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, 2019				148,084,000	148,084,000
Balance, beginning of year 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	301.8									301.8
Change in unrealized appreciation (depreciation) of investments (net of applicable income tax expense (benefit))	$ 813.6							813.6
Common stock repurchased (in shares)	(14,500,000)			(14,471,000)
Common stock repurchased	$ (263.0)			$ (0.2)		(262.8)
Dividends on common stock	(67.4)									(67.4)
Employee benefit plans, net of shares used to pay tax withholdings (in shares)				1,666,000
Employee benefits plans, net of shares used to pay tax withholdings	40.0					40.0
Balance, end of period (in shares) at Dec. 31, 2020				135,279,000
Balance, end of year at Dec. 31, 2020	5,484.2			$ 1.3		2,544.5		2,186.1		752.3
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	441.0									441.0
Change in unrealized appreciation (depreciation) of investments (net of applicable income tax expense (benefit))	$ (239.0)							(239.0)
Common stock repurchased (in shares)	(16,600,000)			(16,561,000)
Common stock repurchased	$ (402.4)			$ (0.1)		(402.3)
Dividends on common stock	(66.1)									(66.1)
Employee benefit plans, net of shares used to pay tax withholdings (in shares)				1,659,000
Employee benefits plans, net of shares used to pay tax withholdings	$ 42.0					42.0
Balance, end of period (in shares) at Dec. 31, 2021	120,377,152			120,377,000
Balance, end of year at Dec. 31, 2021	$ 5,259.7			$ 1.2		2,184.2		1,947.1		1,127.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	396.8									396.8
Change in unrealized appreciation (depreciation) of investments (net of applicable income tax expense (benefit))	$ (4,040.2)							(4,040.2)
Common stock repurchased (in shares)	(7,600,000)			(7,612,000)
Common stock repurchased	$ (180.0)			$ (0.1)		(179.9)
Dividends on common stock	(65.0)									(65.0)
Employee benefit plans, net of shares used to pay tax withholdings (in shares)				1,578,000
Employee benefits plans, net of shares used to pay tax withholdings	$ 29.5					29.5
Balance, end of period (in shares) at Dec. 31, 2022	114,343,070			114,343,000
Balance, end of year at Dec. 31, 2022	$ 1,400.8			$ 1.1		$ 2,033.8		$ (2,093.1)		$ 1,459.0