Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	$ 6,934,709	$ 8,869,423
Investment securities available for sale, gross unrealized gains	89,965	16,528
Investment securities available for sale, gross unrealized losses	39,257	203,442
Investment securities available for sale, estimated fair value	6,985,417	8,682,509
Amortized cost for held to maturity	3,031,299	3,316,640
Gross unrealized gains for held to maturity	65,844	16,532
Gross unrealized losses for held to maturity	21,839	77,689
Estimated fair value for held to maturity	3,075,304	3,255,483
Equity and other securities, Amortized Cost	619,894	677,187
Equity securities, Gross Unrealized Gains	41,152	17,295
Equity securities, Gross Unrealized Losses	179	818
Equity and other securities, Estimated Fair Value	660,867	693,664
Other securities, Amortized cost	510,763	599,747
Other securities, Estimated fair value	510,763	599,747
Total debt securities Amortized cost	9,966,008	12,186,063
Total debt securities Gross unrealized gains	155,809	33,060
Total debt securities Gross unrealized losses	61,096	281,131
Total debt securities Estimated fair value	10,060,721	11,937,992
Readily marketable securities Amortized cost	109,131	77,440
Readily marketable securities Gross unrealized gains	41,152	17,295
Readily marketable securities Gross unrealized losses	179	818
Readily marketable securities Estimated fair value	150,104	93,917
U.S. Treasury and Federal Agencies [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	359,723	1,346,782
Investment securities available for sale, gross unrealized gains	66
Investment securities available for sale, gross unrealized losses	395	9,851
Investment securities available for sale, estimated fair value	359,394	1,336,931
Amortized cost for held to maturity	498,776	446,542
Gross unrealized gains for held to maturity	956
Gross unrealized losses for held to maturity		239
Estimated fair value for held to maturity	499,732	446,303
Obligations of States and Political Subdivisions [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	933	1,660
Investment securities available for sale, gross unrealized gains	4	4
Investment securities available for sale, gross unrealized losses	3	5
Investment securities available for sale, estimated fair value	934	1,659
Amortized cost for held to maturity	4,775	7,494
Gross unrealized gains for held to maturity	21	22
Gross unrealized losses for held to maturity		12
Estimated fair value for held to maturity	4,796	7,504
Government Issued or Guaranteed [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	6,435,730	7,383,340
Investment securities available for sale, gross unrealized gains	88,522	15,754
Investment securities available for sale, gross unrealized losses	29,352	182,103
Investment securities available for sale, estimated fair value	6,494,900	7,216,991
Amortized cost for held to maturity	2,424,716	2,745,776
Gross unrealized gains for held to maturity	53,076	4,165
Gross unrealized losses for held to maturity	1,879	55,111
Estimated fair value for held to maturity	2,475,913	2,694,830
Privately Issued [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	16	24
Investment securities available for sale, gross unrealized losses		2
Investment securities available for sale, estimated fair value	16	22
Amortized cost for held to maturity	99,679	113,160
Gross unrealized gains for held to maturity	11,791	12,345
Gross unrealized losses for held to maturity	19,960	22,327
Estimated fair value for held to maturity	91,510	103,178
Other Debt Securities [Member]
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	138,307	137,617
Investment securities available for sale, gross unrealized gains	1,373	770
Investment securities available for sale, gross unrealized losses	9,507	11,481
Investment securities available for sale, estimated fair value	130,173	126,906
Amortized cost for held to maturity	3,353	3,668
Estimated fair value for held to maturity	$ 3,353	$ 3,668

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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