Derivative Financial Instruments - Information about Interest Rate Swap Agreements (Parenthetical) (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Derivative [Line Items]
Notional Amount	$ 51,050,000,000	$ 57,550,000,000
Forward-Starting Interest Rate Swap Agreements [Member]
Derivative [Line Items]
Notional Amount	40,400,000,000
Fair Value Hedges [Member]
Derivative [Line Items]
Reduction of estimated fair value losses on hedging instruments	101,500,000	45,100,000
Cash Flow Hedges [Member]
Derivative [Line Items]
Reduction of estimated fair value losses on hedging instruments	372,200,000	$ 140,700,000
Forward-Starting Interest Rate Swap Agreements [Member]
Derivative [Line Items]
Notional Amount	$ 32,100,000,000