Financial Assets and Liabilities - Gross Obligations For Securities Lending Transactions (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	$ 390,642	$ 222,203
Remaining contractual maturity for securities lending transactions	754,687
Overnight and Continuous
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	642	$ 222,203
Remaining contractual maturity for securities lending transactions	754,687
Less than 30 Days
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	100,000
30 To 60 Days
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	90,000
61 To 90 Days
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	200,000
Equity securities
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	390,000
Remaining contractual maturity for securities lending transactions	754,687
Equity securities | Overnight and Continuous
Fair value measurements measured on a recurring basis
Remaining contractual maturity for securities lending transactions	754,687
Equity securities | Less than 30 Days
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	100,000
Equity securities | 30 To 60 Days
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	90,000
Equity securities | 61 To 90 Days
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	200,000
US and non-US government obligations
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	642
US and non-US government obligations | Overnight and Continuous
Fair value measurements measured on a recurring basis
Remaining contractual maturity for repurchase agreements	$ 642