Derivative Financial Instruments - Derivative Notional Values (Detail) - USD ($)	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Total Derivative Notional Values	$ 120,600,000,000	$ 131,400,000,000
Designated as Hedging Instrument [Member] | Cash Flow [Member]
Derivative [Line Items]
Total Derivative Notional Values	22,400,000,000	21,400,000,000
Designated as Hedging Instrument [Member] | Fair Value [Member]
Derivative [Line Items]
Total Derivative Notional Values	13,800,000,000	14,800,000,000
Trading [Member]
Derivative [Line Items]
Total Derivative Notional Values	84,400,000,000	95,200,000,000
Interest Rate Swaps [Member]
Derivative [Line Items]
Total Derivative Notional Values	95,100,000,000	98,600,000,000
Interest Rate Swaps [Member] | Designated as Hedging Instrument [Member] | Cash Flow [Member]
Derivative [Line Items]
Total Derivative Notional Values	22,400,000,000	21,400,000,000
Interest Rate Swaps [Member] | Designated as Hedging Instrument [Member] | Fair Value [Member]
Derivative [Line Items]
Total Derivative Notional Values	9,600,000,000	10,300,000,000
Interest Rate Swaps [Member] | Trading [Member]
Derivative [Line Items]
Total Derivative Notional Values	63,100,000,000	66,900,000,000
Floor Income Contracts [Member]
Derivative [Line Items]
Total Derivative Notional Values	21,200,000,000	27,900,000,000
Floor Income Contracts [Member] | Trading [Member]
Derivative [Line Items]
Total Derivative Notional Values	21,200,000,000	27,900,000,000
Cross-Currency Interest Rate Swaps [Member]
Derivative [Line Items]
Total Derivative Notional Values	4,200,000,000	4,700,000,000
Cross-Currency Interest Rate Swaps [Member] | Designated as Hedging Instrument [Member] | Fair Value [Member]
Derivative [Line Items]
Total Derivative Notional Values	4,200,000,000	4,500,000,000
Cross-Currency Interest Rate Swaps [Member] | Trading [Member]
Derivative [Line Items]
Total Derivative Notional Values		200,000,000
Other [Member]
Derivative [Line Items]
Total Derivative Notional Values	100,000,000	200,000,000
Other [Member] | Trading [Member]
Derivative [Line Items]
Total Derivative Notional Values	$ 100,000,000	$ 200,000,000