Indebtedness - Bank Debt - Additional Information (Detail) - Bank Credit Facility	3 Months Ended
	Mar. 31, 2016 USD ($) CommercialBank	Mar. 31, 2015	Mar. 17, 2016 USD ($)
Debt Instrument [Line Items]
Bank Credit facility, maximum amount	$ 4,000,000,000
Bank Credit facility, borrowing base			$ 3,000,000,000
Bank commitments			$ 2,000,000,000
Percentage holding of commercial banks, Maximum	5.80%
Number of commercial banks included in current bank group | CommercialBank	29
Maturity date of loan	Oct. 16, 2019
Bank debt	$ 31,000,000
Undrawn letters of credit	230,800,000
Borrowing capacity available under the commitment amount	$ 1,700,000,000
Weighted average interest rate on the bank credit facility	2.10%	1.80%
Annual rate of commitment fee paid on the undrawn balance	0.30%
Alternate Base Rate
Debt Instrument [Line Items]
Minimum range of base rate	0.25%
Maximum range of base rate	1.25%
Interest rate margin	0.25%
Alternate Base Rate | Credit Facility
Debt Instrument [Line Items]
Minimum range of base rate	0.125%
Maximum range of base rate	0.75%
LIBOR Rate
Debt Instrument [Line Items]
Minimum range of base rate	1.25%
Maximum range of base rate	2.25%
Interest rate margin	1.25%
LIBOR Rate | Credit Facility
Debt Instrument [Line Items]
Minimum range of base rate	1.125%
Maximum range of base rate	1.75%
Minimum
Debt Instrument [Line Items]
Annual rate of commitment fee paid on the undrawn balance	0.30%
Minimum | Credit Facility
Debt Instrument [Line Items]
Annual rate of commitment fee paid on the undrawn balance	0.15%
Maximum
Debt Instrument [Line Items]
Annual rate of commitment fee paid on the undrawn balance	0.375%
Maximum | Credit Facility
Debt Instrument [Line Items]
Annual rate of commitment fee paid on the undrawn balance	0.30%