Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	$ 1,020,838	$ 1,106,057
Held-to-maturity Securities, Gross Unrealized Gains	26,588	11,533
Held-to-maturity Securities, Gross Unrealized Losses	3	637
Held to Maturity, Estimated Fair Value	1,047,423	1,116,953
Available for Sale, Amortized Cost	11,979,766	11,410,447
Available for Sale Securities, Gross Unrealized Gains	443,604	85,611
Available for Sale Securities, Gross Unrealized Losses	2,924	108,737
Available for Sale, Estimated Fair Value	12,420,446	11,387,321
U.S. Treasury [Member]
Schedule of Investments [Line Items]
Available for Sale, Amortized Cost	2,806,802	3,455,417
Available for Sale Securities, Gross Unrealized Gains	20,725	1,772
Available for Sale Securities, Gross Unrealized Losses	1,731	29,500
Available for Sale, Estimated Fair Value	2,825,796	3,427,689
Residential mortgage-backed securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	2,472	2,737
Held-to-maturity Securities, Gross Unrealized Gains	25	8
Held-to-maturity Securities, Gross Unrealized Losses	2	85
Held to Maturity, Estimated Fair Value	2,495	2,660
Available for Sale, Amortized Cost	2,198,466	823,208
Available for Sale Securities, Gross Unrealized Gains	42,766	13,079
Available for Sale Securities, Gross Unrealized Losses	780	6,547
Available for Sale, Estimated Fair Value	2,240,452	829,740
States and political subdivisions [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,016,866	1,101,820
Held-to-maturity Securities, Gross Unrealized Gains	26,563	11,525
Held-to-maturity Securities, Gross Unrealized Losses	0	552
Held to Maturity, Estimated Fair Value	1,043,429	1,112,793
Available for Sale, Amortized Cost	6,931,673	7,089,132
Available for Sale Securities, Gross Unrealized Gains	380,113	70,760
Available for Sale Securities, Gross Unrealized Losses	413	72,690
Available for Sale, Estimated Fair Value	7,311,373	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	1	0
Held to Maturity, Estimated Fair Value	1,499	1,500
Available for Sale, Amortized Cost	42,825	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	$ 42,825	$ 42,690

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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