Derivative Financial Instruments - Information about Interest Rate Swap Agreements (Detail) - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Amount	$ 12,400,000,000	$ 9,400,000,000
Average Maturity (in years)	1 year 10 months 24 days	2 years 6 months
Estimated Fair Value Gain (Loss)	$ (129,000)	$ 639,000
Interest Payments On Variable Rate Commercial Real Estate Loans [Member] | Cash Flow Hedges [Member]
Derivative [Line Items]
Notional Amount	$ 7,700,000,000	$ 4,850,000,000
Average Maturity (in years)	1 year 4 months 24 days	2 years
Weighted-Average Rate, Fixed	1.52%	1.52%
Weighted-Average Rate, Variable	1.98%	1.36%
Estimated Fair Value Gain (Loss)	$ 56,000	$ 66,000
Fixed Rate Long-Term Borrowings [Member] | Fair Value Hedges [Member]
Derivative [Line Items]
Notional Amount	$ 4,700,000,000	$ 4,550,000,000
Average Maturity (in years)	2 years 8 months 12 days	2 years 10 months 25 days
Weighted-Average Rate, Fixed	2.41%	2.27%
Weighted-Average Rate, Variable	2.72%	2.09%
Estimated Fair Value Gain (Loss)	$ (185,000)	$ 573,000