Statutory Requirements - Statutory Capital and Surplus (Details) - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Bermuda
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	$ 8,275,191	$ 7,462,710
Required statutory capital and surplus	2,028,879	1,753,078
Switzerland
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	1,086,800	874,665
Required statutory capital and surplus	798,900	780,000
U.K.
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	1,012,639	983,449
Required statutory capital and surplus	1,012,639	983,449
U.S.
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	1,078,042	819,811
Required statutory capital and surplus	$ 858,282	$ 716,118