Income Taxes - Income Tax Reconciliation (Details) - USD ($) $ in Thousands		12 Months Ended
	May 01, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosures [Line Items]
Current Federal Tax Expense (Benefit)		$ 10,382	$ 1,127	$ 0
Current State and Local Tax Expense (Benefit)		263	17	20
Federal		19,451	254,420	140,315
State		(4)	(421)	(131)
Income Tax Reconciliation [Abstract]
Income taxes at the statutory rate		55,800	148,847	158,586
Production tax credit1		0	0	(12,925)
Utility plant differences2		(25,871)	0	3,966
Income Tax Reconciliation, Executive Compensation		4,439	0	0
Income Tax Reconciliation, Treasury Grant		(4,861)	(9,537)	(9,788)
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount		0	117,185	0
Other–net		585	(1,352)	365
Total income tax expense		$ 30,092	$ 255,143	$ 140,204
Effective tax rate		11.30%	60.00%	30.90%
Deferred Tax Assets, Regulatory Assets and Liabilities, Accelerated Tax Depreciation		$ 919,800
Deferred Tax Assets, Regulatory Assets and Liabilities, Undetermined refund		92,500
Subsidiaries [Member]
Income Tax Disclosures [Line Items]
Current Federal Tax Expense (Benefit)		19,283	$ 1,127	$ 0
Current State and Local Tax Expense (Benefit)		438	17	20
Federal		30,979	210,842	175,327
State		0	0	0
Income Tax Reconciliation [Abstract]
Income taxes at the statutory rate		77,251	185,430	194,572
Production tax credit1		0	0	(12,925)
Utility plant differences2		(25,871)	0	3,966
Income Tax Reconciliation, Executive Compensation		4,439	0	0
Income Tax Reconciliation, Treasury Grant		(4,861)	(9,537)	(9,788)
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount		0	36,328	0
Other–net		(258)	(235)	(478)
Total income tax expense	$ 34,600	$ 50,700	$ 211,986	$ 175,347
Effective tax rate		13.80%	40.00%	31.50%
Deferred Tax Assets, Regulatory Assets and Liabilities		$ (1,012,300)		$ (71,500)