Estimated Fair Values Determined Using Available Market Information and Valuation Methodologies (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Average Balance	$ 3,805,397	$ 3,249,230
Revolving Credit Facility
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Average Balance	150,000	343,500
Level 2 \| Revolving Credit Facility
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, Fair Value	150,000	343,500
Premium on 2021 Notes
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Premium on Long-term Debt	5,397	5,730
Premium on 2021 Notes \| Level 2
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, Fair Value	5,397	5,730
5.875 Percent Senior Notes due April 2021
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Average Balance	750,000	750,000
Premium on Long-term Debt	5,397	5,730
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	805,313	801,098
6.125 Percent Senior Notes due July 2022
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Average Balance	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	827,813	804,848
4.875 Percent Senior Notes due May 2023
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Average Balance	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	1,477,882	1,355,382
4.875 Percent Senior Notes due March 2024
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Average Balance	750,000
4.875 Percent Senior Notes due March 2024 \| Level 2
Fair Value Balance Sheet Grouping Financial Statement Captions [Line Items]
Long-term debt, Fair Value	$ 794,063

The amount of debt premium that was originally recognized at the issuance of the instrument that has yet to be amortized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 45

-Paragraph 1A

-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 30

-Section 55

-Paragraph 8

-URI http://asc.fasb.org/extlink&oid=6584090&loc=d3e28878-108400

+ Details

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Balance Type:	credit
Period Type:	instant

- Details

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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

+ References

No definition available.

+ Details

Name:	us-gaap_LongTermDebtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Balance Type:	credit
Period Type:	instant