Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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Name:	us-gaap_RestrictedCashAndCashEquivalents
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Cash Collateral Pledged (Details) - USD ($)	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Cash collateral pledged	$ 110,303,000	$ 110,743,000	$ 7,538,000
Letter of credit, borrowing capacity	320,000,000
Letters of Credit Outstanding, Amount	0	0
Letter of Credit \| Letter of Credit, NY Office 1, Expires April 2022
Letter of credit, borrowing capacity	215,000
Letter of Credit \| Letter of Credit, NY Office 3, Expires October 2022
Letter of credit, borrowing capacity	1,424,000
Letter of Credit \| Letter of Credit, NY Office 5, Expires November 2021
Letter of credit, borrowing capacity	1,252,000
Letter of Credit \| Letter of Credit, Boston Office, Expires March 2028
Letter of credit, borrowing capacity	194,000
Letter of Credit \| Letter of Credit, San Francisco Office, Expires October 2025
Letter of credit, borrowing capacity	458,000
Facility Leases
Cash collateral pledged	3,543,000	4,000,000.0
Collateral Reinsurance Agreement
Cash collateral pledged	$ 106,800,000	$ 106,800,000