Term Loans (Details) - Term Loans	1 Months Ended			12 Months Ended
	Oct. 31, 2018 USD ($)	Jun. 30, 2015 USD ($)	Jan. 31, 2013 USD ($)	Dec. 31, 2018 USD ($) extension	Dec. 31, 2019 USD ($)
Debt
Face amount of loan					$ 250,000,000.0
Deferred finance costs balance				$ 1,400,000	956,000
$250 million senior unsecured term loan due Mar 2024
Debt
Face amount of loan	$ 250,000,000.0				$ 250,000,000.0
Basis spread on variable rate (as a percent)	0.85%
Effective yield (as a percent)	3.89%
Deferred financing costs incurred	$ 1,100,000
$250 million senior unsecured term loan due Jun 2020
Debt
Face amount of loan		$ 250,000,000.0
Basis spread on variable rate (as a percent)		0.90%
Effective yield (as a percent)		2.62%
Deferred financing costs incurred		$ 1,200,000
$70 million senior unsecured term loan due Jan 2018
Debt
Face amount of loan			$ 70,000,000.0
Basis spread on variable rate (as a percent)			1.10%	0.90%
Effective yield (as a percent)			2.05%
Number of optional extension periods | extension				2
Loan extension period				6 months