Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Millions		Total	Serial Preferred Stock	Preferred Shares, No Par Value	Common Shares, $1.00 Par Value	Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Series B Preferred Stock Serial Preferred Stock	Voting Preference Shares
Cash dividends declared on common stock (in dollars per share)		$ 0.10
Cash dividends declared on Serial Preferred Shares, Series B (in dollars per share)									$ 0
Balance, Beginning of period at Dec. 31, 2022				$ 493.9	$ 584.9	$ 1,893.0	$ 15,721.2	$ (2,802.0)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Redemption of Serial Preferred Shares, Series B	[1]			0.0
Net income		$ 447.9					447.9
Treasury shares purchased					(0.2)	(0.8)	(31.7)
Cash dividends declared on common shares ($0.10 and $0.10 per share)	[1]						(58.5)
Cash dividends declared on Serial Preferred Shares, Series B ($15.688377 and $0 per share)	[1]						0.0
Net restricted equity awards issued/vested					0.7	(0.7)
Amortization of equity-based compensation						15.9
Reinvested dividends on restricted stock units						0.3	(0.3)
Other, net							1.5
Other comprehensive income (loss)		603.3						603.3
Balance, End of period at Mar. 31, 2023		$ 16,868.4		493.9	585.4	1,907.7	16,080.1	(2,198.7)
Preferred stock, no par value									$ 0
Common shares, par value (USD per share)		$ 1.00
Preferred stock, shares authorized (shares)			20,000,000.0						500,000	5,000,000.0
Preferred stock, shares issued (shares)									500,000	0
Preferred stock, no par value									$ 0
Common shares, par value (USD per share)		1.00
Preferred stock, shares authorized (shares)			20,000,000.0						500,000
Preferred stock, shares issued (shares)									500,000
Cash dividends declared on common stock (in dollars per share)		$ 0.10
Cash dividends declared on Serial Preferred Shares, Series B (in dollars per share)									$ 15.688377
Balance, Beginning of period at Dec. 31, 2023		$ 20,277.1		493.9	585.3	2,013.1	18,800.5	(1,615.7)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Redemption of Serial Preferred Shares, Series B	[1]			(493.9)
Net income		2,331.4					2,331.4
Treasury shares purchased					(0.2)	(0.8)	(36.1)
Cash dividends declared on common shares ($0.10 and $0.10 per share)	[1]						(58.5)
Cash dividends declared on Serial Preferred Shares, Series B ($15.688377 and $0 per share)	[1]						(7.8)
Net restricted equity awards issued/vested					0.6	(0.6)
Amortization of equity-based compensation						16.8
Reinvested dividends on restricted stock units						0.2	(0.2)
Other, net							(8.8)
Other comprehensive income (loss)		(207.9)						(207.9)
Balance, End of period at Mar. 31, 2024		$ 21,811.3		$ 0.0	$ 585.7	$ 2,028.7	$ 21,020.5	$ (1,823.6)
Common shares, par value (USD per share)		$ 1.00
Preferred stock, shares authorized (shares)			20,000,000.0						0	5,000,000.0
Preferred stock, shares issued (shares)									0	0

[1] See Note 9 – Dividends for further discussion.