Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Common Shares [Member]	Additional Paid-In Capital [Member]	Treasury Shares [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning Balance at Dec. 31, 2016	$ 1,792.7	$ 40.0	$ 1,123.3	$ (378.2)	$ 959.9	$ 47.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	50.3				50.3
Other comprehensive income - change in fair value of fixed maturities, net of taxes	49.5					49.5
Other comprehensive loss, net - other	(0.6)					(0.6)
Repurchase of common shares	(45.2)			(45.2)
Activity under stock incentive plans	12.4	0.4	12.0
Retirement of common shares (tax payments on equity compensation)	(8.2)	(0.1)	(8.1)
Employee stock purchase plan	2.0	0.1	1.9
Cash dividend declared - common shares	(33.2)				(33.2)
Ending Balance at Dec. 31, 2017	1,819.7	40.4	1,129.1	(423.4)	977.0	96.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	63.6				63.6
Other comprehensive income - change in fair value of fixed maturities, net of taxes	(75.5)					(75.5)
Other comprehensive loss, net - other	(2.4)					(2.4)
Repurchase of common shares	(31.7)			(31.7)
Activity under stock incentive plans	15.9	0.6	15.3
Retirement of common shares (tax payments on equity compensation)	(7.4)	(0.1)	(7.3)
Employee stock purchase plan	2.0		2.0
Stock Dividend	0.0	4.4	232.9		(237.3)
Cash dividend declared - common shares	(37.5)				(37.5)
Ending Balance at Dec. 31, 2018	1,746.7	45.3	1,372.0	(455.1)	862.6	(78.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net (loss) income	(8.4)				(8.4)
Other comprehensive income - change in fair value of fixed maturities, net of taxes	82.5					82.5
Other comprehensive loss, net - other	(1.6)					(1.6)
Activity under stock incentive plans	16.3	0.6	15.7
Retirement of common shares (tax payments on equity compensation)	(13.8)	(0.2)	(13.6)
Employee stock purchase plan	2.5		2.5
Cash dividend declared - common shares	(43.1)				(43.1)
Ending Balance at Dec. 31, 2019	$ 1,781.1	$ 45.7	$ 1,376.6	$ (455.1)	$ 811.1	$ 2.8