Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Net income	$ 25,130	$ 40,926
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $36 and $(249) for the three months ended in 2020 and 2019, respectively	111	397
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax of $149 and $100 for the three months ended in 2020 and 2019, respectively	462	310
Net unrealized gain on available-for-sale investments:
Net unrealized gain on available-for-sale investments arising during the period, net of tax of $36 and $10 for the three months ended in 2020 and 2019, respectively	135	39
Reclassification adjustment for (gain) loss on available-for-sale investments included in net income, net of tax of $0 and $7 for the three months ended in 2020 and 2019, respectively	(1)	28
Net unrealized gain on available-for-sale investments	134	67
Other comprehensive income	707	774
Comprehensive income attributable to common stockholders	$ 25,837	$ 41,700