Consolidated Statements Of Capitalization - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021
Stockholders' equity:
Common stock, $0.50 par value	$ 128,140	$ 128,050
Capital in excess of par value	3,713,560	3,705,814
Retained earnings	1,497,757	1,434,201
Treasury stock, at cost	(84,357)	(83,615)
Total stockholders' equity	5,255,100	5,184,450
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Long-term debt of subsidiaries	3,056,836	3,061,887
Total long-term debt	6,042,109	5,947,357
Current portion of long-term debt	131,746	132,146
Long-term debt, excluding current portion	5,910,363	5,815,211
Less: debt issuance costs	38,403	35,707
Long-term debt, excluding current portion, net of debt issuance costs	5,871,960	5,779,504
Total capitalization	11,127,060	10,963,954
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	2,302	2,341
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	9,157	9,341
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	311,863	312,751
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	1,357,393	1,359,284
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	1,284,508	1,286,024
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	15,917	16,119
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	32,446	32,475
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	28,799	28,980
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	2,651	2,772
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	11,800	11,800
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 Dec 2023	410,000	300,000
Amortizing notes at 3.00% due 2022 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	10,273	20,470
Notes at 2.40% due 2031 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	400,000	400,000
Notes at 2.704% due 2030 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	500,000	500,000
Notes ranging from 3.01% to 3.59% due 2029 through 2050 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	1,125,000	1,125,000
Notes at 4.28%, due 2049 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	500,000	500,000
Notes ranging from 5.64% to 5.95%, due 2022 through 2034 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	$ 40,000	$ 40,000