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	91.4			91.4
Other Comprehensive Income (Note 8)	(143.0)				(143.0)
Cash Dividends and Dividend Equivalents to Shareholders	(24.9)			(24.9)
Equity-based Compensation Cost	8.1		8.1
Equity-based Awards, Net of Shares Exchanged (in shares)		0.1
Equity-based Awards, Net of Shares Exchanged	(1.5)	$ 0.1	0.3	(1.9)
Ending balance (in shares) at Jun. 30, 2018		51.6
Ending balance at Jun. 30, 2018	2,045.7	$ 5.2	681.5	1,289.4	69.6
Beginning balance (in shares) at Mar. 31, 2018		51.5
Beginning balance at Mar. 31, 2018	2,063.8	$ 5.1	676.9	1,264.9	116.9
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	37.6			37.6
Other Comprehensive Income (Note 8)	(47.3)				(47.3)
Cash Dividends and Dividend Equivalents to Shareholders	(12.4)			(12.4)
Equity-based Compensation Cost	4.7		4.7
Equity-based Awards, Net of Shares Exchanged (in shares)		0.1
Equity-based Awards, Net of Shares Exchanged	(0.7)	$ 0.1	(0.1)	(0.7)
Ending balance (in shares) at Jun. 30, 2018		51.6
Ending balance at Jun. 30, 2018	2,045.7	$ 5.2	681.5	1,289.4	69.6
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	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	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