Indebtedness - Bank Debt - Additional Information (Detail) (USD $)	3 Months Ended				3 Months Ended
	Mar. 31, 2014 CommercialBank	Mar. 31, 2013	Dec. 31, 2013	Apr. 03, 2014 Subsequent Event	Mar. 31, 2014 Alternate Base Rate	Mar. 31, 2014 LIBOR Rate	Mar. 31, 2014 Minimum	Mar. 31, 2014 Maximum
Debt Instrument [Line Items]
Bank Credit facility, Facility amount	$ 1,750,000,000			$ 1,750,000,000
Bank Credit facility, Borrowing base	2,000,000,000			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	594,000,000		500,000,000
Undrawn letters of credit	127,400,000
Borrowing capacity available under the facility amount	$ 1,000,000,000
Maturity date of loan	Feb. 18, 2016
Weighted average interest rate on the bank credit facility	2.00%	2.10%
Commitment fee	0.375%
Minimum range of base rate					0.50%	1.50%
Maximum range of base rate					1.50%	2.50%
Interest rate margin					0.75%	1.75%
Annual rate of commitment fee paid on the undrawn balance							0.35%	0.50%