Schedule I (Details) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Statement [Abstract]
Operating Revenues		$ 589,583,000	$ 302,480,000	$ 227,330,000	$ 521,945,000	$ 345,738,000	$ 227,127,000	$ 244,374,000	$ 425,829,000	$ 1,641,338,000	$ 1,243,068,000	$ 1,036,500,000
Operating Expenses:
Operations										256,862,000	169,767,000	147,056,000
Depreciation										96,723,000	100,718,000	90,389,000
Energy and Other Taxes		3,260,000	2,595,000	1,243,000	2,439,000	1,348,000	1,517,000	1,551,000	2,071,000	9,537,000	6,487,000	6,342,000
Total Operating Expenses										1,540,593,000	1,234,225,000	842,323,000
Operating Income (See Note 1)										100,745,000	8,843,000	194,177,000
Other Income:
Equity in Earnings (Losses) of Subsidiaries (See Note 1)										5,611,000	5,794,000	5,396,000
Total Other Income										2,404,000	11,041,000	5,088,000
Interest Charges										90,296,000	54,019,000	31,449,000
Income Taxes		12,767,000	(16,649,000)	(31,972,000)	36,415,000	(16,498,000)	(24,765,000)	(5,544,000)	21,870,000	561,000	(24,937,000)	54,151,000
Income (Loss) from Continuing Operations		46,032,000	(45,642,000)	(93,793,000)	111,306,000	4,009,000	(37,548,000)	(7,612,000)	37,747,000	17,903,000	(3,404,000)	119,061,000
Equity in Undistributed Earnings of Discontinued Operations										(240,000)	(86,000)	(251,000)
Net Income (Loss)		45,927,000	(45,685,000)	(93,819,000)	111,240,000	4,045,000	(37,593,000)	(7,659,000)	37,717,000	17,663,000	(3,490,000)	118,810,000
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net Income		45,927,000	(45,685,000)	(93,819,000)	111,240,000	4,045,000	(37,593,000)	(7,659,000)	37,717,000	17,663,000	(3,490,000)	118,810,000
Other Comprehensive Income (Loss) - Net of Tax
Postretirement Liability Adjustment	[1]									10,636,000	(10,920,000)	(3,197,000)
Unrealized Gain on Available-for-Sale Securities	[2]									0	0	118,000
Unrealized Gain on Derivatives - Other	[2]									34,000	1,536,000	197,000
Other Comprehensive Income (Loss) - Net of Tax										10,670,000	(9,384,000)	(2,882,000)
Comprehensive Income										28,333,000	(12,874,000)	115,928,000
Retained Earnings [Roll Forward]
Retained Earnings - Beginning					420,351,000					420,351,000
Net Income		45,927,000	$ (45,685,000)	$ (93,819,000)	111,240,000	4,045,000	$ (37,593,000)	$ (7,659,000)	37,717,000	17,663,000	(3,490,000)	118,810,000
Dividends Declared - Common Stock										(94,756,000)	(87,308,000)	(82,380,000)
Retained Earnings - Ending		343,258,000				420,351,000				343,258,000	420,351,000
CASH FLOWS FROM INVESTING ACTIVITIES:
Capital Expenditures										(341,120,000)	(272,965,000)	(279,423,000)
Cash Paid for Acquisition										(1,740,285,000)	0	0
Proceeds from Sale of PPE										310,644,000	3,547,000	0
Purchase of Company-Owned Life Insurance										(1,298,000)	(9,180,000)	(2,398,000)
Investment in Affiliate										(9,524,000)	(29,636,000)	(12,943,000)
Net Cash Used in Investing Activities										(1,788,785,000)	(287,264,000)	(280,298,000)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from Issuance of Long Term Debt										2,432,500,000	450,000,000	61,000,000
Principal Repayments of Long Term Debt										(768,909,000)	(293,309,000)	(49,366,000)
Payments for Issuance of Long Term Debt										(21,574,000)	(14,204,000)	(147,000)
Net Borrowings from (Repayments of) Short-Term Credit Facilities										(75,900,000)	50,300,000	(135,600,000)
Dividends on Common Stock										(94,756,000)	(87,308,000)	(82,380,000)
Net Settlement of Restricted Stock										(776,000)	(751,000)	(387,000)
Proceeds from Sale of Common Stock										173,750,000	0	214,426,000
Payments for the Issuance of Common Stock										(7,149,000)	0	0
Net Cash Provided by (Used in) Financing Activities										1,637,186,000	104,728,000	(3,054,000)
Net (Decrease) Increase in Cash, Cash Equivalents and Restricted Cash										(8,016,000)	7,785,000	(20,725,000)
Cash, Cash Equivalents and Restricted Cash at Beginning of Year					39,695,000				31,910,000	39,695,000	31,910,000	52,635,000
Cash, Cash Equivalents and Restricted Cash at End of Year		31,679,000				39,695,000				31,679,000	39,695,000	31,910,000
Property Plant and Equipment:
Nonutility Property, Plant and Equipment, at cost													$ 152,232,000	$ 741,027,000
Accumulated Depreciation													(52,629,000)	(194,913,000)
Property, Plant and Equipment - Net													3,653,473,000	2,700,197,000
Investments:
Available-for-Sale Securities													41,000	36,000
