Investment Information - Restricted assets (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Restricted Assets [Line Items]
Restricted assets	$ 8,138,636	$ 8,733,309
Collateral or guarantees - affiliated transactions
Restricted Assets [Line Items]
Restricted assets	4,271,208	4,643,334
Collateral or guarantees - third party agreements
Restricted Assets [Line Items]
Restricted assets	2,594,053	3,083,324
Deposits with US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	803,878	827,552
Deposits with non-US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	$ 469,497	$ 179,099