CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY - USD ($) $ in Millions	Total	Common stock and additional paid-in capital [Member]	Accumulated other comprehensive income [Member]	Retained earnings (accumulated deficit) [Member]
Balance, beginning of period at Dec. 31, 2013	$ 4,955.2	$ 4,095.0	$ 731.8	$ 128.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	51.4			51.4
Change in unrealized appreciation (depreciation) of investments (net of applicable income tax expense (benefit))	94.2		94.2
Change in noncredit component of impairment losses on fixed maturities, available for sale (net of applicable income tax expense (benefit))	(0.7)		(0.7)
Cost of common stock and warrants repurchased	(376.5)	(376.5)
Dividends on common stock	(51.3)			(51.3)
Stock options, restricted stock and performance units	15.9	15.9
Balance, end of period at Dec. 31, 2014	4,688.2	3,734.4	825.3	128.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	270.7			270.7
Change in unrealized appreciation (depreciation) of investments (net of applicable income tax expense (benefit))	(420.4)		(420.4)
Change in noncredit component of impairment losses on fixed maturities, available for sale (net of applicable income tax expense (benefit))	(2.1)		(2.1)
Cost of common stock and warrants repurchased	(365.2)	(365.2)
Dividends on common stock	(52.1)			(52.1)
Stock options, restricted stock and performance units	19.4	19.4
Balance, end of period at Dec. 31, 2015	4,138.5	3,388.6	402.8	347.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	358.2			358.2
Change in unrealized appreciation (depreciation) of investments (net of applicable income tax expense (benefit))	221.1		221.1
Change in noncredit component of impairment losses on fixed maturities, available for sale (net of applicable income tax expense (benefit))	(1.5)		(1.5)
Cost of common stock and warrants repurchased	(203.0)	(203.0)
Dividends on common stock	(54.6)			(54.6)
Stock options, restricted stock and performance units	28.2	28.2
Balance, end of period at Dec. 31, 2016	$ 4,486.9	$ 3,213.8	$ 622.4	$ 650.7