Income Taxes Effective Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Effective tax rate reconciliation
Income Before Income Taxes	$ 74,379	$ 60,413
Income tax calculated at federal statutory rate	$ 15,620	$ 12,687
Income tax calculated at federal statutory rate	21.00%	21.00%
State income, net of federal provisions	$ 928	$ 732
State income, net of federal provisions	1.20%	1.20%
Flow-through repairs deductions	$ (7,935)	$ (6,586)
Flow-through repairs deductions	(10.70%)	(10.90%)
Production tax credits	$ (4,432)	$ (3,888)
Production tax credits	(6.00%)	(6.40%)
Plant and depreciation of flow-through items	$ (1,523)	$ (916)
Plant and depreciation of flow-through items	(2.00%)	(1.60%)
Amortization of excess deferred income tax	$ (1,376)	$ (384)
Amortization of excess deferred income tax	(1.80%)	(0.60%)
Share-based compensation	$ 186	$ 275
Share-based compensation	0.30%	0.50%
Other, net	$ 105	$ (6)
Other, net	0.10%	0.00%
Total Other Reconciling Items	$ (14,047)	$ (10,773)
Total Other Reconciling Items	(18.90%)	(17.80%)
Income Tax Expense	$ 1,573	$ 1,914
Income Tax Expense	2.10%	3.20%