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Fair Value Measurements (Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis) (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | $ 10,437,565 | $ 11,269,591
Trading account securities | 24,456 | 24,298
Derivative assets: | 11,111 | 5,266
Derivative liabilities: | 69,323 | 26,640
Interest Rate Swaps Caps And Floors [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets: | 85,665 | 44,196
Derivative liabilities: | 35,187 | 20,197
Commodity Swaps And Options [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets: | 45,991 | 12,218
Derivative liabilities: | 45,099 | 11,677
Foreign Exchange Forward [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets: | 0 | 45
Derivative liabilities: | 0 | 33
US Treasury Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | 1,119,633 | 1,948,133
Trading account securities | 23,996 | 24,298
Residential Mortgage Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | 1,987,679 | 2,207,594
US States and Political Subdivisions Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | 7,287,902 | 7,070,997
Trading account securities | 460 |
Other Investments [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | 42,351 | 42,867
Level 1 [Member] | Foreign Exchange Forward [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets: | 0 | 45
Derivative liabilities: | 0 | 33
Level 1 [Member] | US Treasury Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | 1,119,633 | 1,948,133
Trading account securities | 23,996 | 24,298
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: | 85,665 | 44,196
Derivative liabilities: | 35,187 | 20,197
Level 2 [Member] | Commodity Swaps And Options [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets: | 45,535 | 12,218
Derivative liabilities: | 45,099 | 11,677
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, at estimated fair value | 1,987,679 | 2,207,594
Level 2 [Member] | US States and Political Subdivisions Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | 7,287,902 | 7,070,997
Trading account securities | 460 |
Level 2 [Member] | Other Investments [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | 42,351 | 42,867
Level 3 [Member] | Commodity Swaps And Options [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets: | 456 | 0
US Treasury Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | 1,119,633 | 1,948,133
Trading account securities | 23,996 | 24,298
Residential Mortgage Backed Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | 1,987,679 | 2,207,594
US States and Political Subdivisions Debt Securities [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | 7,287,902 | 7,070,997
Trading account securities | 460 | 0
Other Investments [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | $ 42,351 | $ 42,867

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