Securities (Year End Securities Held to Maturity and Available for Sale) (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	$ 2,030,005	$ 1,106,057
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	28,931	11,533
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	10,261	637
Held to Maturity, Estimated Fair Value	2,048,675	1,116,953
Available for Sale, Amortized Cost	10,877,769	11,410,447
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	405,399	85,611
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	13,577	108,737
Available for Sale, Estimated Fair Value	11,269,591	11,387,321
US Treasury [Member]
Schedule of Investments [Line Items]
Available for Sale, Amortized Cost	1,941,283	3,455,417
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	18,934	1,772
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	12,084	29,500
Available for Sale, Estimated Fair Value	1,948,133	3,427,689
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	530,861	2,737
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	22	8
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	9,365	85
Held to Maturity, Estimated Fair Value	521,518	2,660
Available for Sale, Amortized Cost	2,176,275	823,208
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	32,608	13,079
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	1,289	6,547
Available for Sale, Estimated Fair Value	2,207,594	829,740
States and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,497,644	1,101,820
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	28,909	11,525
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	896	552
Held to Maturity, Estimated Fair Value	1,525,657	1,112,793
Available for Sale, Amortized Cost	6,717,344	7,089,132
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	353,857	70,760
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	204	72,690
Available for Sale, Estimated Fair Value	7,070,997	7,087,202
Other [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,500	1,500
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Held to Maturity, Estimated Fair Value	1,500	1,500
Available for Sale, Amortized Cost	42,867	42,690
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Available for Sale, Estimated Fair Value	$ 42,867	$ 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 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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