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Condensed Consolidated Financial Statements - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2016
Sep. 30, 2017
Summary Of Significant Accounting Policies [Line Items]    
Amount of off balance sheet settlement funds $ 270,000,000 $ 223,900,000
Goodwill impairment 0  
Level 2 [Member]    
Summary Of Significant Accounting Policies [Line Items]    
Fair value senior note $ 309,800,000 $ 305,400,000