Income Taxes Effective Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal statutory rate	35.00%	35.00%	35.00%
State income tax, net of federal provisions	(2.40%)	0.10%	(1.80%)
Flow-through repairs deductions	(26.30%)	(13.30%)	(22.90%)
Production tax credits	(7.00%)	(3.20%)	(2.80%)
Plant and depreciation of flow through items	(2.90%)	(1.60%)	0.10%
Share-based compensation	(1.10%)	(0.00%)	(0.00%)
Prior year permanent return to accrual adjustments	(0.10%)	0.10%	(4.70%)
Recognition of unrecognized tax benefit	(0.00%)	(0.00%)	(11.40%)
Other, net	(0.10%)	(0.50%)	(0.80%)
Effective income tax rate	(4.90%)	16.60%	(9.30%)
Income Before Income Taxes	$ 156,525	$ 181,246	$ 110,414
Income tax calculated at 35% federal statutory rate	54,784	63,436	38,645
State tax income, net of federal provisions	(3,714)	301	(1,969)
Flow-through repairs deductions	(41,111)	(24,079)	(25,268)
Production tax credits	(10,941)	(5,721)	(3,136)
Plant and depreciation of flow through items	(4,604)	(2,893)	74
Share-based compensation	(1,646)	0	0
Prior year permanent return to accrual adjustments	(128)	207	(5,172)
Recognition of unrecognized tax benefit	0	0	(12,607)
Other, net	(287)	(1,214)	(839)
Total reconciling items	(62,431)	(33,399)	(48,917)
Income Tax (Benefit) Expense	$ (7,647)	$ 30,037	$ (10,272)