Indebtedness - Bank Debt - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013 CommercialBank	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Bank Credit facility, Facility amount	$ 1,750,000,000
Bank Credit facility, Borrowing base	2,000,000,000
Percentage holding of commercial banks, Maximum	9.00%
Number of commercial banks included in current bank group	28
Notice period for increase in facility amount	20 days
Bank debt	500,000,000	739,000,000
Undrawn letters of credit	84,900,000
Borrowing capacity available under the facility amount	$ 1,200,000,000
Maturity date of loan	Feb. 18, 2016
Weighted average interest rate on the bank credit facility	2.00%	2.20%	2.20%
Commitment fee	0.375%
Alternate Base Rate
Debt Instrument [Line Items]
Minimum range of base rate	0.50%
Maximum range of base rate	1.50%
Interest rate margin	0.50%
LIBOR Rate
Debt Instrument [Line Items]
Minimum range of base rate	1.50%
Maximum range of base rate	2.50%
Interest rate margin	1.50%
Minimum
Debt Instrument [Line Items]
Annual rate of commitment fee paid on the undrawn balance	0.375%
Maximum
Debt Instrument [Line Items]
Annual rate of commitment fee paid on the undrawn balance	0.50%