| Fair Value of Financial Instruments - Fair Values and Related Carrying Values of Debt Instruments (Detail) (USD $) In Thousands, unless otherwise specified | Sep. 30, 2013 | Dec. 31, 2012 | 
|---|---|---|
| Carrying Amount [Member] | ||
| Long-term Debt | ||
| Long term debt Fair value | $ 650,000 | $ 425,000 | 
| Carrying Amount [Member] | 7.50% Senior Notes, due August 2020 [Member] | ||
| Long-term Debt | ||
| Long term debt Fair value | 225,000 | 0 | 
| Carrying Amount [Member] | 9.125% Senior Notes, due April 2018 [Member] | ||
| Long-term Debt | ||
| Long term debt Fair value | 425,000 | 425,000 | 
| Fair Value [Member] | ||
| Long-term Debt | ||
| Long term debt Fair value | 679,750 | 453,688 | 
| Fair Value [Member] | 7.50% Senior Notes, due August 2020 [Member] | ||
| Long-term Debt | ||
| Long term debt Fair value | 225,000 | 0 | 
| Fair Value [Member] | 9.125% Senior Notes, due April 2018 [Member] | ||
| Long-term Debt | ||
| Long term debt Fair value | $ 454,750 | $ 453,688 |