Notes payable and line of credit - Summary of Notes Payable (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Debt Disclosure [Abstract]
Banks – varying payments and rates	$ 98,875	$ 92,875
Other than banks	10,437	13,933
Notes payable	$ 109,312	$ 106,808
Average interest rate	2.59%	1.97%