Term Loans (Details)	1 Months Ended			3 Months Ended
	Jan. 31, 2018 item	Jun. 30, 2015 USD ($)	Jan. 31, 2013 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Promissory Note | $125.9 million promissory note
Term Loans
Face amount of loans					$ 125,900,000
Stated interest rate (as a percent)					1.52%
Senior Unsecured Term Loans
Term Loans
Deferred finance costs				$ 559,000	$ 580,000
Senior Unsecured Term Loans | $250 million senior unsecured term loan
Term Loans
Face amount of loans		$ 250,000,000
Variable reference rate		one-month LIBOR
Basis spread on variable rate (as a percent)		0.90%
Deferred financing costs incurred				1,200,000
Senior Unsecured Term Loans | Interest rate swaps | $250 million senior unsecured term loan
Term Loans
Effective yield (as a percent)		2.62%
Senior Unsecured Term Loans | ARCT | $70 million senior unsecured term loan
Term Loans
Face amount of loans			$ 70,000,000
Variable reference rate	one-month LIBOR		one-month LIBOR
Basis spread on variable rate (as a percent)	0.90%		1.10%
Loan extension period	6 months
Number of additional optional extensions | item	2
Deferred financing costs incurred				$ 368,000
Senior Unsecured Term Loans | ARCT | Interest rate swaps | $70 million senior unsecured term loan
Term Loans
Effective yield (as a percent)			2.05%