Consolidated Statements of Shareholders’ Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income
Beginning balance (in shares) at Dec. 31, 2018		64.7
Beginning balance at Dec. 31, 2018	$ 3,050.1	$ 6.5	$ 1,666.3	$ 1,355.5	$ 21.8
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	277.4			277.4
Other Comprehensive Income (Note 8)	255.1				255.1
Cash Dividends and Dividend Equivalents to Shareholders	(32.8)			(32.8)
Issuances of Common Stock, Net of Transaction Costs (in shares)		1.6
Issuance of Common Stock, Net of Transaction Costs (Note 9)	127.2	$ 0.2	127.0
Equity-based Compensation Cost	14.1		14.1
Equity-based Awards, Net of Shares Exchanged (in shares)		0.3
Equity-based Awards, Net of Shares Exchanged	(7.4)		(1.0)	(6.4)
Ending balance (in shares) at Jun. 30, 2019		66.6
Ending balance at Jun. 30, 2019	3,683.7	$ 6.7	1,806.4	1,593.7	276.9
Beginning balance (in shares) at Mar. 31, 2019		64.9
Beginning balance at Mar. 31, 2019	3,320.1	$ 6.5	1,673.0	1,489.7	150.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	122.1			122.1
Other Comprehensive Income (Note 8)	126.0				126.0
Cash Dividends and Dividend Equivalents to Shareholders	(16.4)			(16.4)
Issuances of Common Stock, Net of Transaction Costs (in shares)		1.6
Issuance of Common Stock, Net of Transaction Costs (Note 9)	127.2	$ 0.2	127.0
Equity-based Compensation Cost	6.5		6.5
Equity-based Awards, Net of Shares Exchanged (in shares)		0.1
Equity-based Awards, Net of Shares Exchanged	(1.8)		(0.1)	(1.7)
Ending balance (in shares) at Jun. 30, 2019		66.6
Ending balance at Jun. 30, 2019	3,683.7	$ 6.7	1,806.4	1,593.7	276.9
Beginning balance (in shares) at Dec. 31, 2019		66.7
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 (Note 8)	161.2				161.2
Cash Dividends and Dividend Equivalents to Shareholders	(39.9)			(39.9)
Repurchases of Common Stock (in shares)		(1.6)
Repurchases of Common Stock (Note 9)	(110.4)	$ (0.2)	(44.2)	(66.0)
Shares Issued Under Employee Stock Purchase Plan	1.9		1.9
Equity-based Compensation Cost	13.6		13.6
Equity-based Awards, Net of Shares Exchanged (in shares)		0.2
Equity-based Awards, Net of Shares Exchanged	(0.9)	$ 0.0	2.0	(2.9)
Ending balance (in shares) at Jun. 30, 2020		65.3
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		65.4
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 (Note 8)	322.4				322.4
Cash Dividends and Dividend Equivalents to Shareholders	(19.7)			(19.7)
Repurchases of Common Stock (in shares)		(0.1)
Repurchases of Common Stock (Note 9)	(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		65.3
Ending balance at Jun. 30, 2020	$ 4,187.9	$ 6.5	$ 1,792.5	$ 1,891.6	$ 497.3