Fair Value Summary of Long-Term Debt with Fixed Interest Rates (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt with fixed interest rates	$ 1,042,750	$ 603,000
Fair Value | 10.375% Senior Unsecured Notes due 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt with fixed interest rates	0	316,500
Fair Value | 7.25% Senior Unsecured Notes due 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt with fixed interest rates	291,000	286,500
Fair Value | Senior Notes due 2020 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt with fixed interest rates	751,750	0
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt with fixed interest rates	1,075,000	594,759
Carrying Value | 10.375% Senior Unsecured Notes due 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt with fixed interest rates	0	294,759
Carrying Value | 7.25% Senior Unsecured Notes due 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt with fixed interest rates	300,000	300,000
Carrying Value | Senior Notes due 2020 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt with fixed interest rates	$ 775,000	$ 0