Leases - Supplemental balance sheet information (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Right-of-Use Assets under Operating Leases
Operating lease right-of-use assets	$ 17,813
Operating lease right-of-use assets		$ 20,819
Operating lease right-of-use liabilities, current portion	5,119
Operating lease right-of-use liabilities, current portion		5,649
Operating lease right-of-use liabilities, long-term portion	12,694
Operating lease right-of-use liabilities, long-term portion		15,170
Total operating lease liabilities	17,813
Total operating lease liabilities		20,819
Right-of-Use Assets under Finance Leases
Property and equipment, at cost	81,956
Property and equipment, at cost		102,852
Less accumulated amortization	26,221
Less accumulated amortization		31,954
Property and equipment under finance lease, net	55,735
Property and equipment under finance lease, net		70,898
Current portion of long-term debt	24,384
Current portion of long-term debt		27,519
Long-term debt	26,779
Long-term debt		33,390
Total finance lease liabilities	$ 51,163
Total finance lease liabilities		$ 60,909