Consolidated Statements Of Capitalization - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Stockholders' equity:
Common stock, $0.50 par value	$ 127,990	$ 124,285
Capital in excess of par value	3,698,095	3,379,057
Retained earnings	1,385,631	1,261,862
Treasury stock, at cost	(83,854)	(81,327)
Total stockholders' equity	5,127,862	4,683,877
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Long-term debt of subsidiaries	3,091,789	3,014,280
Total long-term debt	5,702,379	5,630,243
Current portion of long-term debt	63,418	84,353
Long-term debt, excluding current portion	5,638,961	5,545,890
Less: debt issuance costs	40,247	38,146
Long-term debt, excluding current portion, net of debt issuance costs	5,598,714	5,507,744
Total capitalization	10,726,576	10,191,621
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	2,341	2,805
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	9,578	10,260
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	313,458	265,557
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	1,361,263	1,316,872
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	1,310,107	1,315,812
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	17,222	17,804
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	33,868	33,955
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	29,212	29,890
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	2,940	4,425
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	16,900
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	15,000	385,000
Amortizing notes at 3.00% due 2022 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	30,590	60,502
Notes at 2.40% due 2031 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	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 2021 through 2034 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	$ 40,000	$ 45,461