Condensed Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total	Cumulative Effect, Period of Adoption, Adjustment	Preferred Shares and Additional Paid-in Capital	Common Shares	Additional Paid-In Capital	Treasury Shares	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Cumulative Effect, Period of Adoption, Adjustment
Beginning Balance at Dec. 31, 2019	$ 1,763.7	$ (2.2)	$ 0.0	$ 45.7	$ 1,376.6	$ (455.1)	$ 793.7	$ (7.9)	$ 2.8	$ 5.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(53.2)						(53.2)
Preferred shares issued	144.0		144.0
Other comprehensive income - Change in fair value of fixed maturities, net of taxes	40.7								40.7
Other comprehensive loss, net - Other	(12.3)								(12.3)
Activity under stock incentive plans	6.8			0.4	6.4
Retirement of common shares (tax payments on equity compensation)	(6.6)			(0.1)	(6.5)
Employee stock purchase plan	2.0				2.0
Dividends on preferred shares	(2.0)						(2.0)
Cash dividend declared - common shares	(32.2)						(32.2)
Ending Balance at Sep. 30, 2020	1,848.7		144.0	46.0	1,378.5	(455.1)	698.4		36.9
Beginning Balance at Jun. 30, 2020	1,707.7		0.0	46.0	1,376.5	(455.1)	734.4		5.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(23.1)						(23.1)
Preferred shares issued	144.0		144.0
Other comprehensive income - Change in fair value of fixed maturities, net of taxes	35.7								35.7
Other comprehensive loss, net - Other	(4.7)								(4.7)
Activity under stock incentive plans	1.5				1.5
Retirement of common shares (tax payments on equity compensation)	(0.1)				(0.1)
Employee stock purchase plan	0.6				0.6
Dividends on preferred shares	(2.0)						(2.0)
Cash dividend declared - common shares	(10.9)						(10.9)
Ending Balance at Sep. 30, 2020	1,848.7		144.0	46.0	1,378.5	(455.1)	698.4		36.9
Beginning Balance at Dec. 31, 2020	1,857.8		144.0	46.0	1,380.2	(455.1)	684.1		58.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	122.0						122.0
Other comprehensive income - Change in fair value of fixed maturities, net of taxes	(58.3)								(58.3)
Other comprehensive loss, net - Other	1.7								1.7
Activity under stock incentive plans	6.0			0.2	5.8
Retirement of common shares (tax payments on equity compensation)	(2.5)				(2.5)
Employee stock purchase plan	1.5				1.5
Dividends on preferred shares	(7.9)						(7.9)
Cash dividend declared - common shares	(32.7)						(32.7)
Ending Balance at Sep. 30, 2021	1,887.6		144.0	46.2	1,385.0	(455.1)	765.5		2.0
Beginning Balance at Jun. 30, 2021	1,897.9		144.0	46.2	1,383.1	(455.1)	756.5		23.2
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	22.4						22.4
Other comprehensive income - Change in fair value of fixed maturities, net of taxes	(18.9)								(18.9)
Other comprehensive loss, net - Other	(2.3)								(2.3)
Activity under stock incentive plans	1.5				1.5
Retirement of common shares (tax payments on equity compensation)	(0.1)				(0.1)
Employee stock purchase plan	0.5				0.5
Dividends on preferred shares	(2.6)						(2.6)
Cash dividend declared - common shares	(10.8)						(10.8)
Ending Balance at Sep. 30, 2021	$ 1,887.6		$ 144.0	$ 46.2	$ 1,385.0	$ (455.1)	$ 765.5		$ 2.0