Securities (Year End Securities Held to Maturity) (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	$ 1,945,833	$ 2,030,005
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	107,063	28,931
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	10,261
Debt Securities, Held to Maturity, Estimated Fair Value	2,052,896	2,048,675
Debt Securities, Held-to-maturity, Allowance for Credit Loss	(160)	0
Debt Securities, Held-to-Maturity Net Carrying Amount	1,945,673	2,030,005
Residential Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	528,784	530,861
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	41,742	22
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	9,365
Debt Securities, Held to Maturity, Estimated Fair Value	570,526	521,518
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Debt Securities, Held-to-Maturity Net Carrying Amount	528,784	530,861
States and Political Subdivisions [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,415,549	1,497,644
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	65,321	28,909
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	896
Debt Securities, Held to Maturity, Estimated Fair Value	1,480,870	1,525,657
Debt Securities, Held-to-maturity, Allowance for Credit Loss	(160)	0
Debt Securities, Held-to-Maturity Net Carrying Amount	1,415,389	1,497,644
Other [Member]
Schedule of Investments [Line Items]
Held to Maturity, Amortized Cost	1,500	1,500
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0	0
Debt Securities, Held to Maturity, Estimated Fair Value	1,500	1,500
Debt Securities, Held-to-maturity, Allowance for Credit Loss	0	0
Debt Securities, Held-to-Maturity Net Carrying Amount	$ 1,500	$ 1,500