Indebtedness - Bank Debt - Additional Information (Detail) - Bank Credit Facility	3 Months Ended
	Mar. 31, 2018 USD ($) CommercialBank	Mar. 31, 2017	Apr. 13, 2018 USD ($) CommercialBank	Mar. 21, 2017 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	$ 1,200,000,000
Undrawn letters of credit	281,400,000
Borrowing capacity available under the commitment amount	$ 536,600,000
Weighted average interest rate on the bank credit facility	3.40%	2.40%
Annual rate of commitment fee paid on the undrawn balance	0.30%
Credit Facility
Debt Instrument [Line Items]
Line of credit facility, initiation date	Apr. 13, 2018
Line of credit facility, maturity date	Apr. 13, 2023
Credit Facility | Subsequent Event
Debt Instrument [Line Items]
Bank Credit facility, maximum amount			$ 4,000,000,000
Percentage holding of commercial banks, Maximum			5.80%
Number of commercial banks included in current bank group | CommercialBank			27
Alternate Base Rate
Debt Instrument [Line Items]
Interest rate margin	0.50%
LIBOR Rate
Debt Instrument [Line Items]
Interest rate margin	1.50%
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%
Minimum | Alternate Base Rate
Debt Instrument [Line Items]
Base rate on debt	0.25%
Minimum | Alternate Base Rate | Credit Facility
Debt Instrument [Line Items]
Base rate on debt	0.125%
Minimum | LIBOR Rate
Debt Instrument [Line Items]
Base rate on debt	1.25%
Minimum | LIBOR Rate | Credit Facility
Debt Instrument [Line Items]
Base rate on debt	1.125%
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%
Maximum | Alternate Base Rate
Debt Instrument [Line Items]
Base rate on debt	1.25%
Maximum | Alternate Base Rate | Credit Facility
Debt Instrument [Line Items]
Base rate on debt	0.75%
Maximum | LIBOR Rate
Debt Instrument [Line Items]
Base rate on debt	2.25%
Maximum | LIBOR Rate | Credit Facility
Debt Instrument [Line Items]
Base rate on debt	1.75%