Financial Assets and Liabilities - Gross Obligations For Securities Lending Transactions (Details) $ in Thousands	Jun. 30, 2017 USD ($)
Fair value measurements measured on a recurring basis
Remaining contractual maturity for securities lending transactions	$ 345,184
Overnight and Continuous
Fair value measurements measured on a recurring basis
Remaining contractual maturity for securities lending transactions	345,184
Equity securities
Fair value measurements measured on a recurring basis
Remaining contractual maturity for securities lending transactions	345,184
Equity securities | Overnight and Continuous
Fair value measurements measured on a recurring basis
Remaining contractual maturity for securities lending transactions	$ 345,184