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Capital Stock - (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 15, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2013
Nov. 27, 2013
Class of Stock [Line Items]          
Settlement of tangible equity units, Shares 7,857,000        
Number of shares repurchased     283,976    
Value of share repurchased   $ 0 $ 3,537    
Common stock repurchased since announcement of strategic initiative     3,777,308    
Value of common stock repurchased since announcement of strategic initiative     $ 57,696    
Maximum percentage of shares repurchased under strategic initiative     20.00%    
Maximum number of shares repurchased under strategic initiative     7,000,000    
6% Tangible Equity Units          
Class of Stock [Line Items]          
Interest rate percentage       6.00% 6.00%