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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(2))
-URI http://asc.fasb.org/extlink&oid=119400593&loc=d3e572229-122910

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=98513485&loc=SL98516268-108586

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(1)(a))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3044-108585

Name:	us-gaap_RestrictedCashAndCashEquivalents
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Cash Collateral Pledged (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Cash collateral pledged	$ 110,666,000	$ 6,563,000	$ 18,609,000
Letter of credit, borrowing capacity	395,000,000
Letter of Credit \| Letter of Credit, NY Office 1, Expires April 2021
Letter of credit, borrowing capacity	358,000
Letter of Credit \| Letter of Credit, NY Office 3, Expires October 2020
Letter of credit, borrowing capacity	1,424,000
Letter of Credit \| Letter of Credit, NY Office 5, Expires November 2020
Letter of credit, borrowing capacity	1,637,000
Letter of Credit \| Letter of Credit, Boston Office, Expires March 2021
Letter of credit, borrowing capacity	386,000
Letter of Credit \| Letter of Credit, San Francisco Office, Expires October 2025
Letter of credit, borrowing capacity	712,000
Facility Leases
Cash collateral pledged	4,517,000	4,600,000
Collateral Reinsurance Agreement
Cash collateral pledged	$ 106,100,000	$ 2,000,000