Summary of Information on Plans' Assets, Projected Benefit Obligations (PBO), and Funded Status (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Beginning of Year	$ 87,194	$ 86,744
Actual Return on Plan Assets	3,618	645
Employer Contributions	5,146	4,215	$ 4,191
Benefits Paid	(4,900)	(4,410)	(4,246)
Plan Assets at End of Year	91,058	87,194	86,744
PBO at Beginning of Year	140,816	136,662
Service Cost	3,402	3,689	3,006
Interest Cost	5,945	5,392	5,092
Plan Amendments		474
Benefits Paid	(4,900)	(4,410)	(4,246)
Actuarial (Gain) or Loss	5,176	(991)
PBO at End of Year	150,439	140,816	136,662
Funded Status: Assets vs PBO	(59,381)	(53,622)
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Plan Assets at Beginning of Year	14,174	12,840
Actual Return on Plan Assets	792	(214)
Employer Contributions	4,000	4,000	3,650
Participant Contributions	61	63	59
Benefits Paid	(2,421)	(2,515)	(2,184)
Plan Assets at End of Year	16,606	14,174	12,840
PBO at Beginning of Year	76,249	73,923
Service Cost	2,610	2,622	1,988
Interest Cost	3,232	2,918	2,686
Participant Contributions	61	63	59
Benefits Paid	(2,421)	(2,515)	(2,184)
Actuarial (Gain) or Loss	16,928	(762)
PBO at End of Year	96,659	76,249	73,923
Funded Status: Assets vs PBO	(80,053)	(62,075)
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Employer Contributions	34	40	53
Benefits Paid	(34)	(40)	(53)
PBO at Beginning of Year	9,177	7,965
Service Cost	162	120	57
Interest Cost	386	330	272
Plan Amendments		608
Benefits Paid	(34)	(40)	(53)
Actuarial (Gain) or Loss	(125)	194
PBO at End of Year	9,566	9,177	$ 7,965
Funded Status: Assets vs PBO	$ (9,566)	$ (9,177)