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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, 2017 | Dec. 31, 2016
--- | --- | ---
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | $ 10,489,009 | $ 10,203,277
Trading account securities | 21,098 | 16,703
Derivative assets: | 3,810 | 3,798
Derivative liabilities: | 27,375 | 32,534
Interest Rate Swaps Caps And Floors [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets: | 20,022 | 29,059
Derivative liabilities: | 18,754 | 29,968
Commodity Swaps And Options [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets: | 15,641 | 6,179
Derivative liabilities: | 15,327 | 5,963
Foreign Exchange Forward [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets: | 415 | 29
Derivative liabilities: | 239 | 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 | 3,445,153 | 4,019,731
Trading account securities | 19,210 | 16,594
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 | 665,086 | 785,167
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 | 6,336,209 | 5,355,885
Trading account securities | 1,888 | 109
Other Investments [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | 42,561 | 42,494
Level 1 [Member] | Foreign Exchange Forward [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets: | 415 | 29
Derivative liabilities: | 239 | 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 | 3,445,153 | 4,019,731
Trading account securities | 19,210 | 16,594
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: | 20,022 | 29,059
Derivative liabilities: | 18,754 | 29,968
Level 2 [Member] | Commodity Swaps And Options [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets: | 14,408 | 6,179
Derivative liabilities: | 15,327 | 5,963
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 | 665,086 | 785,167
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 | 6,336,209 | 5,355,885
Trading account securities | 1,888 | 109
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 | 42,561 | 42,494
Level 3 [Member] | Commodity Swaps And Options [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Derivative assets: | 1,233 | 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 | 3,445,153 | 4,019,731
Trading account securities | 19,210 | 16,594
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 | 665,086 | 785,167
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 | 6,336,209 | 5,355,885
Trading account securities | 1,888 | 109
Other Investments [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Securities available for sale, at estimated fair value | $ 42,561 | $ 42,494

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