Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Net income	$ 99,703	$ 61,825	$ 124,833	$ 102,751
Other comprehensive income:
Reclassification adjustment for loss on derivative instruments included in net income, tax	36	36	72	(213)
Reclassification adjustment for loss on derivative instruments included in net income, net of tax	112	111	223	508
Postretirement liability adjustment:
Amortization of postretirement liability losses included in net periodic benefit cost, tax	155	89	304	189
Amortization of postretirement liability losses included in net periodic benefit cost, net of tax	480	276	942	586
Net unrealized gain (loss) on available-for-sale investments:
Net unrealized gain (loss) on available-for-sale investments arising during the period, tax	(4)	21	32	31
Net unrealized gain (loss) on available-for-sale investments arising during the period, net of tax	(13)	79	122	118
Reclassification adjustment for loss on available-for-sale investments included in net income, tax	2	3	2	10
Reclassification adjustment for loss on available-for-sale investments included in net income, net of tax	7	12	6	40
Net unrealized gain (loss) on available-for-sale investments	(6)	91	128	158
Other comprehensive income	586	478	1,293	1,252
Comprehensive income attributable to common stockholders	$ 100,289	$ 62,303	$ 126,126	$ 104,003