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Derivative Financial Instruments - Information about Interest Rate Swap Agreements (Detail) - USD ($)
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Derivative [Line Items]    
Notional Amount $ 15,750,000,000 $ 9,400,000,000
Average Maturity (in years) 1 year 9 months 18 days 2 years 6 months
Estimated Fair Value Gain $ 2,446,000 $ 639,000
Interest Payments On Variable Rate Commercial Real Estate Loans [Member] | Cash Flow Hedges [Member]    
Derivative [Line Items]    
Notional Amount $ 10,550,000,000 $ 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 2.11% 1.36%
Estimated Fair Value Gain $ 988,000 $ 66,000
Fixed Rate Long-Term Borrowings [Member] | Fair Value Hedges [Member]    
Derivative [Line Items]    
Notional Amount $ 5,200,000,000 $ 4,550,000,000
Average Maturity (in years) 2 years 8 months 12 days 2 years 10 months 25 days
Weighted-Average Rate, Fixed 2.45% 2.27%
Weighted-Average Rate, Variable 2.76% 2.09%
Estimated Fair Value Gain $ 1,458,000 $ 573,000