Debt (Details 1) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Disclosure [Abstract]
Maximum amount borrowed	$ 367,000	$ 412,000	$ 308,500
Weighted average interest rate	2.56%	1.94%	1.64%