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]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjustment	[1]	Retained Earnings [Member] Cumulative Effect, Period of Adoption, Adjusted Balance	AOCI Attributable to Parent [Member]	AOCI Attributable to Parent [Member] Cumulative Effect, Period of Adoption, Adjustment	[1]	AOCI Attributable to Parent [Member] Cumulative Effect, Period of Adoption, Adjusted Balance	Unallocated ESSOP Shares [Member]
Beginning Balance at Dec. 31, 2017		$ 0.0	$ 269.2	$ 815.2	$ 3,206.9	$ 502.1		$ 3,708.9	$ 474.2	$ (502.1)		$ (27.9)	$ (32.4)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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
Adjustments to Additional Paid in Capital, Other				0.0
Net income (loss)	$ 370.5				370.5
Dividends on common shares ($1.84, $1.80 and $.78 per common share)					(229.6)
Common Stock, Dividends, Per Share, Cash Paid	$ 0.78
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		0.0	302.7	1,277.6	3,849.8	18.4		3,868.3	(210.0)	0.0		(210.0)	(73.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
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
Adjustments to Additional Paid in Capital, Other				0.0
Net income (loss)	$ 1,056.4				1,056.4
Dividends on common shares ($1.84, $1.80 and $.78 per common share)					(538.7)
Common Stock, Dividends, Per Share, Cash Paid	$ 1.80
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	$ (2.3)		$ 4,383.6	77.7	$ 0.0		$ 77.7	(64.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend reinvestment plan			0.0	0.9
Net issuance of shares under stock based compensation plans			0.4	5.2
Stock Issued During Period, Value, Conversion of Convertible Securities			0.0	0.0
Stock based compensation				2.4
ESSOP shares released				0.9									11.5
Adjustments to Additional Paid in Capital, Other				(0.2)
Net income (loss)	$ 558.6				558.6
Dividends on common shares ($1.84, $1.80 and $.78 per common share)					(547.5)
Common Stock, Dividends, Per Share, Cash Paid	$ 1.84
Net unrealized gains (losses) on securities, net of tax	$ 270.3								270.3
Net adjustment related to defined benefit pension plans, net of tax	(67.0)								(67.0)
Foreign currency translation and other adjustments	2.9								2.9
Origination of Loans to Employee Stock Ownership Plans	50.0												(50.0)
Ending Balance at Dec. 31, 2020	$ 6,186.6	$ 0.0	$ 304.1	$ 1,306.9	$ 4,394.8				$ 284.0				$ (103.2)

[1] Reflects the Company's adoption of new accounting principles relating to credit losses, lease accounting, and equity securities effective January 1, 2020, 2019 and 2018, respectively. Refer to additional discussion in Notes 1 and 4 to the Consolidated Financial Statements.