Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Net income	$ 61,825	$ 43,802	$ 102,751	$ 86,239
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $36 and $53 for the three months ended and $(213) and $109 for the six months ended in 2019 and 2018, respectively	111	95	508	187
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax of $89 and $145 for the three months ended and $189 and $300 for the six months ended in 2019 and 2018, respectively	276	449	586	867
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $0 and $(13) for the three months ended and $0 and $(14) for the six months ended in 2019 and 2018, respectively	0	(59)	0	(61)
Reclassification adjustment for foreign currency translation adjustment included in net income, net of tax of $0 and $75 for the three months ended and $0 and $75 for the six months ended in 2019 and 2018, respectively	0	249	0	249
Foreign currency translation adjustment	0	190	0	188
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 $21 and $(12) for the three months ended and $31 and $(39) for the six months ended in 2019 and 2018, respectively	79	(43)	118	(148)
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax of $3 and $10 for the three months ended and $10 and $17 for the six months ended in 2019 and 2018, respectively	12	34	40	64
Net unrealized gain (loss) on available-for-sale investments	91	(9)	158	(84)
Other comprehensive income	478	725	1,252	1,158
Comprehensive income attributable to common stockholders	$ 62,303	$ 44,527	$ 104,003	$ 87,397