Financial Assets and Liabilities - Gross Obligations For Securities Lending Transactions (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	$ 281,783	$ 281,861
Remaining contractual maturity for securities loaned	1,093,321	1,130,039
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	6,783	11,861
Remaining contractual maturity for securities loaned	1,093,321	1,130,039
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	75,000	45,000
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	50,000	65,000
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	150,000	160,000
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	275,000	270,000
Remaining contractual maturity for securities loaned	1,093,321	1,130,039
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	1,093,321	1,130,039
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	75,000	45,000
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	50,000	65,000
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	150,000	160,000
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	6,783	11,861
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	6,783	11,861
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