Financial Assets and Liabilities - Gross Obligations For Securities Lending Transactions (Details) $ in Thousands	Sep. 30, 2017 USD ($)
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	$ 620,887
Remaining contractual maturity for securities lending transactions	582,915
Overnight and Continuous
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	887
Remaining contractual maturity for securities lending transactions	582,915
Less than 30 Days
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	205,000
30 To 60 Days
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	175,000
61 To 90 Days
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	240,000
Equity securities
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	620,000
Remaining contractual maturity for securities lending transactions	582,915
Equity securities | Overnight and Continuous
Fair value measurements measured on a recurring basis
Remaining contractual maturity for securities lending transactions	582,915
Equity securities | Less than 30 Days
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	205,000
Equity securities | 30 To 60 Days
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	175,000
Equity securities | 61 To 90 Days
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	240,000
US and non-US government obligations
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	887
US and non-US government obligations | Overnight and Continuous
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	$ 887