Consolidated Statements of Shareholders’ Equity - USD ($) shares in Millions, $ in Millions	Total	Common Stock	Paid-in Capital	Retained Earnings	Accumulated Other Comprehensive Income	Previously Reported	Previously Reported Common Stock	Previously Reported Paid-in Capital	Previously Reported Retained Earnings	Previously Reported Accumulated Other Comprehensive Income	Cumulative Effect of Adoption of New Accounting Standard	Cumulative Effect of Adoption of New Accounting Standard Retained Earnings	Cumulative Effect of Adoption of New Accounting Standard Accumulated Other Comprehensive Income
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Cumulative Effect of Adoption of New Accounting Standard											$ 0.0	$ (18.2)	$ 18.2
Beginning balance (in shares) at Dec. 31, 2017		51.5					51.5
Beginning balance at Dec. 31, 2017	$ 2,115.6	$ 5.1	$ 673.1	$ 1,224.8	$ 212.6	$ 2,115.6	$ 5.1	$ 673.1	$ 1,243.0	$ 194.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	183.6			183.6
Other Comprehensive Income (Note 8)	(181.6)				(181.6)
Cash Dividends and Dividend Equivalents to Shareholders	(40.7)			(40.7)
Issuances of Common Stock, Net of Transaction Costs (in shares)		13.1
Issuance of Common Stock, Net of Transaction Costs	978.5	$ 1.3	977.2
Equity-based Compensation Cost	13.5		13.5
Equity-based Awards, Net of Shares Exchanged (in shares)		0.1
Equity-based Awards, Net of Shares Exchanged	(5.1)	$ 0.1	(2.5)	(2.6)	(0.1)
Ending balance (in shares) at Sep. 30, 2018		64.7
Ending balance at Sep. 30, 2018	3,063.8	$ 6.5	1,661.3	1,365.1	30.9
Beginning balance (in shares) at Jun. 30, 2018		51.6
Beginning balance at Jun. 30, 2018	2,045.7	$ 5.2	681.5	1,289.4	69.6
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	92.2			92.2
Other Comprehensive Income (Note 8)	(38.6)				(38.6)
Cash Dividends and Dividend Equivalents to Shareholders	(15.8)			(15.8)
Issuances of Common Stock, Net of Transaction Costs (in shares)		13.1
Issuance of Common Stock, Net of Transaction Costs	978.5	$ 1.3	977.2
Equity-based Compensation Cost	5.4		5.4
Equity-based Awards, Net of Shares Exchanged	(3.6)		(2.8)	(0.7)	(0.1)
Ending balance (in shares) at Sep. 30, 2018		64.7
Ending balance at Sep. 30, 2018	3,063.8	$ 6.5	1,661.3	1,365.1	30.9
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	406.4			406.4
Other Comprehensive Income (Note 8)	348.3				348.3
Cash Dividends and Dividend Equivalents to Shareholders	(49.5)			(49.5)
Issuances of Common Stock, Net of Transaction Costs (in shares)		1.6
Issuance of Common Stock, Net of Transaction Costs	127.2	$ 0.2	127.0
Shares Issued Under Employee Stock Purchase Plan	1.0		1.0
Equity-based Compensation Cost	20.0		20.0
Equity-based Awards, Net of Shares Exchanged (in shares)		0.3
Equity-based Awards, Net of Shares Exchanged	(9.3)		(1.4)	(7.9)
Ending balance (in shares) at Sep. 30, 2019		66.6
Ending balance at Sep. 30, 2019	3,894.2	$ 6.7	1,812.9	1,704.5	370.1
Beginning balance (in shares) at Jun. 30, 2019		66.6
Beginning balance at Jun. 30, 2019	3,683.7	$ 6.7	1,806.4	1,593.7	276.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	129.0			129.0
Other Comprehensive Income (Note 8)	93.2				93.2
Cash Dividends and Dividend Equivalents to Shareholders	(16.7)			(16.7)
Shares Issued Under Employee Stock Purchase Plan	1.0		1.0
Equity-based Compensation Cost	5.9		5.9
Equity-based Awards, Net of Shares Exchanged	(1.9)		(0.4)	(1.5)
Ending balance (in shares) at Sep. 30, 2019		66.6
Ending balance at Sep. 30, 2019	$ 3,894.2	$ 6.7	$ 1,812.9	$ 1,704.5	$ 370.1