Condensed Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Net income	$ 40,349	$ 15,395	$ 162,267	$ 140,450
Other comprehensive income (loss), net of tax:
Benefit plan liability adjustments - net gain (net of tax of $0,$0, $(17) and $0, respectively)	0	0	55	0
Reclassification adjustments of benefit plan liability - prior service cost (net of tax of $6, $3, $19 and $13, respectively)	(18)	(16)	(60)	(45)
Reclassification adjustments of benefit plan liability - net gain (net of tax of $(149), $(92), $(426) and $(197), respectively)	448	(9)	1,365	327
Other comprehensive income, net of tax	1,512	532	3,733	1,221
Comprehensive income	41,861	15,927	166,000	141,671
Less: comprehensive income attributable to noncontrolling interest	4,066	3,655	11,844	10,319
Comprehensive income available for common stock	37,795	12,272	154,156	131,352
Other Comprehensive Income (Loss), Defined Benefit Plan, Gain (Loss) Arising During Period, Tax	0	0	(17)	0
Other Comprehensive (Income) Loss, Defined Benefit Plan, Prior Service Cost (Credit), Reclassification Adjustment from AOCI, Tax	6	3	19	13
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax	(149)	(92)	(426)	(197)
Interest rate swaps
Other comprehensive income (loss), net of tax:
Reclassification of net realized (gains) losses	544	548	1,630	1,639
Reclassification of net realized gains/losses, (tax) benefit	(168)	(165)	(508)	(500)
Commodity Contracts
Other comprehensive income (loss), net of tax:
Reclassification of net realized (gains) losses	137	124	562	(366)
Net unrealized gains (losses) on commodity derivatives (net of tax of $(112), $35, $(44) and $100, respectively)	401	(115)	181	(334)
Reclassification of net realized gains/losses, (tax) benefit	(41)	(5)	(172)	142
Net unrealized gains/losses on commodity derivatives, (tax) benefit	$ (112)	$ 35	$ (44)	$ 100