Condensed Consolidated Statements of Shareholders’ Equity - USD ($) $ in Millions	Total	Cumulative Effect of Adoption of New Accounting Standards	Common Stock	Paid-in Capital	Retained Earnings	Retained Earnings Cumulative Effect of Adoption of New Accounting Standards	Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) Cumulative Effect of Adoption of New Accounting Standards	Previously Reported	Previously Reported Common Stock	Previously Reported Paid-in Capital	Previously Reported Retained Earnings	Previously Reported Accumulated Other Comprehensive Income (Loss)
Beginning balance (in shares) at Dec. 31, 2017										51,500,000
Beginning balance at Dec. 31, 2017	$ 2,115.6	$ 0.0			$ 1,224.8	$ (18.2)	$ 212.6	$ 18.2	$ 2,115.6	$ 5.1	$ 673.1	$ 1,243.0	$ 194.4
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	190.1				190.1
Other Comprehensive Income (Loss), Net of Taxes (Note 13)	(190.8)						(190.8)
Cash Dividends and Dividend Equivalents to Shareholders	$ (56.4)				(56.4)
Repurchases of Common Stock (in shares)	0
Equity-based Compensation Cost (Note 11)	$ 18.6			$ 18.6
Equity-based Awards, Net of Shares Exchanged (Note 11) (in shares)			100,000
Equity-based Awards, Net of Shares Exchanged (Note 11)	(5.6)			(2.6)	(3.0)
Issuances of Common Stock (in shares)			13,100,000
Issuances of Common Stock	$ 978.6		$ 1.4	977.2	0.0
Ending balance (in shares) at Dec. 31, 2018										64,700,000
Ending 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]
Dividends paid to shareholders per share (in dollars per share)	$ 0.96
Net Income	$ 531.1				531.1
Other Comprehensive Income (Loss), Net of Taxes (Note 13)	314.3						314.3
Cash Dividends and Dividend Equivalents to Shareholders	$ (68.4)				(68.4)
Repurchases of Common Stock (in shares)	0
Equity-based Compensation Cost (Note 11)	$ 25.3			25.3
Equity-based Awards, Net of Shares Exchanged (Note 11) (in shares)			400,000
Equity-based Awards, Net of Shares Exchanged (Note 11)	(9.2)			(1.3)	(7.9)
Issuances of Common Stock (in shares)			1,600,000
Issuances of Common Stock	127.2		$ 0.2	127.0	0.0
Shares Issued Under Employee Stock Purchase Plan (Note 10)	1.9			1.9
Ending balance (in shares) at Dec. 31, 2019			66,700,000
Ending 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]
Dividends paid to shareholders per share (in dollars per share)	$ 1.03
Net Income	$ 409.9				409.9
Other Comprehensive Income (Loss), Net of Taxes (Note 13)	344.4						344.4
Cash Dividends and Dividend Equivalents to Shareholders	(79.4)				(79.4)
Stock Repurchased During Period, Value	(110.4)		$ (0.2)	(44.2)	(66.0)		0.0
Equity-based Compensation Cost (Note 11)	24.9			24.9
Equity-based Awards, Net of Shares Exchanged (Note 11) (in shares)			300,000
Equity-based Awards, Net of Shares Exchanged (Note 11)	(2.7)			0.9	(3.6)
Shares Issued Under Employee Stock Purchase Plan (Note 10)	4.4			4.4
Ending balance (in shares) at Dec. 31, 2020			65,400,000
Ending 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]
Dividends paid to shareholders per share (in dollars per share)	$ 1.20
Stock Repurchased During Period, Shares			(1,600,000)