Acquisitions - Acquisitions Purchase Price Allocation (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Apr. 02, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets:
Goodwill	$ 475,206		$ 395,504	$ 386,772
MXD Group
Assets:
Operating property and equipment		$ 9,803
Goodwill		63,424
Customer relationships and other intangibles		23,651
Other assets, primarily accounts receivable		31,492
Deferred income taxes		12,737
Total assets		141,107
Liabilities:
Accrued liabilities		(18,107)
Other liabilities, primarily accounts payable		(5,052)
Net assets acquired		$ 117,948