Consolidated Statements Of Capitalization - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Stockholders' equity:
Common stock, $.50 par value	$ 124,117	$ 111,935
Capital in excess of par value	3,361,586	2,636,555
Retained earnings	1,209,501	1,210,072
Treasury stock, at cost	(82,041)	(77,702)
Total stockholders' equity	4,613,163	3,880,860
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Long-term debt of subsidiaries	2,854,760	1,887,117
Total long-term debt	4,872,438	3,077,400
Current portion of long-term debt	115,011	105,051
Long-term debt, excluding current portion	4,757,427	2,972,349
Less: debt issuance costs	28,393	29,022
Long-term debt, excluding current portion, net of debt issuance costs	4,729,034	2,943,327
Total capitalization	9,342,197	6,824,187
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	3,225	3,474
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	10,682	10,733
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	117,667	15,674
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	1,148,133	655,685
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	1,419,071	1,054,791
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	65,852	60,683
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	35,410	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	30,540	30,751
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	4,880	5,026
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	19,300
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	802,000
Bank Note At 3.50% Due 2020 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	50,000	50,000
Amortizing notes at 3.00% due 2022 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	89,751	99,356
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	490,000
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 2020 through 2034 [Member]
Long-term debt of subsidiaries (substantially collateralized by utility plant):
Unsecured notes payable	$ 50,927	$ 50,927