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Debt - $350 Million 5% Senior Notes - Additional Information (Detail) - $350 Million 5% Senior Notes [Member]
12 Months Ended
Dec. 31, 2017
USD ($)
Debt Instrument [Line Items]  
Maturity date Apr. 15, 2021
Senior notes, principal amount $ 350,000,000
First semi-annual interest payment date --04-15
Second semi-annual interest payment date --10-15
Interest rate of Senior Notes 5.00%
Redemption Period One [Member]  
Debt Instrument [Line Items]  
Redemption date Apr. 15, 2017
Redemption price expressed as percentage 102.50%
Redemption Period Two [Member]  
Debt Instrument [Line Items]  
Redemption date Apr. 15, 2018
Redemption price expressed as percentage 101.25%
Redemption Period Three [Member]  
Debt Instrument [Line Items]  
Redemption date Apr. 15, 2019
Redemption price expressed as percentage 100.00%