STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Comprehensive income:
Net income	$ 81,541	$ 67,781	$ 173,091	$ 128,198
Pension and retiree medical benefits:
Amortization of losses included in net periodic benefit cost, net of tax of $5, $9, $15 and $27, respectively	18	16	55	46
Derivative instruments:
Reclassification of losses to net income, net of tax of $3, $6, $10 and $18, respectively	13	10	37	29
Other comprehensive income	31	26	92	75
Comprehensive income	$ 81,572	$ 67,807	$ 173,183	$ 128,273