Interest Rate Hedging - Derivatives' Cash Flow Hedging Relationships (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Amount of after tax of gain/(loss) recognized in Other Comprehensive Income on Derivatives (effective portion)	$ (1,995)	$ (431)
Pre-tax amount of gain/(loss) reclassified from Accumulated Other Comprehensive Income into Income (effective portion)	0	0
Interest Expense [Member] | Forward Starting Interest Rate Swap Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of after tax of gain/(loss) recognized in Other Comprehensive Income on Derivatives (effective portion)	(1,995)	(431)
Pre-tax amount of gain/(loss) reclassified from Accumulated Other Comprehensive Income into Income (effective portion)	$ 0	$ 0