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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Reference 4: 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

Convertible Debt and Notes Payable - Letters of Credit (Details) - USD ($)	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Debt and Capital Lease Obligations [Line Items]
Cash collateral pledged	$ 110,666,000	$ 6,563,000	$ 18,609,000
Letter of credit, borrowing capacity	395,000,000
Letter of Credit, Boston Office, Expires March 2021 \| Letter of Credit
Debt and Capital Lease Obligations [Line Items]
Letter of credit, borrowing capacity	386,000
Letter of Credit, NY Office 1, Expires April 2021 \| Letter of Credit
Debt and Capital Lease Obligations [Line Items]
Letter of credit, borrowing capacity	358,000
Letter of Credit, NY Office 3, Expires October 2020 \| Letter of Credit
Debt and Capital Lease Obligations [Line Items]
Letter of credit, borrowing capacity	1,424,000
Letter of Credit, NY Office 5, Expires November 2020 \| Letter of Credit
Debt and Capital Lease Obligations [Line Items]
Letter of credit, borrowing capacity	1,637,000
Letter of Credit, San Francisco Office, Expires October 2025 \| Letter of Credit
Debt and Capital Lease Obligations [Line Items]
Letter of credit, borrowing capacity	712,000
Collateral Reinsurance Agreement
Debt and Capital Lease Obligations [Line Items]
Cash collateral pledged	$ 106,100,000	$ 2,000,000