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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, 2019 | Dec. 31, 2018
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, estimated fair value | $ 12,420,446 | $ 11,387,321
Trading account securities | 27,669 | 24,086
Derivative assets | 15,250 | 27,849
Derivative liabilities | 26,012 | 11,185
Interest rate swaps, caps and floors [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 55,842 | 19,806
Derivative liabilities | 24,855 | 18,520
Commodity swaps and options [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 16,534 | 26,768
Derivative liabilities | 16,086 | 26,320
Foreign Exchange Forward [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 24 | 193
Derivative liabilities | 11 | 147
U.S. Treasury [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, estimated fair value | 2,825,796 | 3,427,689
Trading account securities | 23,747 | 21,928
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 | 2,240,452 | 829,740
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 | 7,311,373 | 7,087,202
Trading account securities | 3,922 | 2,158
Other [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, estimated fair value | 42,825 | 42,690
Level 1 [Member] | Foreign Exchange Forward [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 24 | 193
Derivative liabilities | 11 | 147
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 | 2,825,796 | 3,427,689
Trading account securities | 23,747 | 21,928
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 | 55,842 | 19,806
Derivative liabilities | 24,855 | 18,520
Level 2 [Member] | Commodity swaps and options [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets | 16,534 | 26,768
Derivative liabilities | 16,086 | 26,320
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 | 2,240,452 | 829,740
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 | 7,311,373 | 7,087,202
Trading account securities | 3,922 | 2,158
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 | $ 42,825 | $ 42,690

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