Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	$ 1,111,762	$ 1,432,098
Held-to-maturity Securities, Gross Unrealized Gains	7,240	26,477
Held-to-maturity Securities, Gross Unrealized Losses	2,157	2,784
Held to Maturity, Estimated Fair Value	1,116,845	1,455,791
Available for Sale, Amortized Cost	11,306,355	10,329,951
Available for Sale Securities, Gross Unrealized Gains	52,013	193,835
Available for Sale Securities, Gross Unrealized Losses	179,777	34,777
Available for Sale, Estimated Fair Value	11,178,591	10,489,009
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Amortized Cost	3,454,899	3,453,391
Available for Sale Securities, Gross Unrealized Gains	0	7,494
Available for Sale Securities, Gross Unrealized Losses	52,020	15,732
Available for Sale, Estimated Fair Value	3,402,879	3,445,153
Residential mortgage-backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	3,040	3,610
Held-to-maturity Securities, Gross Unrealized Gains	8	15
Held-to-maturity Securities, Gross Unrealized Losses	94	38
Held to Maturity, Estimated Fair Value	2,954	3,587
Available for Sale, Amortized Cost	756,649	648,288
Available for Sale Securities, Gross Unrealized Gains	11,254	19,048
Available for Sale Securities, Gross Unrealized Losses	11,532	2,250
Available for Sale, Estimated Fair Value	756,371	665,086
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Amortized Cost	1,107,222	1,428,488
Held-to-maturity Securities, Gross Unrealized Gains	7,232	26,462
Held-to-maturity Securities, Gross Unrealized Losses	2,057	2,746
Held to Maturity, Estimated Fair Value	1,112,397	1,452,204
Available for Sale, Amortized Cost	7,052,156	6,185,711
Available for Sale Securities, Gross Unrealized Gains	40,759	167,293
Available for Sale Securities, Gross Unrealized Losses	116,225	16,795
Available for Sale, Estimated Fair Value	6,976,690	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	6	0
Held to Maturity, Estimated Fair Value	1,494	0
Available for Sale, Amortized Cost	42,651	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	$ 42,651	$ 42,561

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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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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