Accounting For Income Taxes - Schedule of Deferred Income Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred income tax assets:
Regulatory liabilities	$ 179,871	$ 181,455
Tax credits and NOL carryforwards	57,516	22,331
Provisions for pensions	37,501	41,648
Other	42,641	36,542
Total gross deferred income tax assets	317,529	281,976
Valuation allowances for deferred tax assets	(10,021)	(16,551)
Total deferred income tax assets after valuation allowances	307,508	265,425
Deferred income tax liabilities:
Utility property, plant, and equipment	666,639	581,387
Regulatory assets	232,697	210,231
Other	2,884	2,320
Total deferred income tax liabilities	902,220	793,938
Net long-term deferred income tax liability	$ 594,712	$ 528,513