CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY - USD ($) $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	Cumulative Effect, Period of Adoption, Adjusted Balance	Common stock	Common stock Cumulative Effect, Period of Adoption, Adjusted Balance	Additional paid-in capital	Additional paid-in capital Cumulative Effect, Period of Adoption, Adjusted Balance	Accumulated other comprehensive income	Accumulated other comprehensive income Cumulative Effect, Period of Adoption, Adjusted Balance	Retained earnings	Retained earnings Cumulative Effect, Period of Adoption, Adjustment	Retained earnings Cumulative Effect, Period of Adoption, Adjusted Balance
Balance, beginning of period (in shares) at Dec. 31, 2018				162,202,000
Balance, beginning of period at Dec. 31, 2018	$ 3,370.9	$ (3.1)	$ 3,367.8	$ 1.6	$ 1.6	$ 2,995.0	$ 2,995.0	$ 177.7	$ 177.7	$ 196.6	$ (3.1)	$ 193.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	131.4									131.4
Change in unrealized appreciation (depreciation) of investments (net of applicable income tax expense)	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)	5.6							5.6
Common stock repurchased (in shares)				(11,033,000)
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				152,183,000
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	42.0									42.0
Change in unrealized appreciation (depreciation) of investments (net of applicable income tax expense)	339.2							339.2
Change in noncredit component of impairment losses on fixed maturities, available for sale (net of applicable income tax expense)	5.5							5.5
Common stock repurchased (in shares)				(4,798,000)
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				152,183,000
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 Dec. 31, 2019	148,084,178			148,084,000
Balance, beginning of period at Dec. 31, 2019	$ 4,677.0	$ (17.8)	$ 4,659.2	$ 1.5	$ 1.5	2,767.3	$ 2,767.3	1,372.5	$ 1,372.5	535.7	$ (17.8)	$ 517.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	190.0									190.0
Change in unrealized appreciation (depreciation) of investments (net of applicable income tax expense)	429.1							429.1
Common stock repurchased (in shares)				(10,048,000)
Common stock repurchased	(163.0)			$ (0.1)		(162.9)
Dividends on common stock	(50.4)									(50.4)
Employee benefit plans, net of shares used to pay tax withholdings (in shares)				895,000
Employee benefit plans, net of shares used to pay tax withholdings	$ 19.0					19.0
Balance, end of period (in shares) at Sep. 30, 2020	138,931,352			138,931,000
Balance, end of period at Sep. 30, 2020	$ 5,083.9			$ 1.4		2,623.4		1,801.6		657.5
Balance, beginning of period (in shares) at Jun. 30, 2020				141,719,000
Balance, beginning of period at Jun. 30, 2020	4,731.2			$ 1.4		2,664.3		1,520.2		545.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	129.2									129.2
Change in unrealized appreciation (depreciation) of investments (net of applicable income tax expense)	281.4							281.4
Common stock repurchased (in shares)				(2,997,000)
Common stock repurchased	(50.0)					(50.0)
Dividends on common stock	(17.0)									(17.0)
Employee benefit plans, net of shares used to pay tax withholdings (in shares)				209,000
Employee benefit plans, net of shares used to pay tax withholdings	$ 9.1					9.1
Balance, end of period (in shares) at Sep. 30, 2020	138,931,352			138,931,000
Balance, end of period at Sep. 30, 2020	$ 5,083.9			$ 1.4		$ 2,623.4		$ 1,801.6		$ 657.5