Financial Instruments and Fair Value Disclosures - Fair Value and Related Carrying Values of Our Debt Instruments (Detail) (USD $)	Dec. 31, 2012	Dec. 31, 2011
4.875% Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value	$ 249,837,000	$ 249,779,000
5.15% Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value	249,882,000	249,811,000
5.70% Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value	496,867,000	496,819,000
5.875% Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value	499,480,000	499,414,000
Fair Value [Member] | 4.875% Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	275,500,000	272,900,000
Fair Value [Member] | 5.15% Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	269,000,000	272,700,000
Fair Value [Member] | 5.70% Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	641,400,000	555,000,000
Fair Value [Member] | 5.875% Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	617,100,000	575,400,000
Carrying Value [Member] | 4.875% Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value	249,800,000	249,800,000
Carrying Value [Member] | 5.15% Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value	249,900,000	249,800,000
Carrying Value [Member] | 5.70% Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value	496,900,000	496,800,000
Carrying Value [Member] | 5.875% Senior Notes [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying Value	$ 499,500,000	$ 499,400,000