Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Net income	$ 107,251	$ 87,351	$ 193,490	$ 165,891
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $55 and $56 for the three months ended and $164 and $168 for the nine months ended in 2018 and 2017, respectively	92	92	279	275
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit cost (credit), net of tax of $142 and $203 for the three months ended and $442 and $609 for the nine months ended in 2018 and 2017, respectively	442	333	1,309	1,002
Reclassification of postretirement liability adjustment from regulatory asset, net of tax of $0 and $0 for the three months ended and $0 and $(725) for the nine months ended in 2018 and 2017, respectively	0	0	0	(917)
Postretirement liability adjustment	442	333	1,309	85
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $0 and $9 for the three months ended and $(14) and $5 for the nine months ended in 2018 and 2017, respectively	0	15	(61)	9
Reclassification adjustment for foreign currency translation adjustment included in net income, net of tax of $0 and $0 for the three months ended and $75 and $0 for the nine months ended in 2018 and 2017, respectively	0	0	249	0
Foreign currency translation adjustment	0	15	188	9
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized loss on available-for-sale investments arising during the period, net of tax of $(13) and $(10) for the three months ended and $(52) and $(38) for the nine months ended in 2018 and 2017, respectively	(51)	(19)	(199)	(70)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $9 and $14 for the three months ended and $26 and $50 for the nine months ended in 2018 and 2017, respectively	33	27	97	93
Net unrealized gain (loss) on available-for-sale investments	(18)	8	(102)	23
Other comprehensive income	516	448	1,674	392
Comprehensive income attributable to common stockholders	$ 107,767	$ 87,799	$ 195,164	$ 166,283