CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Cash Flows from Operating Activities
Net Income	$ 186,515	$ 188,323	$ 176,668
Adjustments to Reconcile Net Income to Net Cash Flows from Operating Activities:
Depreciation Expense	169,042	158,310	152,874
Amortization Expense	27,706	26,052	22,255
Amortization of Debt Issuance Costs	2,326	2,339	2,349
Use of Renewable Energy Credits for Compliance	37,141	32,350	25,453
Deferred Income Taxes	41,614	56,066	100,762
Pension and Other Postretirement Benefits Expense	17,762	15,303	16,039
Pension and Other Postretirement Benefits Funding	(16,749)	(26,673)	(14,430)
Allowance for Equity Funds Used During Construction	(15,222)	(8,117)	(5,322)
FERC Transmission Refund Payable	0	0	(4,878)
Regulatory Deferral, ACC Refund Order	7,705	(1,562)	0
Changes in Current Assets and Current Liabilities:
Accounts Receivable	9,238	(26,729)	(13,219)
Materials, Supplies, and Fuel Inventory	(16,236)	(2,357)	175
Regulatory Assets	(20,934)	(4,080)	(3,942)
Other Current Assets	(475)	(1,746)	(751)
Accounts Payable and Accrued Charges	(27,776)	33,536	9,790
Income Taxes Receivable	6,072	(13,004)	0
Regulatory Liabilities	(1,626)	14,028	(20,227)
Other, Net	8,140	15,187	4,728
Net Cash Flows—Operating Activities	414,243	457,226	448,324
Cash Flows from Investing Activities
Capital Expenditures	(607,593)	(392,522)	(345,617)
Purchase Intangibles, Renewable Energy Credits	(51,699)	(51,327)	(51,179)
Contributions in Aid of Construction	6,607	10,817	4,983
Note Receivable	(1,000)	0	0
Net Cash Flows—Investing Activities	(653,685)	(433,032)	(391,813)
Cash Flows from Financing Activities
Proceeds from Borrowings, Revolving Credit Facility	0	171,000	70,000
Repayments of Borrowings, Revolving Credit Facility	0	(206,000)	(35,000)
Proceeds from Borrowings, Term Loan	165,000	0	0
Proceeds from Issuance, Long-Term Debt—Net of Discount	0	298,869	0
Repayments of Long-Term Debt	(14,700)	(136,700)	0
Dividends Paid to Parent	(75,000)	(85,000)	(70,000)
Payments of Finance Lease Obligations	(10,890)
Payments of Finance Lease Obligations		(10,930)	(15,571)
Payment of Debt Issuance Costs	(757)	(3,265)	(245)
Contribution from Parent	50,000	50,000	0
Other, Net	1,514	1,078	481
Net Cash Flows—Financing Activities	115,167	79,052	(50,335)
Net Increase (Decrease) in Cash, Cash Equivalents, and Restricted Cash	(124,275)	103,246	6,176
Cash, Cash Equivalents, and Restricted Cash, Beginning of Period	152,747	49,501	43,325
Cash, Cash Equivalents, and Restricted Cash, End of Period	$ 28,472	$ 152,747	$ 49,501