Estimated Fair Value of Outstanding Debt Balances (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Carrying value of Debt	$ 615,000	$ 855,000
Fair value of Debt	574,052	793,441
Revolving Credit Facility
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Carrying value of Debt	15,000
Fair value of Debt	15,000
Term Loan Facility
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Carrying value of Debt		70,000
Fair value of Debt		70,000
2015 Term Loan
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Carrying value of Debt		185,000
Fair value of Debt		185,000
4.97% Series A Senior Notes, Due October 5th 2020
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Carrying value of Debt	300,000	300,000
Fair value of Debt	288,203	279,635
4.27% Series B Senior Notes, Due June 14th 2022
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Carrying value of Debt	300,000	300,000
Fair value of Debt	$ 270,849	$ 258,806