CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY (unaudited) - USD ($) $ in Millions	Total	Common stock and additional paid-in capital	Accumulated other comprehensive income	Retained earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of accounting change	$ 0.3	$ 0.9		$ (0.6)
Balance, beginning of period at Dec. 31, 2016	4,486.9	3,213.8	$ 622.4	650.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	62.3			62.3
Change in unrealized appreciation (depreciation) of investments (net of applicable income tax expense (benefit))	106.1		106.1
Change in noncredit component of impairment losses on fixed maturities, available for sale (net of applicable income tax expense)	1.1		1.1
Cost of common stock repurchased	(43.0)	(43.0)
Dividends on common stock	(14.0)			(14.0)
Stock options, restricted stock and performance units	7.1	7.1
Balance, end of period at Mar. 31, 2017	4,606.8	3,178.8	729.6	698.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of accounting change	0.0	0.0	(16.3)	16.3
Balance, beginning of period at Dec. 31, 2017	4,847.5	3,075.0	1,212.1	560.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	84.3			84.3
Change in unrealized appreciation (depreciation) of investments (net of applicable income tax expense (benefit))	(302.1)		(302.1)
Change in noncredit component of impairment losses on fixed maturities, available for sale (net of applicable income tax expense)	0.6		0.6
Dividends on common stock	(15.3)			(15.3)
Stock options, restricted stock and performance units	2.2	2.2
Balance, end of period at Mar. 31, 2018	$ 4,617.2	$ 3,077.2	$ 894.3	$ 645.7