CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension and retiree medical benefits:
Net pension and retiree medical benefit (losses) gains arising during the period, tax	$ (4,944)	$ (5,026)	$ (4,687)
Amortization of losses included in net periodic benefit cost, tax	2,185	2,249	2,159
Derivative instruments:
Net fair value (decrease) increase, tax	2	(46)	(103)
Reclassification of losses to net income, tax	2,342	1,810	1,493
Marketable securities:
Net fair value increase, tax	$ 0	$ 0	$ 21