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Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities (Detail) - USD ($) $ in Thousands | Jun. 30, 2019 | Dec. 31, 2018
--- | --- | ---
Schedule of Investments [Line Items] |  |
Held to Maturity, Amortized Cost | $ 1,035,299 | $ 1,106,057
Held-to-maturity Securities, Gross Unrealized Gains | 26,683 | 11,533
Held-to-maturity Securities, Gross Unrealized Losses | 4 | 637
Held to Maturity, Estimated Fair Value | 1,061,978 | 1,116,953
Available for Sale, Amortized Cost | 11,946,522 | 11,410,447
Available for Sale Securities, Gross Unrealized Gains | 342,209 | 85,611
Available for Sale Securities, Gross Unrealized Losses | 9,218 | 108,737
Available for Sale, Estimated Fair Value | 12,279,513 | 11,387,321
U.S. Treasury [Member] |  |
Schedule of Investments [Line Items] |  |
Available for Sale, Amortized Cost | 2,907,476 | 3,455,417
Available for Sale Securities, Gross Unrealized Gains | 18,187 | 1,772
Available for Sale Securities, Gross Unrealized Losses | 4,943 | 29,500
Available for Sale, Estimated Fair Value | 2,920,720 | 3,427,689
Residential mortgage-backed securities [Member] |  |
Schedule of Investments [Line Items] |  |
Held to Maturity, Amortized Cost | 2,554 | 2,737
Held-to-maturity Securities, Gross Unrealized Gains | 13 | 8
Held-to-maturity Securities, Gross Unrealized Losses | 4 | 85
Held to Maturity, Estimated Fair Value | 2,563 | 2,660
Available for Sale, Amortized Cost | 2,212,761 | 823,208
Available for Sale Securities, Gross Unrealized Gains | 36,354 | 13,079
Available for Sale Securities, Gross Unrealized Losses | 1,403 | 6,547
Available for Sale, Estimated Fair Value | 2,247,712 | 829,740
States and political subdivisions [Member] |  |
Schedule of Investments [Line Items] |  |
Held to Maturity, Amortized Cost | 1,031,245 | 1,101,820
Held-to-maturity Securities, Gross Unrealized Gains | 26,670 | 11,525
Held-to-maturity Securities, Gross Unrealized Losses | 0 | 552
Held to Maturity, Estimated Fair Value | 1,057,915 | 1,112,793
Available for Sale, Amortized Cost | 6,783,505 | 7,089,132
Available for Sale Securities, Gross Unrealized Gains | 287,668 | 70,760
Available for Sale Securities, Gross Unrealized Losses | 2,872 | 72,690
Available for Sale, Estimated Fair Value | 7,068,301 | 7,087,202
Other [Member] |  |
Schedule of Investments [Line Items] |  |
Held to Maturity, Amortized Cost | 1,500 | 1,500
Held-to-maturity Securities, Gross Unrealized Gains | 0 | 0
Held-to-maturity Securities, Gross Unrealized Losses | 0 | 0
Held to Maturity, Estimated Fair Value | 1,500 | 1,500
Available for Sale, Amortized Cost | 42,780 | 42,690
Available for Sale Securities, Gross Unrealized Gains | 0 | 0
Available for Sale Securities, Gross Unrealized Losses | 0 | 0
Available for Sale, Estimated Fair Value | $ 42,780 | $ 42,690

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