Total Investments													77,812,000	94,204,000
Current Assets:
Cash and Cash Equivalents													30,030,000	7,819,000
Accounts Receivable													337,502,000	202,379,000
Other													26,548,000	28,247,000
Total Current Assets													663,192,000	438,993,000
Other Noncurrent Assets													116,119,000	110,906,000
Total Assets													5,956,577,000	3,865,086,000
Equity:
Common Stock													106,883,000	99,436,000	$ 99,347,000
Premium on Common Stock													843,268,000	709,658,000
Treasury Stock (at par)													(292,000)	(271,000)
Accumulated Other Comprehensive Loss													(26,095,000)	(36,765,000)
Retained Earnings		343,258,000			420,351,000	420,351,000				420,351,000	420,351,000		343,258,000	420,351,000
Total Equity													1,267,022,000	1,192,409,000	$ 1,289,240,000	$ 1,037,539,000
Long-Term Debt													2,106,863,000	1,122,999,000
Current Liabilities:
Notes Payable - Banks													270,500,000	346,400,000
Current Portion of Long-Term Debt													733,909,000	63,809,000
Accounts Payable													410,463,000	284,899,000
Other Current Liabilities													36,102,000	15,860,000
Total Current Liabilities													1,580,838,000	883,082,000
Other Noncurrent Liabilities													26,645,000	9,436,000
Total Capitalization and Liabilities													$ 5,956,577,000	$ 3,865,086,000
Common stock, par value (in USD per share)													$ 1.25	$ 1.25
Common stock, authorized (in shares)													120,000,000	120,000,000
Common Stock, outstanding (in shares)													85,506,218	79,549,080	79,478,055		70,965,622
Equity in Earnings From Subsidiaries [Abstract]
Equity in Earnings (Losses) of Subsidiaries										5,611,000	5,794,000	5,396,000
Impact of Tax Adjustments										(588,000)	(13,521,000)	0
Parent Company
Income Statement [Abstract]
Operating Revenues										42,934,000	34,321,000	25,463,000
Operating Expenses:
Operations										67,869,000	43,513,000	22,194,000
Depreciation										600,000	311,000	377,000
Energy and Other Taxes										1,517,000	1,324,000	1,033,000
Total Operating Expenses										69,986,000	45,148,000	23,604,000
Operating Income (See Note 1)										(27,052,000)	(10,827,000)	1,859,000
Other Income:
Equity in Earnings (Losses) of Subsidiaries (See Note 1)										65,327,000	(2,793,000)	119,061,000
Other										17,608,000	15,083,000	10,295,000
Total Other Income										82,935,000	12,290,000	129,356,000
Interest Charges										54,678,000	23,818,000	12,148,000
Income Taxes										(16,698,000)	(18,951,000)	6,000
Income (Loss) from Continuing Operations										17,903,000	(3,404,000)	119,061,000
Equity in Undistributed Earnings of Discontinued Operations										(240,000)	(86,000)	(251,000)
Net Income (Loss)										17,663,000	(3,490,000)	118,810,000
Comprehensive Income (Loss), Net of Tax, Attributable to Parent [Abstract]
Net Income										17,663,000	(3,490,000)	118,810,000
Other Comprehensive Income (Loss) - Net of Tax
Postretirement Liability Adjustment										10,636,000	(10,920,000)	(3,197,000)
Unrealized Gain on Available-for-Sale Securities										0	0	118,000
Unrealized Gain on Derivatives - Other										34,000	1,536,000	197,000
Other Comprehensive Income (Loss) - Net of Tax										10,670,000	(9,384,000)	(2,882,000)
Comprehensive Income										28,333,000	(12,874,000)	115,928,000
Retained Earnings [Roll Forward]
Retained Earnings - Beginning					420,351,000				510,597,000	420,351,000	510,597,000	474,167,000
Net Income										17,663,000	(3,490,000)	118,810,000
Retained Earnings After Net Income Before Dividends Declared													$ 438,014,000	$ 507,107,000	$ 592,977,000
Dividends Declared - Common Stock										(94,756,000)	(87,308,000)	(82,380,000)
Excess Tax Benefit on Restricted Stock										0	552,000	0
Retained Earnings - Ending		343,258,000				420,351,000				343,258,000	420,351,000	510,597,000
Statement of Cash Flows [Abstract]
CASH (USED IN) PROVIDED BY OPERATING ACTIVITIES										(6,447,000)	17,339,000	20,507,000
CASH FLOWS FROM INVESTING ACTIVITIES:
Net Repayments from (Advances to) Associated Companies										366,342,000	(16,096,000)	32,300,000
Capital Expenditures										(24,155,000)	(801,000)	(345,000)
Cash Paid for Acquisition										(1,740,291,000)	0	0
Proceeds from Sale of PPE										51,000	0	0
Purchase of Company-Owned Life Insurance										(1,298,000)	(9,180,000)	(2,398,000)
Investment in Affiliate										0	(40,000,000)	(65,000,000)
