Derivative Financial Instruments - Notional Amount of Interest Rate Swap Agreements Outstanding Maturity (Detail)	Dec. 31, 2017 USD ($)
Derivative Instruments And Hedging Activities Disclosures [Line Items]
Total	$ 9,400,000,000
Interest Rate Swap Agreements [Member]
Derivative Instruments And Hedging Activities Disclosures [Line Items]
2018	500,000,000
2019	4,250,000,000
2020	3,500,000,000
2022	650,000,000
2023 through 2027	500,000,000
Total	$ 9,400,000,000