Financial Assets and Liabilities - Gross Obligations For Securities Lending Transactions (Details) $ in Thousands	Dec. 31, 2015 USD ($)
Fair value measurements measured on a recurring basis
Remaining contractual maturity for securities lending transactions	$ 524,603
Overnight and Continuous
Fair value measurements measured on a recurring basis
Remaining contractual maturity for securities lending transactions	524,603
Equity securities
Fair value measurements measured on a recurring basis
Remaining contractual maturity for securities lending transactions	524,603
Equity securities | Overnight and Continuous
Fair value measurements measured on a recurring basis
Remaining contractual maturity for securities lending transactions	$ 524,603