Supplemental Cash Flow Information and Non-cash Activity (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash paid during the period for:
Interest	$ 6,085	$ 6,733
Income taxes	7,607	2,257
Non-cash items from investing and financing activities:
Purchases of computer equipment financed through capital leases	0	3,298
Investment in multi-client data library in accounts payable and accrued expenses	$ 4,741	$ 6,657