Securities (Year End Securities Held to Maturity and Available for Sale) (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	$ 1,106,057	$ 1,432,098
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	11,533	26,477
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	637	2,784
Held to Maturity, Estimated Fair Value	1,116,953	1,455,791
Available for Sale, Amortized Cost	11,410,447	10,329,951
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	85,611	193,835
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	108,737	34,777
Available for Sale, Estimated Fair Value	11,387,321	10,489,009
US Treasury [Member]
Schedule of Investments [Line Items]
Available for Sale, Amortized Cost	3,455,417	3,453,391
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	1,772	7,494
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	29,500	15,732
Available for Sale, Estimated Fair Value	3,427,689	3,445,153
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	2,737	3,610
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	8	15
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	85	38
Held to Maturity, Estimated Fair Value	2,660	3,587
Available for Sale, Amortized Cost	823,208	648,288
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	13,079	19,048
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	6,547	2,250
Available for Sale, Estimated Fair Value	829,740	665,086
States and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,101,820	1,428,488
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	11,525	26,462
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	552	2,746
Held to Maturity, Estimated Fair Value	1,112,793	1,452,204
Available for Sale, Amortized Cost	7,089,132	6,185,711
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	70,760	167,293
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	72,690	16,795
Available for Sale, Estimated Fair Value	7,087,202	6,336,209
Other [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,500	0
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	0
Available for Sale, Amortized Cost	42,690	42,561
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	$ 42,690	$ 42,561

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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