Fair Value Measurements - Face Value, Carrying Amount and Fair Value of Financial Instruments (Details) - Senior unsecured notes payable - USD ($)	Mar. 31, 2021	Dec. 31, 2020
Financial liabilities:
Senior unsecured notes payable - face value	$ 300,000,000	$ 300,000,000
Carrying Amount | Level 2
Financial liabilities:
Senior unsecured notes payable	296,858,000	296,669,000
Fair Value | Level 2
Financial liabilities:
Senior unsecured notes payable	$ 309,000,000	$ 311,430,000