Income Taxes Effective Tax Rate Reconciliation (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Effective tax rate reconciliation
Income Before Income Taxes	$ 25,436,000	$ 46,889,000	$ 99,815,000	$ 107,302,000
Income tax calculated at federal statutory rate	$ 5,341,000	$ 9,846,000	$ 20,961,000	$ 22,533,000
Income tax calculated at federal statutory rate	21.00%	21.00%	21.00%	21.00%
State income, net of federal provisions	$ 237,000	$ 801,000	$ 1,165,000	$ 1,533,000
State income, net of federal provisions	0.90%	1.70%	1.20%	1.50%
Effective Income Tax Rate Reconciliation, Reduction Resulting from Lapse of Applicable Statute of Limitations	$ (23,201,000)	$ 0	$ (22,825,000)	$ 0
Effective Income Tax Rate Reconciliation, Reduction Resulting from Lapse of Applicable Statute of Limitations, Percentage	(91.20%)	0.00%	(22.90%)	0.00%
Flow-through repairs deductions	$ (2,153,000)	$ (4,095,000)	$ (10,088,000)	$ (10,681,000)
Flow-through repairs deductions	(8.50%)	(8.70%)	(10.10%)	(10.00%)
Production tax credits	$ (1,406,000)	$ (2,559,000)	$ (5,838,000)	$ (6,447,000)
Production tax credits	(5.50%)	(5.50%)	(5.80%)	(6.00%)
Plant and depreciation of flow-through items	$ (663,000)	$ (571,000)	$ (2,186,000)	$ (1,487,000)
Plant and depreciation of flow-through items	(2.60%)	(1.20%)	(2.20%)	(1.40%)
Amortization of excess deferred income tax	$ (189,000)	$ 0	$ (1,565,000)	$ (384,000)
Amortization of excess deferred income tax	(0.70%)	0.00%	(1.60%)	(0.40%)
Share-based compensation			$ 186,000	$ 275,000
Share-based compensation			0.20%	0.30%
Other, net	$ (192,000)	$ (320,000)	$ (463,000)	$ (326,000)
Other, net	(0.80%)	(0.70%)	(0.50%)	(0.30%)
Total Other Reconciling Items	$ (27,567,000)	$ (6,744,000)	$ (41,614,000)	$ (17,517,000)
Total Other Reconciling Items	(108.40%)	(14.40%)	(41.70%)	(16.30%)
Income tax (benefit) expense	$ (22,226,000)	$ 3,102,000	$ (20,653,000)	$ 5,016,000
Income tax (benefit) expense	(87.40%)	6.60%	(20.70%)	4.70%