Summary of Significant Accounting Policies - Significant Estimates Used by Management in Determining Fair Values of Customer Relationships Acquired (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Minimum
Goodwill And Intangible Assets [Line Items]
Discount rates	20.00%	17.00%	20.00%
Customer attrition rates	20.00%	15.00%	10.00%
Maximum
Goodwill And Intangible Assets [Line Items]
Discount rates	27.00%	25.00%	23.00%
Customer attrition rates	33.00%	78.00%	70.00%