Long-Lived Assets - Summary of Aggregate Amortization Related to Client Contract Costs (Parenthetical) (Details)	6 Months Ended
	Jun. 30, 2018
Minimum | Client contract incentives
Finite Lived Intangible Assets [Line Items]
Intangible assets, amortization year	2019
Minimum | Capitalized costs
Finite Lived Intangible Assets [Line Items]
Intangible assets, amortization year	2019
Minimum | Capitalized commission fees
Finite Lived Intangible Assets [Line Items]
Intangible assets, amortization year	2019
Maximum | Client contract incentives
Finite Lived Intangible Assets [Line Items]
Intangible assets, amortization year	2025
Maximum | Capitalized costs
Finite Lived Intangible Assets [Line Items]
Intangible assets, amortization year	2023
Maximum | Capitalized commission fees
Finite Lived Intangible Assets [Line Items]
Intangible assets, amortization year	2021
Maximum life of incremental commission fees expensed when incurred	1 year