Condensed Consolidated Statements of Cash Flows (unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Operating activities:
Net income	$ 97,224	$ 107,362
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, depletion and amortization	56,402	51,028
Deferred financing cost amortization	2,237	2,007
Impairment of investment	6,859	0
Stock compensation	291	3,296
Deferred income taxes	21,876	19,602
Employee benefit plans	1,235	3,137
Other adjustments, net	892	4,428
Changes in certain operating assets and liabilities:
Materials, supplies and fuel	19,222	29,387
Accounts receivable and other current assets	8,171	(15,857)
Accounts payable and other current liabilities	(43,297)	(41,689)
Regulatory assets - current	20,679	13,031
Regulatory liabilities - current	1,316	(1,635)
Other operating activities, net	(1,138)	1,796
Net cash provided by operating activities	191,969	175,893
Investing activities:
Property, plant and equipment additions	(171,882)	(144,126)
Other investing activities	(1,202)	(901)
Net cash (used in) investing activities	(173,084)	(145,027)
Financing activities:
Dividends paid on common stock	(32,902)	(30,332)
Common stock issued	99,321	19,949
Net (payments) borrowings of short-term debt	(30,375)	(20,970)
Long-term debt - repayments	(4,291)	(1,436)
Distributions to noncontrolling interest	(4,741)	(4,846)
Other financing activities	(1,391)	(1,657)
Net cash provided by (used in) financing activities	25,621	(39,292)
Net change in cash, restricted cash and cash equivalents	44,506	(8,426)
Cash, restricted cash and cash equivalents at beginning of period	13,658	24,145
Cash, restricted cash and cash equivalents at end of period	58,164	15,719
Supplemental Cash Flow Information [Abstract]
Interest (net of amounts capitalized)	(21,776)	(30,672)
Income taxes	0	8
Accrued property, plant and equipment purchases at March 31	$ 53,011	$ 56,571