Summary of Significant Accounting Policies - Significant Estimates Used by Management in Determining Fair Values of Customer Relationships Acquired (Details)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Indefinite-lived Intangible Assets [Line Items]
Discount rates	21.00%
Customer attrition rates	27.00%
Minimum
Indefinite-lived Intangible Assets [Line Items]
Discount rates		20.00%
Customer attrition rates		20.00%
Maximum
Indefinite-lived Intangible Assets [Line Items]
Discount rates		27.00%
Customer attrition rates		33.00%