Condensed Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total		Common Stock [Member]	Additional Paid-In Capital [Member]	Treasury Shares [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) Member]
Beginning 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 income	107.2					107.2
Other comprehensive income (loss) - change in fair value of fixed maturities, net of taxes	(58.2)						(58.2)
Other comprehensive income (loss), net - Other	0.3						0.3
Repurchase of common share	(30.0)				(30.0)
Activity under stock incentive plans	12.7		0.5	12.2
Retirement of common shares (tax payments on equity compensation)	(7.0)		(0.1)	(6.9)
Employee stock purchase plan	1.4			1.4
15% Stock Dividend			4.4	232.9		(237.3)
Cash dividends declared - common shares	(28.0)					(28.0)
Ending Balance at Sep. 30, 2018	1,818.1		45.2	1,368.7	(453.4)	915.7	(58.1)
Beginning Balance at Dec. 31, 2018	1,746.7	[1]	45.3	1,372.0	(455.1)	862.6	(78.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	94.9					94.9
Other comprehensive income (loss) - change in fair value of fixed maturities, net of taxes	82.9						82.9
Other comprehensive income (loss), net - Other	(0.1)						(0.1)
Activity under stock incentive plans	11.0		0.4	10.6
Retirement of common shares (tax payments on equity compensation)	(11.2)		(0.1)	(11.1)
Employee stock purchase plan	1.7			1.7
Cash dividends declared - common shares	(32.5)					(32.5)
Ending Balance at Sep. 30, 2019	$ 1,893.4		$ 45.6	$ 1,373.2	$ (455.1)	$ 925.0	$ 4.7

[1] Derived from audited consolidated financial statements.