The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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Goodwill and Acquired Client Relationships - Additional Information (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill
Goodwill impairment	$ 0
Definite-lived and indefinite-lived intangibles impairments		$ 0
Acquired Client Relationships (Net)
Goodwill
Intangible amortization expense		114,400,000	$ 86,400,000	$ 107,700,000
Intangible future amortization expense in 2019		110,000,000
Intangible future amortization expense in 2020		55,000,000
Intangible future amortization expense in 2021		30,000,000
Intangible future amortization expense in 2022		30,000,000
Intangible future amortization expense in 2023		$ 30,000,000