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Summary of Significant Accounting Policies - Goodwill and Intangible Assets - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Accounting Policies [Abstract]    
Net book value of intangible assets $ 74,800,000 $ 79,700,000
Goodwill impairment charges 0 0
Finite-Lived Intangible Assets [Line Items]    
Net book value of finite-lived intangible assets $ 38,200,000 $ 43,100,000
Minimum    
Finite-Lived Intangible Assets [Line Items]    
Intangible assets with defined useful lives, amortization period 1 year  
Maximum    
Finite-Lived Intangible Assets [Line Items]    
Intangible assets with defined useful lives, amortization period 20 years