Fair Value Measurements - Estimated Fair Value of Financial Instruments (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	$ 5,877,806	$ 5,191,015
Securities available for sale	9,753,392	9,299,688
Securities held to maturity (exclusive of allowance for credit losses)	1,042,670	1,014,614
Trading securities	29,099	35,020
Other securities	298,209	296,053
Loans (exclusive of allowance for credit losses)	16,507,660	16,110,359
Derivatives	65,539	103,554
Demand and savings deposits	27,616,227	26,175,156
Time deposits	664,565	876,095
Other borrowings	2,759,818	2,315,497
Long-term debt	270,074	269,595
Derivatives	17,379	9,375
Estimate of Fair Value Measurement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	5,877,806	5,191,015
Securities available for sale	9,753,392	9,299,688
Securities held to maturity (exclusive of allowance for credit losses)	1,012,129	1,029,444
Trading securities	29,099	35,020
Other securities	298,299	296,053
Loans (exclusive of allowance for credit losses)	16,820,506	16,413,132
Derivatives	65,539	103,554
Demand and savings deposits	27,616,227	26,175,156
Time deposits	664,565	879,841
Other borrowings	2,759,818	2,315,497
Long-term debt	293,386	299,858
Derivatives	17,379	9,375
Commitments to extend credit for loans	1,583	6,405
Commercial letters of credit	76	150
Standby letters of credit	777	3,365
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	4,247,993	3,540,680
Securities available for sale	145,940	111,939
Trading securities	7,870	14,903
Other securities	133,979	134,197
Demand and savings deposits	27,616,227	26,175,156
Other borrowings	217,301	65,636
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and short-term investments	1,629,813	1,650,335
Securities available for sale	9,607,452	9,187,749
Securities held to maturity (exclusive of allowance for credit losses)	1,012,129	1,029,444
Trading securities	21,229	20,117
Other securities	164,320	161,856
Loans (exclusive of allowance for credit losses)	16,820,506	16,413,132
Derivatives	65,539	103,554
Time deposits	664,565	879,841
Other borrowings	2,542,517	2,249,861
Long-term debt	293,386	299,858
Derivatives	$ 17,379	$ 9,375