STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Comprehensive income:
Net income	$ 58,451	$ 35,362	$ 91,550	$ 60,417
Pension and retiree medical benefits:
Amortization of losses included in net periodic benefit cost, net of tax of $5, $9, $10 and $18, respectively	18	15	37	30
Derivative instruments:
Reclassification of losses to net income, net of tax of $4, $6, $7 and $12, respectively	12	10	24	19
Other comprehensive income	30	25	61	49
Comprehensive income	$ 58,481	$ 35,387	$ 91,611	$ 60,466