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Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities (Detail) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Schedule of Investments [Line Items]    
Held to Maturity, Amortized Cost $ 1,077,279 $ 1,106,057
Held-to-maturity Securities, Gross Unrealized Gains 20,634 11,533
Held-to-maturity Securities, Gross Unrealized Losses 55 637
Held to Maturity, Estimated Fair Value 1,097,858 1,116,953
Available for Sale, Amortized Cost 11,838,943 11,410,447
Available for Sale Securities, Gross Unrealized Gains 207,094 85,611
Available for Sale Securities, Gross Unrealized Losses 32,074 108,737
Available for Sale, Estimated Fair Value 12,013,963 11,387,321
U.S. Treasury [Member]    
Schedule of Investments [Line Items]    
Available for Sale, Amortized Cost 3,455,925 3,455,417
Available for Sale Securities, Gross Unrealized Gains 5,314 1,772
Available for Sale Securities, Gross Unrealized Losses 16,171 29,500
Available for Sale, Estimated Fair Value 3,445,068 3,427,689
Residential mortgage-backed securities [Member]    
Schedule of Investments [Line Items]    
Held to Maturity, Amortized Cost 2,656 2,737
Held-to-maturity Securities, Gross Unrealized Gains 6 8
Held-to-maturity Securities, Gross Unrealized Losses 53 85
Held to Maturity, Estimated Fair Value 2,609 2,660
Available for Sale, Amortized Cost 1,210,179 823,208
Available for Sale Securities, Gross Unrealized Gains 20,610 13,079
Available for Sale Securities, Gross Unrealized Losses 3,354 6,547
Available for Sale, Estimated Fair Value 1,227,435 829,740
States and political subdivisions [Member]    
Schedule of Investments [Line Items]    
Held to Maturity, Amortized Cost 1,073,123 1,101,820
Held-to-maturity Securities, Gross Unrealized Gains 20,628 11,525
Held-to-maturity Securities, Gross Unrealized Losses 2 552
Held to Maturity, Estimated Fair Value 1,093,749 1,112,793
Available for Sale, Amortized Cost 7,130,105 7,089,132
Available for Sale Securities, Gross Unrealized Gains 181,170 70,760
Available for Sale Securities, Gross Unrealized Losses 12,549 72,690
Available for Sale, Estimated Fair Value 7,298,726 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 42,734 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 $ 42,734 $ 42,690