Fair Value Measurements - Face Value, Carrying Amount and Fair Value of Financial Instruments (Details) - Notes payable - USD ($)	Sep. 30, 2021	Jun. 17, 2021	Dec. 31, 2020	May 10, 2017
2028 Senior unsecured notes payable
Financial liabilities:
Senior unsecured notes payable - face value	$ 400,000,000	$ 400,000,000	$ 0
2025 Senior unsecured notes payable
Financial liabilities:
Senior unsecured notes payable - face value	0		300,000,000	$ 300,000,000
Carrying Amount | Level 2 | 2028 Senior unsecured notes payable
Financial liabilities:
Senior unsecured notes payable	394,063,000		0
Carrying Amount | Level 2 | 2025 Senior unsecured notes payable
Financial liabilities:
Senior unsecured notes payable	0		296,669,000
Fair Value | Level 2 | 2028 Senior unsecured notes payable
Financial liabilities:
Senior unsecured notes payable	415,000,000		0
Fair Value | Level 2 | 2025 Senior unsecured notes payable
Financial liabilities:
Senior unsecured notes payable	$ 0		$ 311,430,000