Significant Service Provider Partners (Details) - Service Provider Concentration Risk - Revenue	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
10 Largest Service Providers
Concentration Risk [Line Items]
Concentration risk percentage (percent)	60.00%	60.00%	63.00%
Minimum | Service Provider A
Concentration Risk [Line Items]
Concentration risk percentage (percent)	15.00%
Minimum | Service Provider B
Concentration Risk [Line Items]
Concentration risk percentage (percent)	10.00%	10.00%	15.00%
Maximum | Service Provider A
Concentration Risk [Line Items]
Concentration risk percentage (percent)	20.00%
Maximum | Service Provider B
Concentration Risk [Line Items]
Concentration risk percentage (percent)	15.00%	15.00%	20.00%