STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	$ 100	$ 0	$ 100
Other comprehensive income	100	100	0
Comprehensive income:
Net income	213,300	159,200	152,200
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, after Tax	0	100	(100)
Derivative instruments:
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	$ 213,400	$ 159,300	$ 152,200