CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension and retiree medical benefits:
Amortization of losses included in net periodic benefit cost, tax	$ 536	$ 559	$ 1,227	$ 1,689
Derivative instruments:
Net fair value increase (decrease), tax	(2)	(28)	3	(24)
Reclassification of losses to net income, tax	588	446	1,786	1,210
Marketable securities:
Net fair value increase (decrease), tax	$ 0	$ 0	$ 0	$ 1