Term Loans - Narrative (Details) - Term Loans	1 Months Ended				6 Months Ended
	Oct. 31, 2018 USD ($)	Jan. 31, 2018 extension	Jun. 30, 2015 USD ($)	Jan. 31, 2013 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Debt
Deferred finance costs balance						$ 1,300,000	$ 1,400,000
$250 million senior unsecured term loan due March 2024
Debt
Face amount of loan	$ 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 June 2020
Debt
Face amount of loan			$ 250,000,000		$ 250,000,000
Basis spread on variable rate (as a percent)					0.90%
Deferred financing costs incurred			$ 1,200,000
$250 million senior unsecured term loan due June 2020 | Interest rate swaps
Debt
Effective yield (as a percent)			2.62%		2.62%
$70 million senior unsecured term loan | ARCT
Debt
Face amount of loan				$ 70,000,000
Basis spread on variable rate (as a percent)		0.90%		1.10%
Number of optional extension periods | extension		2
Loan extension period		6 months
$70 million senior unsecured term loan | ARCT | Interest rate swaps
Debt
Effective yield (as a percent)				2.05%