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)		$ 0.30
Cash dividends declared on Serial Preferred Shares, Series B (in dollars per share)		$ 39.60163
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
Treasury shares purchased					(0.6)	(2.1)	(77.1)
Net restricted equity awards issued/vested					0.7	(0.7)
Amortization of equity-based compensation						86.7
Reinvested dividends on restricted stock units						1.0	(1.0)
Net income		$ 1,914.6					1,914.6
Cash dividends declared on common shares ($0.10, $0.10, $0.30, and $0.30 per share)	[1]						(175.4)
Cash dividends declared on Serial Preferred Shares, Series B ($0, $20.67700, $15.688377, and $39.60163 per share)	[1]						(19.8)
Other, net							18.4
Other comprehensive income (loss)		(240.4)						(240.4)
Balance, End of period at Sep. 30, 2023		$ 17,395.3		493.9	585.0	1,977.9	17,380.9	(3,042.4)
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)		$ 20.67700
Balance, Beginning of period at Jun. 30, 2023				493.9	585.3	1,935.7	16,350.4	(2,654.0)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Redemption of Serial Preferred Shares, Series B	[1]			0.0
Treasury shares purchased					(0.3)	(1.1)	(38.3)
Net restricted equity awards issued/vested					0.0	0.0
Amortization of equity-based compensation						42.9
Reinvested dividends on restricted stock units						0.4	(0.4)
Net income		$ 1,121.3					1,121.3
Cash dividends declared on common shares ($0.10, $0.10, $0.30, and $0.30 per share)	[1]						(58.5)
Cash dividends declared on Serial Preferred Shares, Series B ($0, $20.67700, $15.688377, and $39.60163 per share)	[1]						(10.3)
Other, net							16.7
Other comprehensive income (loss)		(388.4)						(388.4)
Balance, End of period at Sep. 30, 2023		$ 17,395.3		493.9	585.0	1,977.9	17,380.9	(3,042.4)
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)		0.30
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)
Treasury shares purchased					(0.7)	(2.3)	(130.8)
Net restricted equity awards issued/vested					1.2	(1.2)
Amortization of equity-based compensation						85.8
Reinvested dividends on restricted stock units						0.8	(0.8)
Net income		6,123.5					6,123.5
Cash dividends declared on common shares ($0.10, $0.10, $0.30, and $0.30 per share)	[1]						(175.6)
Cash dividends declared on Serial Preferred Shares, Series B ($0, $20.67700, $15.688377, and $39.60163 per share)	[1]						(7.8)
Other, net							22.7
Other comprehensive income (loss)		1,461.3						1,461.3
Balance, End of period at Sep. 30, 2024		$ 27,159.3		0.0	585.8	2,096.2	24,631.7	(154.4)
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 Jun. 30, 2024				0.0	585.7	2,060.5	22,410.0	(1,715.8)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Redemption of Serial Preferred Shares, Series B	[1]			0.0
Treasury shares purchased					(0.4)	(1.3)	(84.1)
Net restricted equity awards issued/vested					0.5	(0.5)
Amortization of equity-based compensation						37.3
Reinvested dividends on restricted stock units						0.2	(0.2)
Net income		$ 2,333.4					2,333.4
Cash dividends declared on common shares ($0.10, $0.10, $0.30, and $0.30 per share)	[1]						(58.6)
Cash dividends declared on Serial Preferred Shares, Series B ($0, $20.67700, $15.688377, and $39.60163 per share)	[1]						0.0
Other, net							31.2
Other comprehensive income (loss)		1,561.4						1,561.4
Balance, End of period at Sep. 30, 2024		$ 27,159.3		$ 0.0	$ 585.8	$ 2,096.2	$ 24,631.7	$ (154.4)
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.