Consolidated Statements of Preferred Stock and Common Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Retained Earnings Cumulative Effect, Period of Adoption, Adjustment	Retained Earnings Cumulative Effect, Period of Adoption, Adjusted Balance	AOCI Attributable to Parent	Unallocated ESSOP Shares	Convertible Preferred Stock Preferred Stock
Common Stock, Dividends, Per Share, Cash Paid	$ 0.4000
Balance at Dec. 31, 2018		$ 302.7	$ 1,277.6	$ 3,849.8	$ 18.4	$ 3,868.3	$ (210.0)	$ (73.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend reinvestment plan		0.0	0.4
Net issuance of shares under stock based compensation plans		0.4	5.9
Stock based compensation			3.2
ESSOP shares released			1.3					4.5
Net income (loss)	$ 577.7			577.7
Dividends on common stock				(119.4)
Net unrealized gains (losses) on securities, net of tax	249.8						249.8
Net adjustment related to defined benefit pension plans, net of tax	(0.4)						(0.4)
Foreign currency translation and other adjustments	5.4						5.4
Origination of Loans to Employee Stock Ownership Plans	$ 0.0							0.0
Balance at Jun. 30, 2019		303.2	1,288.6	4,326.7			44.8	(69.4)	$ 0.0
Common Stock, Dividends, Per Share, Cash Paid	$ 0.2000
Balance at Mar. 31, 2019		302.9	1,284.1	4,220.8	0.0	4,220.8	(70.6)	(71.7)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend reinvestment plan		0.0	0.2
Net issuance of shares under stock based compensation plans		0.2	3.1
Stock based compensation			0.3
ESSOP shares released			0.7					2.2
Net income (loss)	$ 165.5			165.5
Dividends on common stock				(59.7)
Net unrealized gains (losses) on securities, net of tax	111.4						111.4
Net adjustment related to defined benefit pension plans, net of tax	0.8						(0.8)
Foreign currency translation and other adjustments	$ 3.1						3.1
Origination of Loans to Employee Stock Ownership Plans								0.0
Balance at Jun. 30, 2019		303.2	1,288.6	4,326.7			44.8	(69.4)	0.0
Common Stock, Dividends, Per Share, Cash Paid	$ 0.4200
Balance at Dec. 31, 2019		303.6	1,297.5	4,386.0	(2.3)	4,383.6	77.7	(64.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend reinvestment plan		0.0	0.4
Net issuance of shares under stock based compensation plans		0.3	4.7
Stock based compensation			1.5
ESSOP shares released			0.8					5.7
Net income (loss)	$ (207.0)			(207.0)
Dividends on common stock				(125.3)
Net unrealized gains (losses) on securities, net of tax	234.8						234.8
Net adjustment related to defined benefit pension plans, net of tax	1.4						1.4
Foreign currency translation and other adjustments	(6.5)						(6.5)
Origination of Loans to Employee Stock Ownership Plans	$ (50.0)							50.0
Balance at Jun. 30, 2020		304.0	1,305.1	4,051.2			307.5	(109.0)	0.0
Common Stock, Dividends, Per Share, Cash Paid	$ 0.2100
Balance at Mar. 31, 2020		303.9	1,304.2	3,715.8	$ 0.0	$ 3,715.8	(69.2)	(111.9)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend reinvestment plan		0.0	0.2
Net issuance of shares under stock based compensation plans		0.0	0.3
Stock based compensation			0.3
ESSOP shares released			0.0					2.8
Net income (loss)	$ 397.7			397.7
Dividends on common stock				(62.3)
Net unrealized gains (losses) on securities, net of tax	372.0						372.0
Net adjustment related to defined benefit pension plans, net of tax	0.7						(0.7)
Foreign currency translation and other adjustments	$ 4.0						4.0
Origination of Loans to Employee Stock Ownership Plans								0.0
Balance at Jun. 30, 2020		$ 304.0	$ 1,305.1	$ 4,051.2			$ 307.5	$ (109.0)	$ 0.0