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Fair Value Measurements - Additional Information (Detail) (USD $)
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Financial Instruments [Line Items]      
Estimated fair value of long-term debt $ 2,190,000,000us-gaap_LongTermDebtFairValue $ 2,180,000,000us-gaap_LongTermDebtFairValue $ 1,660,000,000us-gaap_LongTermDebtFairValue
Long-term debt $ 2,100,000,000us-gaap_LongTermDebt $ 2,100,000,000us-gaap_LongTermDebt $ 1,600,000,000us-gaap_LongTermDebt