Consolidated Statements of Shareholders’ Equity - USD ($) shares in Thousands, $ in Millions	Total	Common Stock	Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income
Beginning balance (in shares) at Dec. 31, 2019		66,700
Beginning balance at Dec. 31, 2019	$ 3,972.3	$ 6.7	$ 1,819.2	$ 1,810.3	$ 336.1
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	190.1			190.1
Other Comprehensive Income (Loss), Net of Tax	161.2				161.2
Cash Dividends and Dividend Equivalents to Shareholders	(39.9)			(39.9)
Repurchases of Common Stock (in shares)		1,617
Repurchases of Common Stock (Note 10)	(110.4)	$ (0.2)	(44.2)	(66.0)
Shares Issued Under Employee Stock Purchase Plan			1.9
Shares Issued Under Employee Stock Purchase Plan	1.9
Equity-based Compensation Cost	13.6		13.6
Equity-based Awards, Net of Shares Exchanged (in shares)		200
Equity-based Awards, Net of Shares Exchanged	(0.9)	$ 0.0	2.0	(2.9)
Ending balance (in shares) at Jun. 30, 2020		65,300
Ending balance at Jun. 30, 2020	4,187.9	$ 6.5	1,792.5	1,891.6	497.3
Beginning balance (in shares) at Mar. 31, 2020		65,400
Beginning balance at Mar. 31, 2020	3,760.8	$ 6.5	1,788.2	1,791.2	174.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	126.1			126.1
Other Comprehensive Income (Loss), Net of Tax	322.4				322.4
Cash Dividends and Dividend Equivalents to Shareholders	(19.7)			(19.7)
Repurchases of Common Stock (in shares)		128
Repurchases of Common Stock (Note 10)	(9.2)	$ (0.1)	(3.5)	(5.6)
Shares Issued Under Employee Stock Purchase Plan	0.9		0.9
Equity-based Compensation Cost	6.3		6.3
Equity-based Awards, Net of Shares Exchanged	0.3	$ 0.1	0.6	(0.4)
Ending balance (in shares) at Jun. 30, 2020		65,300
Ending balance at Jun. 30, 2020	4,187.9	$ 6.5	1,792.5	1,891.6	497.3
Beginning balance (in shares) at Dec. 31, 2020		65,400
Beginning balance at Dec. 31, 2020	4,563.4	$ 6.5	1,805.2	2,071.2	680.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	60.6			60.6
Other Comprehensive Income (Loss), Net of Tax	(137.5)				(137.5)
Cash Dividends and Dividend Equivalents to Shareholders	(41.4)			(41.4)
Repurchases of Common Stock (in shares)		2,000
Repurchases of Common Stock (Note 10)	(158.7)	$ (0.2)	(56.7)	(101.8)
Shares Issued Under Employee Stock Purchase Plan	2.5		2.5
Equity-based Compensation Cost	18.5		18.5
Equity-based Awards, Net of Shares Exchanged (in shares)		200
Equity-based Awards, Net of Shares Exchanged	(1.2)	$ 0.1	1.4	(2.7)
Ending balance (in shares) at Jun. 30, 2021		63,600
Ending balance at Jun. 30, 2021	4,306.2	$ 6.4	1,770.9	1,985.9	543.0
Beginning balance (in shares) at Mar. 31, 2021		65,000
Beginning balance at Mar. 31, 2021	4,339.1	$ 6.5	1,802.1	2,140.0	390.5
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	(62.6)			(62.6)
Other Comprehensive Income (Loss), Net of Tax	152.5				152.5
Cash Dividends and Dividend Equivalents to Shareholders	(20.4)			(20.4)
Repurchases of Common Stock (in shares)		1,400
Repurchases of Common Stock (Note 10)	(111.6)	$ (0.2)	(40.3)	(71.1)
Shares Issued Under Employee Stock Purchase Plan	1.3		1.3
Equity-based Compensation Cost	6.2		6.2
Equity-based Awards, Net of Shares Exchanged (in shares)		0
Equity-based Awards, Net of Shares Exchanged	1.7	$ 0.1	1.6	0.0
Ending balance (in shares) at Jun. 30, 2021		63,600
Ending balance at Jun. 30, 2021	$ 4,306.2	$ 6.4	$ 1,770.9	$ 1,985.9	$ 543.0