Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	$ 1,236,511	$ 1,432,098
Held-to-maturity Securities, Gross Unrealized Gains	13,109	26,477
Held-to-maturity Securities, Gross Unrealized Losses	2,477	2,784
Held to Maturity, Estimated Fair Value	1,247,143	1,455,791
Available for Sale, Amortized Cost	10,749,394	10,329,951
Available for Sale Securities, Gross Unrealized Gains	85,241	193,835
Available for Sale Securities, Gross Unrealized Losses	116,892	34,777
Available for Sale, Estimated Fair Value	10,717,743	10,489,009
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Amortized Cost	3,454,387	3,453,391
Available for Sale Securities, Gross Unrealized Gains	0	7,494
Available for Sale Securities, Gross Unrealized Losses	44,306	15,732
Available for Sale, Estimated Fair Value	3,410,081	3,445,153
Residential mortgage-backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	3,317	3,610
Held-to-maturity Securities, Gross Unrealized Gains	9	15
Held-to-maturity Securities, Gross Unrealized Losses	73	38
Held to Maturity, Estimated Fair Value	3,253	3,587
Available for Sale, Amortized Cost	625,897	648,288
Available for Sale Securities, Gross Unrealized Gains	13,540	19,048
Available for Sale Securities, Gross Unrealized Losses	7,779	2,250
Available for Sale, Estimated Fair Value	631,658	665,086
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	1,231,694	1,428,488
Held-to-maturity Securities, Gross Unrealized Gains	13,100	26,462
Held-to-maturity Securities, Gross Unrealized Losses	2,394	2,746
Held to Maturity, Estimated Fair Value	1,242,400	1,452,204
Available for Sale, Amortized Cost	6,626,495	6,185,711
Available for Sale Securities, Gross Unrealized Gains	71,701	167,293
Available for Sale Securities, Gross Unrealized Losses	64,807	16,795
Available for Sale, Estimated Fair Value	6,633,389	6,336,209
Other [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	1,500	0
Held-to-maturity Securities, Gross Unrealized Gains	0	0
Held-to-maturity Securities, Gross Unrealized Losses	10	0
Held to Maturity, Estimated Fair Value	1,490	0
Available for Sale, Amortized Cost	42,615	42,561
Available for Sale Securities, Gross Unrealized Gains	0	0
Available for Sale Securities, Gross Unrealized Losses	0	0
Available for Sale, Estimated Fair Value	$ 42,615	$ 42,561

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

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-Name Accounting Standards Codification
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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 after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

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