Components of Retirement Plan Costs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Service Cost	$ 3,402	$ 3,689	$ 3,006
Interest Cost	5,945	5,392	5,092
Expected Return on Plan Assets	(7,257)	(6,779)	(6,245)
Prior Service Cost Amortization	263	265	211
Actuarial Loss Amortization	4,398	4,714	2,847
Sub-total	6,751	7,281	4,911
Amounts Capitalized or Deferred	(3,008)	(3,397)	(1,881)
NPBC Recognized	3,743	3,884	3,030
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service Cost	2,610	2,622	1,988
Interest Cost	3,232	2,918	2,686
Expected Return on Plan Assets	(1,205)	(1,093)	(920)
Prior Service Cost Amortization	1,486	1,682	1,682
Actuarial Loss Amortization	1,049	1,150	56
Sub-total	7,172	7,279	5,492
Amounts Capitalized or Deferred	(3,351)	(3,423)	(2,270)
NPBC Recognized	3,821	3,856	3,222
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Service Cost	162	120	57
Interest Cost	386	330	272
Prior Service Cost Amortization	189	85	11
Actuarial Loss Amortization	375	327	100
Sub-total	1,112	862	440
NPBC Recognized	$ 1,112	$ 862	$ 440