STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Comprehensive income:
Net income	$ 35,362	$ 32,211	$ 60,417	$ 54,734
Pension and retiree medical benefits:
Amortization of losses included in net periodic benefit cost, net of tax of $9, $6, $18 and $13, respectively	15	11	30	23
Derivative instruments:
Reclassification of losses to net income, net of tax of $6, $24, $12, and $49, respectively	10	43	19	85
Other comprehensive income	25	54	49	108
Comprehensive income	$ 35,387	$ 32,265	$ 60,466	$ 54,842