Consolidated Statements of Preferred Stock and Common Shareholders' Equity - USD ($) $ in Millions	Total	Convertible Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Unallocated ESSOP Shares [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification - change in accounting principle					$ 0.0	$ 0.0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance					3,199.6	323.6
Beginning Balance at Dec. 31, 2016		$ 0.0	$ 262.7	$ 713.8	3,199.6	323.6	$ (39.2)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend reinvestment plan			0.0	0.8
Net issuance of shares under stock based compensation plans			1.3	15.0
Stock Issued During Period, Value, Conversion of Convertible Securities			5.1	73.8
Stock based compensation				4.1
ESSOP shares released				7.3			6.8
Net income (loss)	$ 560.5				560.5
Dividends on common shares ($1.80, .78 and $1.76 per share)					(468.0)
Common Stock, Dividends, Per Share, Cash Paid	$ 1.76
Reclassification of income tax effects of the Tax Cuts and Jobs Act	$ 85.1				(85.1)	85.1
Net unrealized gains (losses) on securities, net of tax	73.9					73.9
Net adjustment related to defined benefit pension plans, net of tax	(18.0)					(18.0)
Foreign currency translation and other adjustments	9.5					9.5
Origination of Loans to Employee Stock Ownership Plans	0.0						0.0
Ending Balance at Dec. 31, 2017		0.0	269.2	815.2	3,206.9	474.2	(32.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Reclassification - change in accounting principle					502.1	(502.1)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance					3,708.9	(27.9)
Dividend reinvestment plan			0.0	1.7
Net issuance of shares under stock based compensation plans			1.1	15.7
Stock Issued During Period, Value, Conversion of Convertible Securities			32.2	438.1
Stock based compensation				4.1
ESSOP shares released				2.6			8.4
Net income (loss)	$ 370.5				370.5
Dividends on common shares ($1.80, .78 and $1.76 per share)					(229.6)
Common Stock, Dividends, Per Share, Cash Paid	$ 0.78
Reclassification of income tax effects of the Tax Cuts and Jobs Act					0.0	0.0
Net unrealized gains (losses) on securities, net of tax	$ (176.3)					(176.3)
Net adjustment related to defined benefit pension plans, net of tax	5.4					5.4
Foreign currency translation and other adjustments	(11.1)					(11.1)
Origination of Loans to Employee Stock Ownership Plans	(50.0)						(50.0)
Ending Balance at Dec. 31, 2018	5,146.2	0.0	302.7	1,277.6	3,849.8	(210.0)	(73.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Special dividend declared in December 2017 and paid in January 2018	269.2
Reclassification - change in accounting principle	502.1				18.4	0.0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance					3,868.3	(210.0)
Dividend reinvestment plan			0.0	1.7
Net issuance of shares under stock based compensation plans			0.8	11.0
Stock Issued During Period, Value, Conversion of Convertible Securities			0.0	0.0
Stock based compensation				4.0
ESSOP shares released				3.0			9.1
Net income (loss)	$ 1,056.4				1,056.4
Dividends on common shares ($1.80, .78 and $1.76 per share)					(538.7)
Common Stock, Dividends, Per Share, Cash Paid	$ 1.80
Reclassification of income tax effects of the Tax Cuts and Jobs Act					0.0	0.0
Net unrealized gains (losses) on securities, net of tax	$ 287.2					287.2
Net adjustment related to defined benefit pension plans, net of tax	(5.4)					(5.4)
Foreign currency translation and other adjustments	5.9					5.9
Origination of Loans to Employee Stock Ownership Plans	0.0						0.0
Ending Balance at Dec. 31, 2019	6,000.1	$ 0.0	$ 303.6	$ 1,297.5	$ 4,386.0	$ 77.7	$ (64.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Special dividend paid in September 2019	$ 303.4