Annual report pursuant to Section 13 and 15(d)

Loss per Share of Common Stock - Basic and Diluted Loss Per Share of Common Stock Narrative (Detail)

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Loss per Share of Common Stock - Basic and Diluted Loss Per Share of Common Stock Narrative (Detail) - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Senior Common Stock      
Summary Of Computation Of Basic And Diluted Earnings Per Share [Line Items]      
Anti-dilutive convertible shares of senior common stock excluded from calculation of diluted earnings per share (in shares) 753,881 800,116 782,957