Convertible Debt and Notes Payable - Letters of Credit (Details)	Mar. 31, 2020 USD ($) letters_of_credit	Dec. 31, 2019 USD ($)	Mar. 31, 2019 USD ($)
Debt and Capital Lease Obligations [Line Items]
Number of letters of credit | letters_of_credit	6
Cash collateral pledged	$ 7,538,000	$ 6,563,000	$ 6,420,000
Letter of credit, borrowing capacity	300,000,000
Letter of Credit, Boston Office, Expires March 2021 | Letter of Credit
Debt and Capital Lease Obligations [Line Items]
Letter of credit, borrowing capacity	379,000
Letter of Credit, NY Office 1, Expires April 2021 | Letter of Credit
Debt and Capital Lease Obligations [Line Items]
Letter of credit, borrowing capacity	360,000
Letter of Credit, NY Office 3, Expires October 2020 | Letter of Credit
Debt and Capital Lease Obligations [Line Items]
Letter of credit, borrowing capacity	398,000
Letter of Credit, NY Office 4, Expires October 2020 | Letter of Credit
Debt and Capital Lease Obligations [Line Items]
Letter of credit, borrowing capacity	1,125,000
Letter of Credit, NY Office 5, Expires November 2020 | Letter of Credit
Debt and Capital Lease Obligations [Line Items]
Letter of credit, borrowing capacity	1,635,000
Letter of Credit, San Francisco Office, Expires October 2025 | Letter of Credit
Debt and Capital Lease Obligations [Line Items]
Letter of credit, borrowing capacity	711,000
Collateral Reinsurance Agreement
Debt and Capital Lease Obligations [Line Items]
Cash collateral pledged	$ 2,900,000	$ 2,000,000.0