Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Net income	$ 137,637	$ 107,251	$ 240,388	$ 193,490
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, net of tax of $36 and $55 for the three months ended and $(177) and $164 for the nine months ended in 2019 and 2018, respectively	112	92	620	279
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax of $95 and $142 for the three months ended and $284 and $442 for the nine months ended in 2019 and 2018, respectively	292	442	878	1,309
Foreign currency translation adjustment:
Foreign currency translation adjustment recognized during the period, net of tax of $0 and $0 for the three months ended and $0 and $(14) for the nine months ended in 2019 and 2018, respectively	0	0	0	(61)
Reclassification adjustment for foreign currency translation adjustment included in net income, net of tax of $0 and $0 for the three months ended and $0 and $75 for the nine months ended in 2019 and 2018, respectively	0	0	0	249
Foreign currency translation adjustment	0	0	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 $3 and $(13) for the three months ended and $35 and $(52) for the nine months ended in 2019 and 2018, respectively	12	(51)	130	(199)
Reclassification adjustment for (gain) loss on available-for-sale investments included in net income, net of tax of $(1) and $9 for the three months ended and $10 and $26 for the nine months ended in 2019 and 2018, respectively	(4)	33	36	97
Net unrealized gain (loss) on available-for-sale investments	8	(18)	166	(102)
Other comprehensive income	412	516	1,664	1,674
Comprehensive income attributable to common stockholders	$ 138,049	$ 107,767	$ 242,052	$ 195,164