Derivative Financial Instruments - Information about Interest Rate Swap Agreements (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Derivative [Line Items]
Notional Amount	$ 34,200,000,000	$ 19,850,000,000
Average Maturity (in years)	1 year 7 months 6 days	1 year 8 months 12 days
Estimated Fair Value Gain (Loss)	$ (16,924,000)	$ 5,530,000
Interest Payments On Variable Rate Commercial Real Estate Loans [Member] | Cash Flow Hedges [Member]
Derivative [Line Items]
Notional Amount	$ 29,750,000,000	$ 15,400,000,000
Average Maturity (in years)	1 year 4 months 24 days	1 year 3 months 18 days
Weighted-Average Rate, Fixed	2.29%	1.52%
Weighted-Average Rate, Variable	2.49%	2.35%
Estimated Fair Value Gain (Loss)	$ (12,534,000)	$ 1,311,000
Fixed Rate Long-Term Borrowings [Member] | Fair Value Hedges [Member]
Derivative [Line Items]
Notional Amount	$ 4,450,000,000	$ 4,450,000,000
Average Maturity (in years)	2 years 7 months 6 days	2 years 9 months 18 days
Weighted-Average Rate, Fixed	2.47%	2.47%
Weighted-Average Rate, Variable	3.04%	3.02%
Estimated Fair Value Gain (Loss)	$ (4,390,000)	$ 4,219,000