Commitments and Contingencies - Narrative (Details)	3 Months Ended	12 Months Ended			14 Months Ended
	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 31, 2016 defendant
Operating Leases, Rent Expense, Net [Abstract]
Operating leases, rent expense		$ 2,000,000	$ 1,700,000	$ 1,000,000
Proceeds from legal settlements	$ 5,500,000
Increase (decrease) in notes receivable, current		800,000
Increase (decrease) in notes receivable		$ 3,000,000
Office building
Operating Leases, Rent Expense, Net [Abstract]
Lease term of assets subject to operating leases		9 years
Vehicles
Operating Leases, Rent Expense, Net [Abstract]
Operating leases, rent expense		$ 1,000,000	$ 700,000	$ 1,500,000
Other facilities | Minimum
Operating Leases, Rent Expense, Net [Abstract]
Lease term of assets subject to operating leases		1 year
Other facilities | Maximum
Operating Leases, Rent Expense, Net [Abstract]
Lease term of assets subject to operating leases		5 years
Houston Police Department, Environmental Enforcement | Pending Litigation
Operating Leases, Rent Expense, Net [Abstract]
Number of defendants | defendant					2
Estimate of possible loss		$ 250,000
Settlement agreement, plea bargain declined		$ 75,000