STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Comprehensive income:
Net income	$ 67,781	$ 68,346	$ 128,198	$ 123,080
Pension and retiree medical benefits:
Amortization of losses included in net periodic benefit cost, net of tax of $9, $6, $27 and $19, respectively	16	12	46	35
Derivative instruments:
Reclassification of losses to net income, net of tax of $6, $25, $18 and $74, respectively	10	44	29	129
Other comprehensive income	26	56	75	164
Comprehensive income	$ 67,807	$ 68,402	$ 128,273	$ 123,244