Statutory Requirements - Statutory Capital and Surplus (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Bermuda
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	$ 12,629,170	$ 8,275,191
Required statutory capital and surplus	2,605,468	2,028,879
Switzerland
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	2,644,998	1,086,800
Required statutory capital and surplus	987,707	798,900
U.K.
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	935,776	1,012,639
Required statutory capital and surplus	935,776	1,012,639
U.S.
Statutory Accounting Practices [Line Items]
Statutory capital and surplus	1,034,603	1,078,042
Required statutory capital and surplus	$ 739,531	$ 858,282