Investment Information - Restricted assets (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Restricted Assets [Line Items]
Restricted assets	$ 6,339,109	$ 5,902,339
Collateral or guarantees - affiliated transactions
Restricted Assets [Line Items]
Restricted assets	4,003,009	3,871,971
Collateral or guarantees - third party agreements
Restricted Assets [Line Items]
Restricted assets	1,672,159	1,513,079
Deposits with US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	618,448	472,890
Deposits with non-US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	$ 45,493	$ 44,399