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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, 2019 | Dec. 31, 2018
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | $ 11,269,591 | $ 11,387,321
Trading account securities | 24,298 | 24,086
Derivative assets: | 5,266 | 27,849
Derivative liabilities: | 26,640 | 11,185
Interest Rate Swaps Caps And Floors [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets: | 44,196 | 19,806
Derivative liabilities: | 20,197 | 18,520
Commodity Swaps And Options [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets: | 12,218 | 26,768
Derivative liabilities: | 11,677 | 26,320
Foreign Exchange Forward [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets: | 45 | 193
Derivative liabilities: | 33 | 147
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 | 1,948,133 | 3,427,689
Trading account securities | 24,298 | 21,928
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 | 2,207,594 | 829,740
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 | 7,070,997 | 7,087,202
Trading account securities |  | 2,158
Other Investments [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | 42,867 | 42,690
Level 1 [Member] | Foreign Exchange Forward [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets: | 45 | 193
Derivative liabilities: | 33 | 147
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 | 1,948,133 | 3,427,689
Trading account securities | 24,298 | 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: | 44,196 | 19,806
Derivative liabilities: | 20,197 | 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: | 12,218 | 26,768
Derivative liabilities: | 11,677 | 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, at estimated fair value | 2,207,594 | 829,740
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 | 7,070,997 | 7,087,202
Trading account securities |  | 2,158
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 | 42,867 | 42,690
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 | 1,948,133 | 3,427,689
Trading account securities | 24,298 | 21,928
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 | 2,207,594 | 829,740
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 | 7,070,997 | 7,087,202
Trading account securities | 0 | 2,158
Other Investments [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | $ 42,867 | $ 42,690

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