Summary of Information on Plans' Assets, Projected Benefit Obligations (PBO), and Funded Status (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Beginning of Year	$ 125,755	$ 107,808
Actual Return on Plan Assets	13,024	17,908
Employer Contributions	4,665	6,916	$ 16,628
Benefits Paid	(6,038)	(6,877)
Plan Assets at End of Year	137,406	125,755	107,808
PBO at Beginning of Year	182,135	156,197
Service Cost	3,322	3,104	3,393
Interest Cost	5,776	6,484	5,878
Plan Amendments	732
Benefits Paid	(6,038)	(6,877)	(4,986)
Actuarial (Gain) or Loss	20,165	23,227
PBO at End of Year	206,092	182,135	156,197
Funded Status: Assets vs PBO	(68,686)	(56,380)
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Beginning of Year	27,280	21,109
Actual Return on Plan Assets	3,739	3,808
Employer Contributions	4,156	4,000	4,000
Participant Contributions	240	121
Benefits Paid	(2,568)	(1,758)
Plan Assets at End of Year	32,847	27,280	21,109
PBO at Beginning of Year	95,657	81,005
Service Cost	2,698	2,304	2,933
Interest Cost	3,121	3,426	3,404
Benefits Paid	(2,568)	(1,758)	(2,193)
Actuarial (Gain) or Loss	7,683	10,559
PBO at End of Year	106,831	95,657	81,005
Funded Status: Assets vs PBO	(73,984)	(68,377)
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions	654	610	401
Benefits Paid	(654)	(610)
PBO at Beginning of Year	17,759	13,754
Service Cost	283	247	487
Interest Cost	549	567	404
Plan Amendments		225
Benefits Paid	(654)	(610)	(401)
Actuarial (Gain) or Loss	2,288	3,576
PBO at End of Year	20,225	17,759	$ 13,754
Funded Status: Assets vs PBO	$ (20,225)	$ (17,759)