Indebtedness - Bank Debt - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended				9 Months Ended
	Sep. 30, 2013 CommercialBank	Sep. 30, 2012	Sep. 30, 2013 CommercialBank	Sep. 30, 2012	Dec. 31, 2012	Oct. 18, 2013 Subsequent Event	Sep. 30, 2013 Minimum	Sep. 30, 2013 Maximum	Sep. 30, 2013 Alternate Base Rate	Sep. 30, 2013 LIBOR Rate
Debt Instrument [Line Items]
Bank Credit facility, Facility amount	$ 1,750,000,000		$ 1,750,000,000			$ 1,750,000,000
Bank Credit facility, Borrowing base	2,000,000,000		2,000,000,000			2,000,000,000
Percentage holding of commercial banks, Maximum	9.00%		9.00%
Number of commercial banks included in current bank group	28		28
Notice period for increase in facility amount	20 days
Bank debt	427,000,000		427,000,000		739,000,000
Undrawn letters of credit	84,900,000		84,900,000
Borrowing capacity available under the facility amount	$ 1,200,000,000		$ 1,200,000,000
Maturity date of loan	Feb. 18, 2016
Minimum range of base rate									0.50%	1.50%
Maximum range of base rate									1.50%	2.50%
Weighted average interest rate on the bank credit facility	1.90%	2.10%	2.00%	2.20%
Annual rate of commitment fee paid on the undrawn balance							0.35%	0.50%
Commitment fee	0.375%
Interest rate margin									0.50%	1.50%