Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Net income	$ 43,802	$ 41,215	$ 86,239	$ 78,539
Other comprehensive income (loss):
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $53 and $56 for the three months ended and $109 and $112 for the six months ended in 2018 and 2017, respectively	95	92	187	183
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit cost (credit), net of tax of $145 and $190 for the three months ended and $300 and $406 for the six months ended in 2018 and 2017, respectively	449	312	867	669
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 six months ended in 2018 and 2017, respectively	0	0	0	(917)
Postretirement liability adjustment	449	312	867	(248)
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $(13) and $(9) for the three months ended and $(14) and $(3) for the six months ended in 2018 and 2017, respectively	(59)	(15)	(61)	(6)
Reclassification adjustment for foreign currency translation adjustment included in net income, net of tax of $75 and $0 for the three months ended and $75 and $0 for the six months ended in 2018 and 2017, respectively	249	0	249	0
Foreign currency translation adjustment	190	(15)	188	(6)
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 $(12) and $(13) for the three months ended and $(39) and $(28) for the six months ended in 2018 and 2017, respectively	(43)	(24)	(148)	(51)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $10 and $17 for the three months ended and $17 and $36 for the six months ended in 2018 and 2017, respectively	34	31	64	66
Net unrealized gain (loss) on available-for-sale investments	(9)	7	(84)	15
Other comprehensive income (loss)	725	396	1,158	(56)
Comprehensive income attributable to common stockholders	$ 44,527	$ 41,611	$ 87,397	$ 78,483