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Debt - 6.375% Senior Unsecured Notes Due 2022 - Additional Information (Detail) - 6.375% Senior Unsecured Notes Due 2022 [Member] - USD ($)
12 Months Ended
Oct. 07, 2017
Aug. 20, 2013
Feb. 17, 2012
Dec. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]          
Senior unsecured notes face amount   $ 150,000,000 $ 200,000,000    
Senior unsecured notes, maturity date     Feb. 15, 2022    
Senior unsecured notes, interest rate     6.375% 6.375% 6.375%
Debt instrument redemption price   102.00%      
Debt instrument effective rate   5.998%      
Senior notes frequency of periodic payment       Interest on these notes was payable semi-annually on February 15 and August 15 of each year  
Senior unsecured notes, redemption description       Offered a redemption option to redeem some or all of the notes at a premium that decreased over time, plus accrued and unpaid interest to, but not including, the redemption date.  
Redemption Premium $ 11,200,000