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Fair Value Measurements - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Sep. 30, 2015
Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities available for sale, estimated fair value $ 8,972,688 $ 8,461,254
Trading account securities 19,672 15,426
Derivative assets 16,036 29,921
Derivative liabilities 52,291 47,898
Interest rate swaps, caps and floors [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 48,022 41,101
Derivative liabilities 50,919 43,853
Commodity swaps and options [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 17,370 32,829
Derivative liabilities 17,314 32,729
Foreign Exchange Forward [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 33 666
Derivative liabilities 19 617
U.S. Treasury [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities available for sale, estimated fair value 3,782,387 3,811,252
Trading account securities 16,244 15,339
Residential mortgage-backed securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities available for sale, estimated fair value 1,130,531 1,398,724
States and political subdivisions [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities available for sale, estimated fair value 4,017,333 3,208,907
Trading account securities 3,428 87
Other [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities available for sale, estimated fair value 42,437 42,371
Level 1 [Member] | Foreign Exchange Forward [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 33 666
Derivative liabilities 19 617
Level 1 [Member] | U.S. Treasury [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities available for sale, estimated fair value 3,782,387 3,811,252
Trading account securities 16,244 15,339
Level 2 [Member] | Interest rate swaps, caps and floors [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 48,022 40,931
Derivative liabilities 50,919 43,853
Level 2 [Member] | Commodity swaps and options [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 17,370 32,829
Derivative liabilities 17,314 32,729
Level 2 [Member] | Residential mortgage-backed securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities available for sale, estimated fair value 1,130,531 1,398,724
Level 2 [Member] | States and political subdivisions [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities available for sale, estimated fair value 4,017,333 3,208,907
Trading account securities 3,428 87
Level 2 [Member] | Other [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities available for sale, estimated fair value 42,437 42,371
Level 3 [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets 0  
Level 3 [Member] | Interest rate swaps, caps and floors [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Derivative assets $ 0 $ 170