Financial Assets and Liabilities - Gross Obligations For Securities Lending Transactions (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Remaining contractual maturity for securities sold under agreements to repurchase	$ 461,235	$ 340,742
Remaining contractual maturity for securities loaned	948,256	1,600,099
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	86,235	65,742
Remaining contractual maturity for securities loaned	948,256	1,600,099
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	125,000	75,000
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	50,000
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	200,000	150,000
Remaining contractual maturity for securities loaned	0	0
Greater than 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
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	375,000	275,000
Remaining contractual maturity for securities loaned	948,256	1,600,099
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	948,256	1,600,099
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	125,000	75,000
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	50,000
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	200,000	150,000
Remaining contractual maturity for securities loaned	0	0
Equity securities | Greater than 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
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	86,235	65,742
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	86,235	65,742
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
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
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
U.S. and Non-U.S. government obligations | Greater than 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