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	66.6					66.6
Other comprehensive income (loss)- Change in fair value of investment, net of tax	(49.7)						(49.7)
Other comprehensive income (loss)- Other	(4.0)						(4.0)
Repurchase of common shares	(20.4)				(20.4)
Activity under stock incentive plans	9.7		0.4	9.3
Retirement of common shares (tax payments on equity compensation)	(6.6)		(0.1)	(6.5)
Employee stock purchase plan	0.5			0.5
15% stock dividend			4.4	232.9		(237.3)
Cash dividends declared - common shares	(18.7)					(18.7)
Ending Balance at Jun. 30, 2018	1,797.1		45.1	1,365.3	(443.8)	884.4	(53.9)
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	66.6
Ending 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	120.0					120.0
Other comprehensive income (loss)- Change in fair value of investment, net of tax	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

[1] Derived from audited consolidated financial statements.