Commitments and Contingencies (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / Person	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Commitments and Contingencies [Line Items]
Rent expense, operating leases	$ 6,600	$ 6,400	$ 6,500
Operating Leases, Rent Expense, Sublease Rentals	1,100	977	580
Self-insured health insurance, liability	332	346
Derivative Liability	37,096	8,510	$ 4,277
Contingent liability for accrued earn-out acquisition consideration	33,096	6,589
Contingent liability for accrued earn-out acquisition consideration	$ 4,000	1,921
Maximum [Member]
Commitments and Contingencies [Line Items]
Self-insured health insurance limit, per person | $ / Person	120,000
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 | Minimum [Member]
Commitments and Contingencies [Line Items]
Term of Lease	3 years
Computer equipment | Maximum [Member]
Commitments and Contingencies [Line Items]
Term of Lease	5 years
Contingent Accrued Earn-out Acquisition Consideration
Commitments and Contingencies [Line Items]
Derivative Liability	$ 37,100	$ 8,500