Securities - Amortized Cost and Estimated Fair Value of Securities, Excluding Trading Securities, Presented by Contractual Maturity (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	$ 131,389
Held to Maturity, Due after one year through five years, Amortized Cost	381,014
Held to Maturity, Due after five years through ten years, Amortized Cost	712,265
Held to Maturity, Due after ten years, Amortized Cost	596,057
Held to Maturity, Amortized Cost	1,820,725	$ 1,945,833
Held to Maturity, Due in one year or less, Estimated Fair Value	133,608
Held to Maturity, Due after one year through five years, Estimated Fair Value	394,107
Held to Maturity, Due after five years through ten years, Estimated Fair Value	730,223
Held to Maturity, Due after ten years, Estimated Fair Value	626,364
Held to Maturity, Estimated Fair Value	1,884,302	2,052,896
Available-for-sale Securities, Due in one year or less, Amortized Cost	930,146
Available for Sale, Due after one year through five years, Amortized Cost	1,663,943
Available for Sale, Due after five years through ten years, Amortized Cost	1,011,837
Available for Sale, Due after ten years, Amortized Cost	6,178,779
Available for Sale, Amortized Cost	9,827,059	9,727,401
Available for Sale, Due in one year or less, Estimated Fair Value	941,444
Available for Sale, Due after one year through five years, Estimated Fair Value	1,795,824
Available for Sale, Due after five years through ten years, Estimated Fair Value	1,059,105
Available for Sale, Due after ten years, Estimated Fair Value	6,538,717
Securities available for sale, estimated fair value	10,377,444	10,437,565
US Treasury Securities [Member]
Schedule of Investments [Line Items]
Available-for-sale Securities, Due in one year or less, Amortized Cost	799,130
Available for Sale, Due after one year through five years, Amortized Cost	285,739
Available for Sale, Due after five years through ten years, Amortized Cost	197,014
Available for Sale, Due after ten years, Amortized Cost	0
Available for Sale, Amortized Cost	1,281,883	1,084,542
Available for Sale, Due in one year or less, Estimated Fair Value	808,469
Available for Sale, Due after one year through five years, Estimated Fair Value	304,750
Available for Sale, Due after five years through ten years, Estimated Fair Value	188,812
Available for Sale, Due after ten years, Estimated Fair Value	0
Securities available for sale, estimated fair value	1,302,031	1,119,633
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	16
Held to Maturity, Due after one year through five years, Amortized Cost	129
Held to Maturity, Due after five years through ten years, Amortized Cost	515,865
Held to Maturity, Due after ten years, Amortized Cost	12,462
Held to Maturity, Amortized Cost	528,472	528,784
Held to Maturity, Due in one year or less, Estimated Fair Value	16
Held to Maturity, Due after one year through five years, Estimated Fair Value	130
Held to Maturity, Due after five years through ten years, Estimated Fair Value	529,086
Held to Maturity, Due after ten years, Estimated Fair Value	12,698
Held to Maturity, Estimated Fair Value	541,930	570,526
Available-for-sale Securities, Due in one year or less, Amortized Cost	7,147
Available for Sale, Due after one year through five years, Amortized Cost	22,151
Available for Sale, Due after five years through ten years, Amortized Cost	22,162
Available for Sale, Due after ten years, Amortized Cost	1,808,967
Available for Sale, Amortized Cost	1,860,427	1,916,581
Available for Sale, Due in one year or less, Estimated Fair Value	7,468
Available for Sale, Due after one year through five years, Estimated Fair Value	23,421
Available for Sale, Due after five years through ten years, Estimated Fair Value	22,634
Available for Sale, Due after ten years, Estimated Fair Value	1,841,282
Securities available for sale, estimated fair value	1,894,805	1,987,679
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	129,873
Held to Maturity, Due after one year through five years, Amortized Cost	380,885
Held to Maturity, Due after five years through ten years, Amortized Cost	196,400
Held to Maturity, Due after ten years, Amortized Cost	583,595
Held to Maturity, Amortized Cost	1,290,753	1,415,549
Held to Maturity, Due in one year or less, Estimated Fair Value	132,092
Held to Maturity, Due after one year through five years, Estimated Fair Value	393,977
Held to Maturity, Due after five years through ten years, Estimated Fair Value	201,137
Held to Maturity, Due after ten years, Estimated Fair Value	613,666
Held to Maturity, Estimated Fair Value	1,340,872	1,480,870
Available-for-sale Securities, Due in one year or less, Amortized Cost	123,869
Available for Sale, Due after one year through five years, Amortized Cost	1,356,053
Available for Sale, Due after five years through ten years, Amortized Cost	792,661
Available for Sale, Due after ten years, Amortized Cost	4,369,812
Available for Sale, Amortized Cost	6,642,395	6,683,927
Available for Sale, Due in one year or less, Estimated Fair Value	125,507
Available for Sale, Due after one year through five years, Estimated Fair Value	1,467,653
Available for Sale, Due after five years through ten years, Estimated Fair Value	847,659
Available for Sale, Due after ten years, Estimated Fair Value	4,697,435
Securities available for sale, estimated fair value	7,138,254	7,287,902
Other Investments [Member]
Schedule of Investments [Line Items]
Held to Maturity, Due in one year or less, Amortized Cost	1,500
Held to Maturity, Due after one year through five years, Amortized Cost	0
Held to Maturity, Due after five years through ten years, Amortized Cost	0
Held to Maturity, Due after ten years, Amortized Cost	0
Held to Maturity, Amortized Cost	1,500	1,500
Held to Maturity, Due in one year or less, Estimated Fair Value	1,500
Held to Maturity, Due after one year through five years, Estimated Fair Value	0
Held to Maturity, Due after five years through ten years, Estimated Fair Value	0
Held to Maturity, Due after ten years, Estimated Fair Value	0
Held to Maturity, Estimated Fair Value	1,500	1,500
Available-for-sale Securities, Due in one year or less, Amortized Cost	0
Available for Sale, Due after one year through five years, Amortized Cost	0
Available for Sale, Due after five years through ten years, Amortized Cost	0
Available for Sale, Due after ten years, Amortized Cost	0
Available for Sale, Amortized Cost	42,354	42,351
Available for Sale, Due in one year or less, Estimated Fair Value	0
Available for Sale, Due after one year through five years, Estimated Fair Value	0
Available for Sale, Due after five years through ten years, Estimated Fair Value	0
Available for Sale, Due after ten years, Estimated Fair Value	0
Securities available for sale, estimated fair value	$ 42,354	$ 42,351

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

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