Net Cash Used in Investing Activities										(1,399,351,000)	(66,077,000)	(35,443,000)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from Issuance of Long Term Debt										1,592,500,000	50,000,000	0
Principal Repayments of Long Term Debt										0	(16,000,000)	0
Payments for Issuance of Long Term Debt										(15,513,000)	(12,174,000)	(84,000)
Net Borrowings from (Repayments of) Short-Term Credit Facilities										(217,400,000)	102,600,000	(105,500,000)
Dividends on Common Stock										(94,756,000)	(87,308,000)	(82,380,000)
Net Settlement of Restricted Stock										(776,000)	(751,000)	(387,000)
Proceeds from Sale of Common Stock										173,750,000	0	214,426,000
Payments for the Issuance of Common Stock										(7,149,000)	0	0
Net Cash Provided by (Used in) Financing Activities										1,430,656,000	36,367,000	26,075,000
Net (Decrease) Increase in Cash, Cash Equivalents and Restricted Cash										24,858,000	(12,371,000)	11,139,000
Cash, Cash Equivalents and Restricted Cash at Beginning of Year					476,000				12,847,000	476,000	12,847,000	1,708,000
Cash, Cash Equivalents and Restricted Cash at End of Year		25,334,000				476,000				25,334,000	476,000	12,847,000
Property Plant and Equipment:
Nonutility Property, Plant and Equipment, at cost													4,188,000	3,318,000
Accumulated Depreciation													(2,488,000)	(2,194,000)
Property, Plant and Equipment - Net													1,700,000	1,124,000
Investments:
Investments in Subsidiaries													2,458,680,000	1,209,308,000
Available-for-Sale Securities													41,000	36,000
Total Investments													2,458,721,000	1,209,344,000
Current Assets:
Cash and Cash Equivalents													25,334,000	476,000
Receivable from Associated Companies													270,478,000	636,327,000
Accounts Receivable													38,000	52,000
Other													19,100,000	5,017,000
Total Current Assets													314,950,000	641,872,000
Other Noncurrent Assets													53,838,000	50,735,000
Total Assets													2,829,209,000	1,903,075,000
Equity:
Common Stock													106,883,000	99,436,000
Premium on Common Stock													843,268,000	709,658,000
Treasury Stock (at par)													(292,000)	(271,000)
Accumulated Other Comprehensive Loss													(26,095,000)	(36,765,000)
Retained Earnings		$ 343,258,000			$ 420,351,000	$ 420,351,000			$ 510,597,000	420,351,000	510,597,000	474,167,000	343,258,000	420,351,000	$ 510,597,000	$ 474,167,000
Total Equity													1,267,022,000	1,192,409,000
Long-Term Debt													708,360,000	364,946,000
Current Liabilities:
Notes Payable - Banks													77,000,000	294,400,000
Current Portion of Long-Term Debt													715,000,000	0
Payable to Associated Companies													899,000	404,000
Accounts Payable													6,378,000	17,316,000
Other Current Liabilities													27,895,000	7,763,000
Total Current Liabilities													827,172,000	319,883,000
Other Noncurrent Liabilities													26,655,000	25,837,000
Total Capitalization and Liabilities													$ 2,829,209,000	$ 1,903,075,000
Common stock, par value (in USD per share)													$ 1.25	$ 1.25
Common stock, authorized (in shares)													120,000,000	120,000,000
Common Stock, outstanding (in shares)													85,506,218	79,549,080
Dividends received from subsidiaries										0	20,000,000	0
Equity in Earnings From Subsidiaries [Abstract]
Equity in Earnings (Losses) of Subsidiaries										65,327,000	(2,793,000)	119,061,000
Acquisition Costs, net of tax										(29,727,000)	(12,031,000)	0
Interest Charges, net of tax										(17,697,000)	0	0
Impact of Tax Adjustments										0	11,420,000	0
Income (Loss) From Continuing Operations										$ 17,903,000	$ (3,404,000)	$ 119,061,000
Unrealized Gain (Loss)
Other Comprehensive Income (Loss) - Net of Tax
Combined average statutory tax rate										25.00%	39.00%	40.00%
Unrealized Gain (Loss) | Parent Company
Other Comprehensive Income (Loss) - Net of Tax
Combined average statutory tax rate											39.00%	40.00%
Postretirement Liability
Other Comprehensive Income (Loss) - Net of Tax
Combined average statutory tax rate										25.00%	27.00%	40.00%
Postretirement Liability | Parent Company
Other Comprehensive Income (Loss) - Net of Tax
Combined average statutory tax rate										25.00%	27.00%	40.00%

[1] Determined using a combined average statutory tax rate of 25% for 2018; 27% for 2017; and 40% for 2016.

[2] Determined using a combined average statutory tax rate of 25% for 2018; 39% for 2017; and 40% for 2016.