Consolidated Statements Of Capitalization - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Stockholders' equity:
Common stock, $.50 par value	$ 109,471	$ 90,576
Capital in excess of par value	2,633,193	820,378
Retained earnings	1,197,600	1,174,245
Treasury stock, at cost	(77,702)	(75,835)
Total stockholders' equity	3,862,562	2,009,364
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Long-term debt of subsidiaries	1,877,205	1,604,233
Total long-term debt	3,112,021	2,563,660
Current portion of long-term debt	178,116	144,545
Long-term debt, excluding current portion	2,933,905	2,419,115
Less: debt issuance costs	35,607	20,651
Long-term debt, excluding current portion, net of debt issuance costs	2,898,298	2,398,464
Total capitalization	6,760,860	4,407,828
Long-Term Debt Of Subsidiaries 0.00% To 0.99% [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Long-term debt of subsidiaries	3,472	3,732
Long-Term Debt Of Subsidiaries 1.00% To 1.99% [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Long-term debt of subsidiaries	10,985	11,588
Long-Term Debt Of Subsidiaries 2.00% To 2.99% [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Long-term debt of subsidiaries	16,132	17,488
Long-Term Debt Of Subsidiaries 3.00% To 3.99% [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Long-term debt of subsidiaries	496,295	497,426
Long-Term Debt Of Subsidiaries 4.00% To 4.99% [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Long-term debt of subsidiaries	1,129,571	831,066
Long-Term Debt Of Subsidiaries 5.00% To 5.99% [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Long-term debt of subsidiaries	134,323	154,788
Long-Term Debt Of Subsidiaries 6.00% To 6.99% [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Long-term debt of subsidiaries	31,000	31,000
Long-Term Debt Of Subsidiaries 7.00% To 7.99% [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Long-term debt of subsidiaries	30,958	31,564
Long-Term Debt Of Subsidiaries 8.00% To 8.99% [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Long-term debt of subsidiaries	5,169	5,581
Long-Term Debt Of Subsidiaries 9.00% To 9.99% [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Long-term debt of subsidiaries	19,300	20,000
Revolving Credit Agreement, Due 2023 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Notes payable to bank under revolving credit agreement, variable rate, due 2023		370,000
Bank Note At 3.50% Due 2020 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	50,000	100,000
Amortizing notes at 3.00% due 2022 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	108,889
Notes ranging from 3.01% to 3.59% due 2029 through 2041 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	525,000	245,000
Notes at 4.28%, due 2049 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	500,000	112,000
Notes ranging from 5.64% to 5.95%, due 2020 through 2034 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	$ 50,927	$ 132,427