Investment Information - Restricted assets (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Restricted Assets [Line Items]
Restricted assets	$ 5,740,388	$ 5,302,042
Collateral or guarantees - affiliated transactions
Restricted Assets [Line Items]
Restricted assets	4,346,385	4,060,533
Collateral or guarantees - third party agreements
Restricted Assets [Line Items]
Restricted assets	951,609	856,890
Deposits with US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	350,284	302,809
Deposits with non-US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	2	6,546
Trust funds
Restricted Assets [Line Items]
Restricted assets	$ 92,108	$ 75,264