Financial Assets and Liabilities - Gross Obligations For Securities Lending Transactions (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	$ 281,861	$ 390,642
Remaining contractual maturity for securities loaned	1,130,039	754,687
Overnight and Continuous
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	11,861	642
Remaining contractual maturity for securities loaned	1,130,039	754,687
Less than 30 days
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	45,000	100,000
Remaining contractual maturity for securities loaned	0	0
30 - 60 days
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	65,000	90,000
Remaining contractual maturity for securities loaned	0	0
61 - 90 Days
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	160,000	200,000
Remaining contractual maturity for securities loaned	0	0
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	270,000	390,000
Remaining contractual maturity for securities loaned	1,130,039	754,687
Equity securities | Overnight and Continuous
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	0	0
Remaining contractual maturity for securities loaned	1,130,039	754,687
Equity securities | Less than 30 days
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	45,000	100,000
Remaining contractual maturity for securities loaned	0	0
Equity securities | 30 - 60 days
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	65,000	90,000
Remaining contractual maturity for securities loaned	0	0
Equity securities | 61 - 90 Days
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	160,000	200,000
Remaining contractual maturity for securities loaned	0	0
U.S. and Non-U.S. government obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	11,861	642
U.S. and Non-U.S. government obligations | Overnight and Continuous
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	11,861	642
U.S. and Non-U.S. government obligations | Less than 30 days
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	0	0
U.S. and Non-U.S. government obligations | 30 - 60 days
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	0	0
U.S. and Non-U.S. government obligations | 61 - 90 Days
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	$ 0	$ 0