Condensed Consolidated Statements of Shareholders' Equity - USD ($) $ in Millions	Total		Common Shares [Member]	Additional Paid-In Capital [Member]	Treasury Shares [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [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 income	21.4					21.4
Other comprehensive income, net of tax	49.3						49.3
Repurchase of common shares	(36.6)				(36.6)
Activity under stock incentive plans	11.7		0.4	11.3
Retirement of common shares (tax payments on equity compensation)	(6.8)		(0.1)	(6.7)
Employee stock purchase plan	1.2			1.2
Cash dividends declared - common shares	(24.9)					(24.9)
Ending Balance at Sep. 30, 2017	1,808.0		40.3	1,129.1	(414.8)	956.4	97.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative effect of adoption of ASU | ASU 2016-01 [Member]	0.0					117.5	(117.5)
Cumulative effect of adoption of ASU | ASU 2018-02 [Member]	0.0					(20.7)	20.7
Beginning Balance at Dec. 31, 2017	1,819.7	[1]	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, net of tax	(57.9)						(57.9)
Repurchase of common shares	(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	0.0		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)

[1] Derived from audited consolidated financial statements.