Investment Information - Restricted assets (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Restricted Assets [Line Items]
Restricted assets	$ 6,845,855	$ 6,670,912
Collateral or guarantees - affiliated transactions
Restricted Assets [Line Items]
Restricted assets	4,549,122	4,323,726
Collateral or guarantees - third party agreements
Restricted Assets [Line Items]
Restricted assets	1,537,336	1,674,304
Deposits with US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	701,026	616,987
Deposits with non-US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	$ 58,371	$ 55,895