Long-Term Debt Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Long Term Debt Maturities Repayments Of Principal [Line Items]
Amount of long-term debt maturing - 2015	$ 0
Amount of long-term debt maturing - 2016	300,000
Amount of long-term debt maturing - 2017	0
Amount of long-term debt maturing - 2018	250,000
Amount of long-term debt maturing - 2019	500,000
Amount of long-term debt maturing - Thereafter	1,050,000
Long-term debt	2,100,000us-gaap_LongTermDebt	1,600,000us-gaap_LongTermDebt
2010 Senior Notes
Long Term Debt Maturities Repayments Of Principal [Line Items]
Amount of long-term debt maturing - 2015	0
Amount of long-term debt maturing - 2016	0
Amount of long-term debt maturing - 2017	0
Amount of long-term debt maturing - 2018	0
Amount of long-term debt maturing - 2019	0
Amount of long-term debt maturing - Thereafter	500,000
Long-term debt	500,000	500,000
2011 Senior Notes
Long Term Debt Maturities Repayments Of Principal [Line Items]
Amount of long-term debt maturing - 2015	0
Amount of long-term debt maturing - 2016	300,000
Amount of long-term debt maturing - 2017	0
Amount of long-term debt maturing - 2018	0
Amount of long-term debt maturing - 2019	0
Amount of long-term debt maturing - Thereafter	300,000
Long-term debt	600,000	600,000
2013 Senior Notes
Long Term Debt Maturities Repayments Of Principal [Line Items]
Amount of long-term debt maturing - 2015	0
Amount of long-term debt maturing - 2016	0
Amount of long-term debt maturing - 2017	0
Amount of long-term debt maturing - 2018	250,000
Amount of long-term debt maturing - 2019	0
Amount of long-term debt maturing - Thereafter	250,000
Long-term debt	500,000	500,000
2014 Senior Notes
Long Term Debt Maturities Repayments Of Principal [Line Items]
Amount of long-term debt maturing - 2015	0
Amount of long-term debt maturing - 2016	0
Amount of long-term debt maturing - 2017	0
Amount of long-term debt maturing - 2018	0
Amount of long-term debt maturing - 2019	500,000
Amount of long-term debt maturing - Thereafter	0
Long-term debt	$ 500,000

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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. 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 03

-Paragraph 16

-Article 7

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

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-Name Accounting Standards Codification

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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-Name Regulation S-X (SX)

-Number 210

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-Article 9

Reference 5: http://www.xbrl.org/2003/role/presentationRef

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-Number 210

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-Paragraph 20, 22

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Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 470

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6802200&loc=d3e1835-112601

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 3

-Subparagraph (SX 210.12-04.(a))

-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e24072-122690

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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