Income Taxes - Deferred Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred Tax Liabilities, Gross
Deferred Tax Liabilities, Property, Plant and Equipment	$ 1,943,730	$ 1,998,721
Deferred Tax Liabilities, Other	133,440	113,051
Deferred Tax Liabilities, Gross, Total	2,077,170	2,111,772
Deferred Tax Assets, Gross
Deferred Tax Assets, Operating Loss Carryforwards	(238,869)	(224,885)
Deferred Tax Assets, Net Regulatory Liability for Income Taxes	(946,179)	(975,974)
Deferred Tax Assets, Tax Credit Carryforwards, Production	(67,402)	(121,616)
Deferred Tax Assets, Gross	(1,252,450)	(1,322,475)
Deferred Tax Liabilities, Net, Total	824,720	789,297
Subsidiaries [Member]
Deferred Tax Liabilities, Gross
Deferred Tax Liabilities, Property, Plant and Equipment	1,943,730	1,998,721
Deferred Tax Liabilities, Other	47,774	25,880
Deferred Tax Liabilities, Gross, Total	1,991,504	2,024,601
Deferred Tax Assets, Gross
Deferred Tax Assets, Net Regulatory Liability for Income Taxes	946,936	976,582
Deferred Tax Assets, Tax Credit Carryforwards, Production	67,405	121,616
Deferred Tax Assets, Gross	1,014,341	1,098,198
Deferred Tax Liabilities, Net, Total	$ 977,163	$ 926,403