Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Amortized Cost And Estimated Fair Value Of Investment Securities [Line Items]
Investment securities available for sale, amortized cost	$ 8,398,686	$ 8,869,423
Investment securities available for sale, gross unrealized gains	31,216	16,528
Investment securities available for sale, gross unrealized losses	104,318	203,442
Investment securities available for sale, estimated fair value	8,325,584	8,682,509
Amortized cost for held to maturity	3,714,699	3,316,640
Gross unrealized gains for held to maturity	25,444	16,532
Gross unrealized losses for held to maturity	40,547	77,689
Estimated fair value for held to maturity	3,699,596	3,255,483
Equity and other securities, Amortized Cost	468,232	677,187
Equity securities, Gross Unrealized Gains	28,913	17,295
Equity securities, Gross Unrealized Losses	588	818
Equity and other securities, Estimated Fair Value	496,557	693,664
Other securities, Amortized cost	419,971	599,747
Other securities, Estimated fair value	419,971	599,747
Total debt securities Amortized cost	12,113,385	12,186,063
Total debt securities Gross unrealized gains	56,660	33,060
Total debt securities Gross unrealized losses	144,865	281,131
Total debt securities Estimated fair value	12,025,180	11,937,992
Readily marketable securities Amortized cost	48,261	77,440
Readily marketable securities Gross unrealized gains	28,913	17,295
Readily marketable securities Gross unrealized losses	588	818
Readily marketable securities Estimated fair value	76,586	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	1,144,288	1,346,782
Investment securities available for sale, gross unrealized gains	4
Investment securities available for sale, gross unrealized losses	4,864	9,851
Investment securities available for sale, estimated fair value	1,139,428	1,336,931
Amortized cost for held to maturity	944,230	446,542
Gross unrealized gains for held to maturity	735
Gross unrealized losses for held to maturity	15	239
Estimated fair value for held to maturity	944,950	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	1,181	1,660
Investment securities available for sale, gross unrealized gains	4	4
Investment securities available for sale, gross unrealized losses	4	5
Investment securities available for sale, estimated fair value	1,181	1,659
Amortized cost for held to maturity	6,810	7,494
Gross unrealized gains for held to maturity	27	22
Gross unrealized losses for held to maturity		12
Estimated fair value for held to maturity	6,837	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	7,115,459	7,383,340
Investment securities available for sale, gross unrealized gains	29,959	15,754
Investment securities available for sale, gross unrealized losses	91,511	182,103
Investment securities available for sale, estimated fair value	7,053,907	7,216,991
Amortized cost for held to maturity	2,652,219	2,745,776
Gross unrealized gains for held to maturity	11,876	4,165
Gross unrealized losses for held to maturity	19,472	55,111
Estimated fair value for held to maturity	2,644,623	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	107,858	113,160
Gross unrealized gains for held to maturity	12,806	12,345
Gross unrealized losses for held to maturity	21,060	22,327
Estimated fair value for held to maturity	99,604	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	137,742	137,617
Investment securities available for sale, gross unrealized gains	1,249	770
Investment securities available for sale, gross unrealized losses	7,939	11,481
Investment securities available for sale, estimated fair value	131,052	126,906
Amortized cost for held to maturity	3,582	3,668
Estimated fair value for held to maturity	$ 3,582	$ 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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