Estimated Fair Value of Outstanding Debt Balances (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Carrying value of Debt	$ 865,000	$ 985,500
Fair value of Debt	$ 851,144	943,019
Revolving Credit Facility
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Carrying value of Debt		303,000
Fair value of Debt		303,000
Term Loan Facility
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Carrying value of Debt	$ 75,000	82,500
Fair value of Debt	75,000	$ 82,500
2015 Term Loan
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Carrying value of Debt	190,000
Fair value of Debt	190,000
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	301,447	288,346
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	$ 284,697	$ 269,173