Derivative Financial Instruments - Information about Interest Rate Swap Agreements (Parenthetical) (Detail) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Amount	$ 39,200,000,000	$ 19,850,000,000
Forward-Starting Interest Rate Swap Agreements [Member]
Derivative [Line Items]
Notional Amount	21,400,000,000	12,600,000,000
Fair Value Hedges [Member]
Derivative [Line Items]
Reduction of estimated fair value gains (loss) on hedging instruments	41,600,000	(54,700,000)
Cash Flow Hedges [Member]
Derivative [Line Items]
Reduction of estimated fair value gains (loss) on hedging instruments	$ 196,200,000	$ (9,100,000)