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Debt - 5.500% Senior Unsecured Notes Due 2024 - Additional Information (Detail) - 5.500% Senior Unsecured Notes Due 2024 [Member] - USD ($)
12 Months Ended
Apr. 17, 2014
Dec. 31, 2020
Dec. 31, 2019
Debt Instrument [Line Items]      
Senior unsecured notes face amount $ 300,000,000    
Senior unsecured notes, payable term   Interest on the notes was payable semi-annually on May 1 and November 1 of each year.  
Senior unsecured notes, interest rate 5.50% 5.50% 5.50%
Debt instrument maturity date May 01, 2024    
Debt instrument, redemption date Dec. 19, 2020