Leases - Supplemental balance sheet information (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Right-of-Use Assets under Operating Leases
Operating lease right-of-use assets	$ 19,321
Operating lease right-of-use assets		$ 20,819
Operating lease right-of-use liabilities, current portion	5,347
Operating lease right-of-use liabilities, current portion		5,649
Operating lease right-of-use liabilities, long-term portion	13,974
Operating lease right-of-use liabilities, long-term portion		15,170
Total operating lease liabilities	19,321
Total operating lease liabilities		20,819
Right-of-Use Assets under Finance Leases
Property and equipment, at cost	81,012
Property and equipment, at cost		102,852
Accumulated depreciation	(22,513)
Accumulated depreciation		(31,954)
Property and equipment under finance lease, net	58,499
Property and equipment under finance lease, net		70,898
Current portion of long-term debt	22,824
Current portion of long-term debt		27,519
Long-term debt	32,806
Long-term debt		33,390
Total finance lease liabilities	$ 55,630
Total finance lease liabilities		$ 60,909