Investment Information (Restricted Assets) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Restricted Assets [Line Items]
Restricted assets	$ 5,431,680	$ 5,467,832
Collateral or guarantees - affiliated transactions
Restricted Assets [Line Items]
Restricted assets	4,226,334	4,062,097
Collateral or guarantees - third party agreements
Restricted Assets [Line Items]
Restricted assets	793,162	771,631
Deposits with US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	302,572	290,441
Deposits with non-US regulatory authorities
Restricted Assets [Line Items]
Restricted assets	6,591	247,321
Trust funds
Restricted Assets [Line Items]
Restricted assets	$ 103,021	$ 96,342