,527			204,527
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period | shares		5,483
Payments to Acquire Interest in Subsidiaries and Affiliates		$ 295.9
Scenario, Forecast [Member] | ARX Holding Corp. [Member]
Redeemable Noncontrolling Interest [Line Items]
Business Combination, Step Acquisition, Equity Interest in Acquiree, Including Subsequent Acquisition, Percentage									100.00%
[1]	See Note 12 – Redeemable Noncontrolling Interest for further discussion.

Goodwill and Intangible Asset57

Goodwill and Intangible Assets Goodwill (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill, Acquired During Period			$ 0
Goodwill	$ 452,700,000	$ 452,700,000	452,700,000	$ 452,700,000	$ 452,700,000	$ 452,700,000
Goodwill, Impaired, Accumulated Impairment Loss	0		0		$ 0
Amortization of intangible assets	$ 18,000,000	$ 15,500,000	$ 36,000,000	$ 31,000,000
Weighted Average [Member]
Goodwill [Line Items]
Finite-Lived Intangible Assets, Remaining Amortization Period			4 years 4 months 25 days
Trade name [Member]
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life				10 years	2 years

Goodwill and Intangible Asset58

Goodwill and Intangible Assets - Net Carrying Amount of Other Intangible Assets (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Schedule of Other Intangible Assets [Line Items]
Indefinite lived intangible assets	$ 12.4	$ 12.4	$ 12.4
Total	330.6	366.6	401.8
Accumulated amortization	211.7	175.7	140.5
Licensing Agreements [Member]
Schedule of Other Intangible Assets [Line Items]
Accumulated amortization	$ 0.6	$ 0.6	$ 0.6

Goodwill and Intangible Asset59

Goodwill and Intangible Assets - Finite-lived Intangible Assets (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Finite-Lived Intangible Assets [Line Items]
Accumulated Amortization	$ 211.7	$ 175.7	$ 140.5
Policies in force [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	256.2	256.2	256.2
Accumulated Amortization	119	100.7	82.4
Net Carrying Amount	137.2	155.5	173.8
Agency relationships [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	159.2	159.2	159.2
Accumulated Amortization	37	31.3	25.6
Net Carrying Amount	122.2	127.9	133.6
Software rights [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	79.1	79.1	79.1
Accumulated Amortization	34.7	29.4	24.1
Net Carrying Amount	44.4	49.7	55
Trade name [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	34.8	34.8	34.8
Accumulated Amortization	20.4	13.7	7.8
Net Carrying Amount	14.4	21.1	27
Finite-Lived Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	529.3	529.3	529.3
Accumulated Amortization	211.1	175.1	139.9
Net Carrying Amount	$ 318.2	$ 354.2	$ 389.4

New Accounting Standards Effect

New Accounting Standards Effect of Standard Change (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017	Mar. 31, 2017
Item Effected [Line Items]
Liabilities		$ 32,144.5	$ 27,196.5	$ 32,144.5	$ 27,196.5		$ 28,912.7
Assets		43,363.5	36,689.8	43,363.5	36,689.8		38,701.2
Provision (benefit) for income taxes		178.9	181.9	359.9	393.1
Cumulative Effect of New Accounting Principle in Period of Adoption							1,300.2
Reclassification of Stranded Amounts in AOCI to Retained Earnings							$ 4.3
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		140
Assets		140		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	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	$ 4.3	$ 0
[1]	See Note 14 – New Accounting Standards for further discussion.