Income Taxes - Summary of net deferred tax assets and (liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred income tax assets:
Self insurance reserves	$ 4,458	$ 4,299
Contract loss reserves	642	350
Stock-based awards	1,164	1,143
Bonus	4,904	3,271
Operating lease liabilities	5,850	0
Non-U.S. operating loss	5,499	5,641
Other	3,439	1,958
Total deferred income tax assets before valuation allowances	25,956	16,662
Less: valuation allowances	(2,508)	(2,672)
Total deferred income tax assets	23,448	13,990
Deferred income tax liabilities:
Property and equipment-tax over book depreciation	(32,220)	(26,030)
Intangible assets-tax over book amortization	(1,856)	(1,890)
Right-of-use operating lease assets	(5,850)	0
Non-U.S. deferred income tax liabilities	(2,280)	(1,443)
Other	(2,187)	(2,025)
Total deferred income tax liabilities	(44,393)	(31,388)
Net deferred income taxes	$ (20,945)	$ (17,398)