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Fair Value Measurements - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands | Jun. 30, 2021 | Dec. 31, 2020
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, estimated fair value | $ 10,840,526 | $ 10,437,565
Trading account securities | 24,137 | 24,456
Derivative assets | 14,211 | 11,111
Derivative liabilities | 177,239 | 69,323
Interest rate swaps, caps and floors [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 58,079 | 85,665
Derivative liabilities | 28,734 | 35,187
Commodity swaps and options [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 164,716 | 45,991
Derivative liabilities | 163,387 | 45,099
U.S. Treasury [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, estimated fair value | 1,497,722 | 1,119,633
Trading account securities | 24,137 | 23,996
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 | 1,761,629 | 1,987,679
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 | 7,538,819 | 7,287,902
Trading account securities |  | 460
Other [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, estimated fair value | 42,356 | 42,351
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 | 1,497,722 | 1,119,633
Trading account securities | 24,137 | 23,996
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 | 58,079 | 85,665
Derivative liabilities | 28,734 | 35,187
Level 2 [Member] | Commodity swaps and options [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 164,716 | 45,535
Derivative liabilities | 163,387 | 45,099
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 | 1,761,629 | 1,987,679
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 | 7,538,819 | 7,287,902
Trading account securities |  | 460
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 | 42,356 | 42,351
Level 3 [Member] | Commodity swaps and options [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | $ 0 | $ 456

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