Goodwill and Intangible Assets - Gross Carrying Amounts and Accumulated Amortization (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Finite-Lived Intangible Assets
Total intangible assets, gross	$ 187,556	$ 172,178
Total accumulated amortization	(46,898)	(42,525)
Total intangible assets, net	$ 140,658	129,653
Customer relationships
Finite-Lived Intangible Assets
Estimated useful life (in years)	10 years 3 months 18 days
Total intangible assets, gross	$ 149,700	146,200
Total accumulated amortization	$ (28,443)	(24,870)
Technology including internally developed software
Finite-Lived Intangible Assets
Estimated useful life (in years)	6 years 1 month 6 days
Total intangible assets, gross	$ 27,456	18,078
Total accumulated amortization	$ (11,469)	(10,879)
Backlog
Finite-Lived Intangible Assets
Estimated useful life (in years)	3 years 8 months 12 days
Total intangible assets, gross	$ 6,860	6,860
Total accumulated amortization	$ (6,020)	(5,827)
Covenants not to compete
Finite-Lived Intangible Assets
Estimated useful life (in years)	5 years 6 months
Total intangible assets, gross	$ 1,040	1,040
Total accumulated amortization	$ (966)	(949)
Trade Names
Finite-Lived Intangible Assets
Estimated useful life (in years)	15 years
Total intangible assets, gross	$ 2,500	$ 0