Estimated Fair Values Determined Using Available Market Information and Valuation Methodologies (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Revolving Credit Facility
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Carrying Amount	$ 104,800
Revolving Credit Facility | Level 2
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, Fair Value	104,800
5.875 Percent Senior Notes due April 2021
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Carrying Amount	755,893	350,000
5.875 Percent Senior Notes due April 2021 | Level 2
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, Fair Value	771,563	370,125
6.125 Percent Senior Notes due July 2022
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Carrying Amount	750,000	750,000
6.125 Percent Senior Notes due July 2022 | Level 2
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, Fair Value	773,910	810,000
4.875 Percent Senior Notes due May 2023
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Debt Instrument, Carrying Amount	1,400,000	1,400,000
4.875 Percent Senior Notes due May 2023 | Level 2
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, Fair Value	$ 1,326,500	$ 1,428,882