Debt (Details) - Line of Credit - USD ($) $ in Millions	Dec. 31, 2022	Jun. 15, 2022	Jun. 14, 2022
Revolving Credit Agreement [Member] | Revolving Credit Agreement Issued June 15, 2022 [Member]
Line of Credit Facility
Credit agreement, term (in years)		5 years
Credit agreement, maximum borrowing capacity		$ 1,000
Credit agreement, covenant terms		Pursuant to the credit agreement covenants, the Company must maintain a minimum consolidated net worth, defined as shareholders’ equity determined in accordance with GAAP (excluding accumulated other comprehensive income (loss)) plus (a) trust preferred securities (not to exceed 15% of total capital) and (b) mandatorily convertible securities (combined with trust preferred securities, not to exceed 25% of total capital), less goodwill and other intangible assets. That threshold is fixed during the term of the credit agreement at an amount equal to $13.9 billion (57.5% of the Company's net worth at March 31, 2022). In addition, the credit agreement contains other customary restrictive covenants as well as certain customary events of default, including with respect to a change in control, which would occur upon the acquisition of 35% or more of the Company’s voting stock or certain changes in the composition of the Company’s Board of Directors.
Maximum percentage of trust preferred securities relative to total capital in determining consolidated net worth		15.00%
Maximum percentage of trust preferred securities and mandatorily convertible securities relative to total capital in determining consolidated net worth		25.00%
Credit agreement, threshold of consolidated net worth		$ 13,900
Credit agreement, threshold of consolidated net worth as a percentage of shareholders' equity as calculated per the agreement		57.50%
Percentage of Company's voting stock acquired by outside entity that would be considered a change in control		35.00%
Credit agreement, compliance	At December 31, 2022, the Company was in compliance with these covenants.
Revolving Credit Agreement [Member] | Revolving Credit Agreement Issued June 15, 2022 [Member] | SOFR [Member]
Line of Credit Facility
Credit agreement, cost of borrowing, basis points above SOFR	1.10%
Revolving Credit Agreement [Member] | Revolving Credit Agreement Issued June 15, 2022 [Member] | SOFR [Member] | Minimum [Member]
Line of Credit Facility
Credit agreement, cost of borrowing, basis points above SOFR		0.85%
Revolving Credit Agreement [Member] | Revolving Credit Agreement Issued June 15, 2022 [Member] | SOFR [Member] | Maximum [Member]
Line of Credit Facility
Credit agreement, cost of borrowing, basis points above SOFR		1.475%
Revolving Credit Agreement [Member] | Revolving Credit Agreement Due to Expire on June 4, 2023 [Member]
Line of Credit Facility
Credit agreement, term (in years)			5 years
Credit agreement, maximum borrowing capacity			$ 1,000
Letter of credit [Member] | Unsecured Debt [Member]
Line of Credit Facility
Credit agreement, maximum borrowing capacity	$ 302
Credit agreement, maximum borrowing capacity, portion utilized to support obligations at Lloyd's	$ 260