Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 3,295	$ 3,402	$ 3,689
Interest Cost	6,057	5,945	5,392
Expected Return on Plan Assets	(7,306)	(7,257)	(6,779)
Prior Service Cost Amortization	263	263	265
Actuarial Loss Amortization	4,662	4,398	4,714
Sub-total	6,971	6,751	7,281
Amounts Capitalized or Deferred	(3,122)	(3,008)	(3,397)
NPBC Recognized	3,849	3,743	3,884
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service Cost	2,974	2,610	2,622
Interest Cost	3,913	3,232	2,918
Expected Return on Plan Assets	(1,347)	(1,205)	(1,093)
Prior Service Cost Amortization	1,399	1,486	1,682
Actuarial Loss Amortization	2,098	1,049	1,150
Sub-total	9,037	7,172	7,279
Amounts Capitalized or Deferred	(4,515)	(3,351)	(3,423)
NPBC Recognized	4,522	3,821	3,856
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service Cost	460	162	120
Interest Cost	392	386	330
Prior Service Cost Amortization	189	189	85
Actuarial Loss Amortization	295	375	327
Sub-total	1,336	1,112	862
NPBC Recognized	$ 1,336	$ 1,112	$ 862