Condensed Consolidated Statements of Cash Flows (unaudited) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Operating activities:
Net income	$ 162,267	$ 140,450
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	169,413	154,507
Deferred financing cost amortization	5,523	6,326
Impairment of investment	6,859	19,741
Stock compensation	2,696	8,332
Deferred income taxes	28,502	24,381
Employee benefit plans	9,294	7,965
Other adjustments, net	7,910	9,192
Changes in certain operating assets and liabilities:
Materials, supplies and fuel	(10,905)	(4,126)
Accounts receivable and other current assets	75,960	115,325
Accounts payable and other current liabilities	(11,136)	(83,436)
Regulatory assets - current	1,954	12,455
Regulatory liabilities - current	(17,686)	(15,644)
Payment for Pension Benefits	(12,700)	(12,700)
Other operating activities, net	1,508	3,307
Net cash provided by operating activities	419,459	386,075
Investing activities:
Property, plant and equipment additions	(535,993)	(592,537)
Other investing activities	6,269	(735)
Net cash (used in) investing activities	(529,724)	(593,272)
Financing activities:
Dividends paid on common stock	(99,999)	(91,779)
Common stock issued	99,316	101,361
Net (payments) borrowings of short-term debt	(265,180)	109,280
Long-term debt - issuances	400,000	400,000
Long-term debt - repayments	(7,163)	(304,307)
Distributions to noncontrolling interest	(12,636)	(12,736)
Other financing activities	(6,519)	(1,992)
Net cash provided by financing activities	107,819	199,827
Net change in cash, restricted cash and cash equivalents	(2,446)	(7,370)
Cash, restricted cash and cash equivalents at beginning of period	13,658	24,145
Cash, restricted cash and cash equivalents at end of period	11,212	16,775
Supplemental Cash Flow Information [Abstract]
Interest (net of amounts capitalized)	(87,453)	(99,375)
Income taxes	1,256	2,255
Accrued property, plant and equipment purchases at September 30	$ 86,474	$ 86,661