Summary of Information on Plans' Assets, Projected Benefit Obligations (PBO), and Funded Status (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Beginning of Year	$ 91,058	$ 87,194
Actual Return on Plan Assets	12,731	3,618
Employer Contributions	4,100	5,146	$ 4,215
Benefits Paid	(5,574)	(4,900)
Plan Assets at End of Year	102,315	91,058	87,194
PBO at Beginning of Year	150,439	140,816
Service Cost	3,295	3,402	3,689
Interest Cost	6,057	5,945	5,392
Plan Amendments	608
Benefits Paid	(5,574)	(4,900)	(4,410)
Actuarial (Gain) or Loss	12,096	5,176
PBO at End of Year	166,921	150,439	140,816
Funded Status: Assets vs PBO	(64,606)	(59,381)
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Beginning of Year	16,606	14,174
Actual Return on Plan Assets	1,907	792
Employer Contributions	4,000	4,000	4,000
Participant Contributions	126	61
Benefits Paid	(2,405)	(2,421)
Plan Assets at End of Year	20,234	16,606	14,174
PBO at Beginning of Year	96,659	76,249
Service Cost	2,974	2,610	2,622
Interest Cost	3,913	3,232	2,918
Participant Contributions	126	61	63
Benefits Paid	(2,405)	(2,421)	(2,515)
Actuarial (Gain) or Loss	(7,145)	16,928
PBO at End of Year	94,122	96,659	76,249
Funded Status: Assets vs PBO	(73,888)	(80,053)
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions	34	34	40
Benefits Paid	(34)	(34)
PBO at Beginning of Year	9,566	9,177
Service Cost	460	162	120
Interest Cost	392	386	330
Benefits Paid	(34)	(34)	(40)
Actuarial (Gain) or Loss	1,339	(125)
PBO at End of Year	11,723	9,566	$ 9,177
Funded Status: Assets vs PBO	$ (11,723)	$ (9,566)