Consolidated Statements of Preferred Stock and Common Shareholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Paid-in Capital [Member]	Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Unallocated 401(k) Plan Shares [Member]	Treasury Stock, Common	Cumulative Effect, Period of Adoption, Adjustment Retained Earnings [Member]	[1]	Cumulative Effect, Period of Adoption, Adjustment AOCI Attributable to Parent [Member]	Cumulative Effect, Period of Adoption, Adjusted Balance Retained Earnings [Member]	Cumulative Effect, Period of Adoption, Adjusted Balance AOCI Attributable to Parent [Member]	Convertible Preferred Stock [Member] Preferred Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Equity, Including Portion Attributable to Noncontrolling Interest					$ 284.0
Beginning Balance at Dec. 31, 2020		$ 304.1	$ 1,306.9	$ 4,394.8		$ (103.2)	$ 0.0	$ 2.0		$ (1.4)	$ 4,396.9	$ 282.6	$ 0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividend reinvestment plan		0.1	3.5
Stock-based compensation		3.2
Stock-based compensation			56.5
401(k) plan shares released			9.1			20.6
Adjustments to Additional Paid in Capital, Other			0.0
Net income	$ 1,534.3			1,534.3
Dividends on common shares ($0.98, $1.92, and $2.38 per common share)				(715.1)
Common Stock, Dividends, Per Share, Cash Paid	$ 2.38
Net unrealized gains (losses) on securities not included in the statements of income, net of tax	$ (287.0)				(287.0)
Net adjustment related to defined benefit pension plans, net of tax	80.5				80.5
Treasury Stock, Value, Acquired, Cost Method							0.0
Treasury Stock, Retired, Cost Method, Amount		0.0	0.0				0.0
Ending Balance at Dec. 31, 2021		307.5	1,376.1	5,216.1	77.4	(82.5)	0.0	0.0		0.0	5,216.1	77.4	0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Foreign currency translation adjustment and other	1.3				1.3
Equity, Including Portion Attributable to Noncontrolling Interest					77.4
Dividend reinvestment plan		0.1	2.2
Stock-based compensation		1.9
Stock-based compensation			31.1
401(k) plan shares released			6.1			13.0
Adjustments to Additional Paid in Capital, Other			(5.1)
Net income	$ 686.4			686.4
Dividends on common shares ($0.98, $1.92, and $2.38 per common share)				(580.7)
Common Stock, Dividends, Per Share, Cash Paid	$ 1.92
Net unrealized gains (losses) on securities not included in the statements of income, net of tax	$ (657.3)				(657.3)
Net adjustment related to defined benefit pension plans, net of tax	68.4				68.4
Treasury Stock, Value, Acquired, Cost Method							(281.2)
Treasury Stock, Retired, Cost Method, Amount		(12.6)	(268.6)				(281.2)
Ending Balance at Dec. 31, 2022	6,173.2	296.9	1,141.8	5,321.8	(517.8)	(69.5)	0.0	$ 0.0		$ 0.0	$ 5,321.8	$ (517.8)	0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Foreign currency translation adjustment and other	(6.4)				(6.4)
Equity, Including Portion Attributable to Noncontrolling Interest					(517.8)
Dividend reinvestment plan		0.0	1.2
Stock-based compensation		2.3
Stock-based compensation			45.5
401(k) plan shares released			4.5			11.2
Adjustments to Additional Paid in Capital, Other			0.0
Net income	$ 598.6			598.6
Dividends on common shares ($0.98, $1.92, and $2.38 per common share)				(276.2)
Common Stock, Dividends, Per Share, Cash Paid	$ 0.98
Net unrealized gains (losses) on securities not included in the statements of income, net of tax	$ 370.8				370.8
Net adjustment related to defined benefit pension plans, net of tax	8.5				8.5
Treasury Stock, Value, Acquired, Cost Method							(535.3)
Treasury Stock, Retired, Cost Method, Amount		(20.9)	(514.4)				(535.3)
Ending Balance at Dec. 31, 2023	6,410.7	$ 278.3	$ 678.7	$ 5,644.3	(132.4)	$ (58.2)	$ 0.0						$ 0.0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Foreign currency translation adjustment and other	$ 6.0				$ 6.0

[1] .