CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY - USD ($) $ in Millions	Total	Common stock	Additional paid-in capital	Accumulated other comprehensive income	Retained earnings
Balance, beginning of period (in shares) at Dec. 31, 2017		166,858,000
Balance, beginning of period at Dec. 31, 2017	$ 4,847.5	$ 1.7	$ 3,073.3	$ 1,212.1	$ 560.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(343.3)				(343.3)
Change in unrealized appreciation (depreciation) of investments (net of applicable income tax expense (benefit))	(791.5)			(791.5)
Change in noncredit component of impairment losses on fixed maturities, available for sale (net of applicable income tax expense (benefit))	(0.8)			(0.8)
Cost of common stock repurchased (in shares)		(2,998,000)
Cost of common stock repurchased	(60.5)	$ (0.1)	(60.4)
Dividends on common stock	(48.6)				(48.6)
Employee benefit plans, net of shares used to pay tax withholdings (in shares)		774,000
Employee benefit plans, net of shares used to pay tax withholdings	17.1		17.1
Balance, end of period (in shares) at Sep. 30, 2018		164,634,000
Balance, end of period at Sep. 30, 2018	3,619.9	$ 1.6	3,030.0	403.5	184.8
Balance, beginning of period (in shares) at Jun. 30, 2018		164,433,000
Balance, beginning of period at Jun. 30, 2018	4,454.9	$ 1.6	3,021.9	700.2	731.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(529.8)				(529.8)
Change in unrealized appreciation (depreciation) of investments (net of applicable income tax expense (benefit))	(296.1)			(296.1)
Change in noncredit component of impairment losses on fixed maturities, available for sale (net of applicable income tax expense (benefit))	(0.6)			(0.6)
Dividends on common stock	(16.6)				(16.6)
Employee benefit plans, net of shares used to pay tax withholdings (in shares)		201,000
Employee benefit plans, net of shares used to pay tax withholdings	8.1		8.1
Balance, end of period (in shares) at Sep. 30, 2018		164,634,000
Balance, end of period at Sep. 30, 2018	$ 3,619.9	$ 1.6	3,030.0	403.5	184.8
Balance, beginning of period (in shares) at Dec. 31, 2018	162,201,692	162,202,000
Balance, beginning of period at Dec. 31, 2018	$ 3,370.9	$ 1.6	2,995.0	177.7	196.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	131.4				131.4
Change in unrealized appreciation (depreciation) of investments (net of applicable income tax expense (benefit))	1,259.6			1,259.6
Change in noncredit component of impairment losses on fixed maturities, available for sale (net of applicable income tax expense (benefit))	5.6			5.6
Cost of common stock repurchased (in shares)		(11,033,000)
Cost of common stock repurchased	(177.3)	$ (0.1)	(177.2)
Dividends on common stock	(50.6)				(50.6)
Employee benefit plans, net of shares used to pay tax withholdings (in shares)		1,014,000
Employee benefit plans, net of shares used to pay tax withholdings	$ 16.8		16.8
Balance, end of period (in shares) at Sep. 30, 2019	152,183,491	152,183,000
Balance, end of period at Sep. 30, 2019	$ 4,553.3	$ 1.5	2,834.6	1,442.9	274.3
Balance, beginning of period (in shares) at Jun. 30, 2019		156,768,000
Balance, beginning of period at Jun. 30, 2019	4,252.2	$ 1.6	2,903.2	1,098.2	249.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	42.0				42.0
Change in unrealized appreciation (depreciation) of investments (net of applicable income tax expense (benefit))	339.2			339.2
Change in noncredit component of impairment losses on fixed maturities, available for sale (net of applicable income tax expense (benefit))	5.5			5.5
Cost of common stock repurchased (in shares)		(4,798,000)
Cost of common stock repurchased	(75.3)	$ (0.1)	(75.2)
Dividends on common stock	(16.9)				(16.9)
Employee benefit plans, net of shares used to pay tax withholdings (in shares)		213,000
Employee benefit plans, net of shares used to pay tax withholdings	$ 6.6		6.6
Balance, end of period (in shares) at Sep. 30, 2019	152,183,491	152,183,000
Balance, end of period at Sep. 30, 2019	$ 4,553.3	$ 1.5	$ 2,834.6	$ 1,442.9	$ 274.3