Stockholders' Equity - Changes in accumulated other comprehensive income (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) swap_agreement	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) swap_agreement
Accumulated Other Comprehensive Income (Loss) [Line Items]
Number of instruments | swap_agreement	2		4
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimate of Time to Transfer			10 years
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Gains arising during the period related to active interest rate swaps	$ 513
Forward-starting Interest Rate Swaps
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(1,299)	$ (1,401)	$ (1,401)
Gains arising during the period related to active interest rate swaps	513	0
Amounts reclassified from accumulated other comprehensive loss	147	43
Net accumulated other comprehensive income	660	43
Ending balance	$ (639)	$ (1,358)	$ (1,299)