Summary of Significant Accounting Policies - Goodwill and Other Intangible Assets (Details)	12 Months Ended
	Dec. 31, 2017
Customer relationships | Minimum
Finite-Lived Intangible Assets [Line Items]
Intangible assets other than goodwill, estimated period of benefit	10 years
Customer relationships | Maximum
Finite-Lived Intangible Assets [Line Items]
Intangible assets other than goodwill, estimated period of benefit	15 years
Goodwill
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Operational forecast period used in fair value inputs	3 years