Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net income	$ 335,453	$ 272,318	$ 281,203
Other comprehensive income (loss):
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $(140), $429 and $224 in 2019, 2018 and 2017, respectively	731	162	366
Postretirement liability adjustment:
Postretirement liability gains (losses) arising during the period, net of tax of $(2,012), $1,471 and $(1,162) in 2019, 2018 and 2017, respectively	(6,151)	4,441	(1,812)
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax of $476, $721 and $645 in 2019, 2018 and 2017, respectively	1,486	2,173	1,013
Reclassification of postretirement liability adjustment from regulatory asset, net of tax of $0, $0 and $(876) in 2019, 2018 and 2017, respectively	0	0	(1,143)
Postretirement liability adjustment	(4,665)	6,614	(1,942)
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $0, $(14) and $(3) in 2019, 2018 and 2017, respectively	0	(61)	(6)
Reclassification adjustment for foreign currency translation adjustment included in net income, net of tax of $0, $75 and $0 in 2019, 2018 and 2017, respectively	0	249	0
Foreign currency translation adjustment	0	188	(6)
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized gain (loss) on available-for-sale investments arising during the period, net of tax of $35, $(38) and $(75) in 2019, 2018 and 2017, respectively	134	(144)	(139)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $10, $35 and $65 in 2019, 2018 and 2017, respectively	40	131	120
Net unrealized gain (loss) on available-for-sale investments	174	(13)	(19)
Other comprehensive income (loss)	(3,760)	6,951	(1,601)
Comprehensive income attributable to common stockholders	$ 331,693	$ 279,269	$ 279,602