Employee Benefit Plans (Disaggregated Information Related to Corporations Defined Benefit Pension Plans) (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 197,593	$ 186,641	$ 167,107	$ 182,607
Accumulated benefit obligation	197,593	186,641	167,107
Fair value of plan assets	182,088	174,173	152,820	$ 168,450
Funded status of the plan at end of year and accrued benefit (liability) recognized	(15,505)	(12,468)	$ (14,287)
Retirement Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	180,986	170,541
Accumulated benefit obligation	180,986	170,541
Fair value of plan assets	182,088	174,173
Funded status of the plan at end of year and accrued benefit (liability) recognized	1,102	3,632
Restoration Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	16,607	16,100
Accumulated benefit obligation	16,607	16,100
Fair value of plan assets	0	0
Funded status of the plan at end of year and accrued benefit (liability) recognized	$ (16,607)	$ (16,100)