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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Amount of write-down of assets recognized in the income statement. Includes, but is not limited to, losses from tangible assets, intangible assets and goodwill.

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Goodwill and Acquired Client Relationships - Additional Information (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Goodwill
Goodwill impairment	$ 0
Definite-lived and indefinite-lived intangibles impairments		$ 0
Acquired Client Relationships (Net)
Goodwill
Intangible amortization expense		86,400,000	$ 107,700,000	$ 115,400,000
Intangible future amortization expense in 2018		85,000,000
Intangible future amortization expense in 2019		85,000,000
Intangible future amortization expense in 2020		50,000,000
Intangible future amortization expense in 2021		30,000,000
Intangible future amortization expense in 2022		$ 30,000,000