Condensed Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total		Cumulative Effect, Period of Adoption, Adjustment	Common Stock	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, 2018	$ 1,746.7			$ 45.3	$ 1,372.0	$ (455.1)	$ 862.6		$ (78.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	120.0						120.0
Other comprehensive income (loss) - change in fair value of fixed maturities, net of taxes	87.1								87.1
Other comprehensive income (loss) - other	(0.1)								(0.1)
Activity under stock incentive plans	7.8			0.4	7.4
Retirement of common shares (tax payments on equity compensation)	(10.9)			(0.1)	(10.8)
Employee stock purchase plan	1.1				1.1
Cash dividends declared - common shares	(22.7)						(22.7)
Ending Balance at Jun. 30, 2019	1,929.0			45.6	1,369.7	(455.1)	959.9		8.9
Beginning 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]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Ending Balance at Dec. 31, 2019	$ 1,781.1	[1]	$ (2.2)	45.7	1,376.6	(455.1)	811.1	$ (7.9)	2.8	$ 5.7
Beginning Balance at Mar. 31, 2019	1,880.6			45.4	1,374.9	(455.1)	943.0		(27.6)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	28.8						28.8
Other comprehensive income (loss) - change in fair value of fixed maturities, net of taxes	36.8								36.8
Other comprehensive income (loss) - other	(0.3)								(0.3)
Activity under stock incentive plans	3.6			0.3	3.3
Retirement of common shares (tax payments on equity compensation)	(9.3)			(0.1)	(9.2)
Employee stock purchase plan	0.7				0.7
Cash dividends declared - common shares	(11.9)						(11.9)
Ending Balance at Jun. 30, 2019	1,929.0			45.6	1,369.7	(455.1)	959.9		8.9
Beginning Balance at Dec. 31, 2019	1,781.1	[1]	$ (2.2)	45.7	1,376.6	(455.1)	811.1	$ (7.9)	2.8	$ 5.7
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	(25.2)						(25.2)
Other comprehensive income (loss) - change in fair value of fixed maturities, net of taxes	5.0								5.0
Other comprehensive income (loss) - other	(7.6)								(7.6)
Activity under stock incentive plans	5.3			0.4	4.9
Retirement of common shares (tax payments on equity compensation)	(6.5)			(0.1)	(6.4)
Employee stock purchase plan	1.4				1.4
Cash dividends declared - common shares	$ (21.3)						(21.3)
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Ending Balance at Jun. 30, 2020	$ 1,730.0			46.0	1,376.5	(455.1)	756.7		5.9
Beginning Balance at Mar. 31, 2020	1,636.8			45.9	1,374.2	(455.1)	773.7		(101.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	(6.4)						(6.4)
Other comprehensive income (loss) - change in fair value of fixed maturities, net of taxes	112.0								112.0
Other comprehensive income (loss) - other	(4.2)								(4.2)
Activity under stock incentive plans	2.2			0.1	2.1
Retirement of common shares (tax payments on equity compensation)	(0.7)			0.0	(0.7)
Employee stock purchase plan	0.9			0.0	0.9
Cash dividends declared - common shares	(10.6)						(10.6)
Ending Balance at Jun. 30, 2020	$ 1,730.0			$ 46.0	$ 1,376.5	$ (455.1)	$ 756.7		$ 5.9

[1] Derived from audited consolidated financial statements.