Long-term Debt - Additional Information (Detail) (Bank credit facility, USD $)	6 Months Ended
	Jun. 30, 2014
Debt Instrument [Line Items]
Bank credit facility	$ 1,000,000,000
Line of credit facility commitment term (in years)	5 years
Maturity of credit facility	Nov. 22, 2018
Borrowing base	700,000,000
Availability of borrowing base	420,000,000
Stated percentage over federal funds rate to calculate base rate	0.50%
Stated percentage over 30 day LIBOR to calculate base rate	1.00%
Minimum spread rate over base rate for interest rate on credit facility	0.50%
Maximum spread rate over base rate for interest rate on credit facility	1.50%
Maximum amount of cash dividends without restriction under credit facility	$ 120,000,000
Minimum
Debt Instrument [Line Items]
Spread rate over LIBOR for interest rate on credit facility	1.50%
Commitment fee on unused borrowing base	0.375%
Maximum
Debt Instrument [Line Items]
Spread rate over LIBOR for interest rate on credit facility	2.50%
Commitment fee on unused borrowing base	0.50%