Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Net income	$ 42,437	$ 37,325
Derivative instruments:
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $56 and $56 for the three months ended in 2018 and 2017, respectively	92	91
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit cost (credit), net of tax of $155 and $217 for the three months ended in 2018 and 2017, respectively	418	357
Reclassification of postretirement liability adjustment from regulatory asset, net of tax of $0 and $(725) for the three months ended in 2018 and 2017, respectively	0	(917)
Postretirement liability adjustment	418	(560)
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $(1) and $5 for the three months ended in 2018 and 2017, respectively	(2)	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 $(28) and $(15) for the three months ended in 2018 and 2017, respectively	(105)	(27)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $7 and $19 for the three months ended in 2018 and 2017, respectively	30	35
Net unrealized gain (loss) on available-for-sale investments	(75)	8
Other comprehensive income (loss)	433	(452)
Comprehensive income attributable to common stockholders	$ 42,870	$ 36,873