Commitments and Contingencies (Narrative) (Details) $ / Person in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) $ / Person	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Commitments and Contingencies [Line Items]
Rent expense, operating leases	$ 14,700	$ 4,300
Reduction of rent expense as a result of purchase accounting adjustment	0	$ (948)
Self-insured health insurance, liability	282
Derivative Liability	29,285		$ 37,096	$ 8,510
Derivative Financial Instrument, Liabilities, Fair Value Disclosure, Noncurrent	15,083		33,096
Derivative Financial Instrument, Liabilities, Fair Value Disclosure, Current	$ 14,202		4,000
Maximum [Member]
Commitments and Contingencies [Line Items]
Self-insured health insurance limit, per person | $ / Person	120
Self-insured health Insurance, aggregate liability based on participants and claims (percentage)	125.00%
Self-insured health insurance, estimated cumulative liability for annual contract	$ 2,800
Computer Equipment [Member] | Minimum [Member]
Commitments and Contingencies [Line Items]
Capital lease obligation, term	3 years
Computer Equipment [Member] | Maximum [Member]
Commitments and Contingencies [Line Items]
Capital lease obligation, term	5 years
Contingent Accrued Earn-out Acquisition Consideration [Member]
Commitments and Contingencies [Line Items]
Derivative Liability	$ 29,285		37,096
Contingent liability for accrued earn-out acquisition consideration	15,100		33,100
Business Combination, Contingent Consideration, Liability, Current	$ 14,200		$ 4,000