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Long-Term Debt - Additional Information - Senior Unsecured Notes (Detail) - USD ($)
$ in Millions
1 Months Ended 3 Months Ended 12 Months Ended
Jan. 27, 2010
Nov. 30, 2015
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2014
Debt Instrument [Line Items]          
Repayments of senior debt       $ 84.1  
Senior Notes (8.5%) due January 15, 2020          
Debt Instrument [Line Items]          
Fixed interest rate on the portion of U. S. Dollar-denominated term loans outstanding 8.50% 8.50%   8.50%  
Debt instrument, principal amount $ 450.0 $ 200.0      
Percentage over par at which notes sold 99.20% 99.01%      
Effective interest rate       8.67%  
Repayments of senior debt       $ 84.1 $ 57.3
Capitalized cost included in long term debt       13.3  
Unamortized balance of the capitalized issuance cost       $ 1.6  
Debt instrument, redemption price as percentage of principal amount       100.00%  
Discount rate for redemption feature       0.50%  
Subsequent Events | Senior Notes (8.5%) due January 15, 2020          
Debt Instrument [Line Items]          
Fixed interest rate on the portion of U. S. Dollar-denominated term loans outstanding     8.50%    
Repayments of senior debt     $ 10.9