Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net income	$ 40,926	$ 42,437
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $(249) and $56 for the three months ended in 2019 and 2018, respectively	397	92
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit cost (credit), net of tax of $100 and $155 for the three months ended in 2019 and 2018, respectively	310	418
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $0 and $(1) for the three months ended in 2019 and 2018, respectively	0	(2)
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 $10 and $(28) for the three months ended in 2019 and 2018, respectively	39	(105)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $7 and $7 for the three months ended in 2019 and 2018, respectively	28	30
Net unrealized gain (loss) on available-for-sale investments	67	(75)
Other comprehensive income	774	433
Comprehensive income attributable to common stockholders	$ 41,700	$ 42,870