Commitments and Contingencies - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	May 31, 2018
Bank line of credit
Operating lease, option to extend	true
Operating lease, option to terminate	true
Right-of-use Assets	$ 190,763,000
Lease Liabilities	197,625,000
Rent expense incurred relating to operating leases		$ 15,965,000	$ 15,102,000
Headquarters At New York
Bank line of credit
Right-of-use Assets	151,000,000
Lease Liabilities	$ 151,000,000
Unsecured Revolving Credit Facility
Bank line of credit
Commitment amount				$ 65,000,000
Fixed rate of interest (as a percent)	3.50%
Borrowings under the credit facility	$ 0	$ 0
Available credit under the facility	$ 60,036,000
Unsecured Revolving Credit Facility | LIBOR
Bank line of credit
Interest rate margin (as a percent)	1.00%
Reference rate (as a percent)	LIBOR
Unsecured Revolving Credit Facility | Prime
Bank line of credit
Interest rate margin (as a percent)	(1.50%)
Reference rate (as a percent)	Prime
Standby Letters of Credit
Bank line of credit
Letters of credit outstanding	$ 4,964,000
Fee on the outstanding balances (as a percent)	1.00%