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Derivative Financial Instruments - Information about Interest Rate Swap Agreements (Detail) - Fixed Rate Long-Term Borrowings [Member] - Fair Value Hedges [Member] - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Derivative [Line Items]    
Notional Amount $ 1,400,000,000 $ 1,400,000,000
Average Maturity (in years) 1 year 8 months 12 days 2 years 8 months 12 days
Weighted-Average Rate, Fixed 4.42% 4.42%
Weighted-Average Rate, Variable 1.39% 1.19%
Estimated Fair Value Gain $ 43,892,000 $ 73,251,000