Income Taxes - Components of Income Tax Expense (Benefit) (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)
Total income tax expense		30,092	255,143	140,204
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Revenue Change		51,200
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, effect on income tax expense		17,900
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Effect on Income Taxes due to revaluation		80,900
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
Total income tax expense	$ 34,600	$ 50,700	211,986	$ 175,347
Income Tax Non-Reg Treatment [Member] | Subsidiaries [Member]
Income Tax Disclosures [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, unregulated portion			3,000
Income Tax Reg Treatment [Member] | Subsidiaries [Member]
Income Tax Disclosures [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, PTC revaluation			$ 33,300