Term Loans (Details) - Term loans - USD ($)	1 Months Ended
	Oct. 31, 2018	Jun. 30, 2015	Dec. 31, 2020	Dec. 31, 2019
Debt
Deferred finance costs balance			$ 642,000	$ 956,000
$250 million senior unsecured term loan due Mar 2024
Debt
Face amount of loan	$ 250,000,000.0		$ 250,000,000.0	250,000,000.0
Effective yield (as a percent)			3.89%
Deferred financing costs incurred	$ 1,100,000
$250 million senior unsecured term loan due Mar 2024 | LIBOR
Debt
Basis spread on variable rate (as a percent)	0.85%
$250 million senior unsecured term loan due Jun 2020
Debt
Face amount of loan		$ 250,000,000.0		$ 250,000,000.0
Effective yield (as a percent)		2.62%
Deferred financing costs incurred		$ 1,200,000
$250 million senior unsecured term loan due Jun 2020 | LIBOR
Debt
Basis spread on variable rate (as a percent)		0.90%