Consolidated Statements Of Capitalization - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Stockholders' equity:
Common stock, $.50 par value	$ 90,283	$ 90,155
Capital in excess of par value	800,420	797,513
Retained earnings	1,048,963	1,032,844
Treasury stock, at cost	(73,166)	(71,113)
Accumulated other comprehensive income	727	669
Total stockholders' equity	1,867,227	1,850,068
Long-term debt:
Long-term debt of subsidiaries	1,247,455	1,247,224
Total long-term debt	1,938,864	1,910,633
Current portion of long-term debt	119,296	150,671
Long-term debt, excluding current portion	1,819,568	1,759,962
Less: debt issuance costs	22,057	22,357
Long-term debt, excluding current portion, net of debt issuance costs	1,797,511	1,737,605
Total capitalization	3,664,738	3,587,673
Long-Term Debt Of Subsidiaries 0.00% To 0.99% [Member]
Long-term debt:
Long-term debt of subsidiaries	4,612	4,661
Long-Term Debt Of Subsidiaries 1.00% To 1.99% [Member]
Long-term debt:
Long-term debt of subsidiaries	14,679	15,539
Long-Term Debt Of Subsidiaries 2.00% To 2.99% [Member]
Long-term debt:
Long-term debt of subsidiaries	19,813	19,668
Long-Term Debt Of Subsidiaries 3.00% To 3.99% [Member]
Long-term debt:
Long-term debt of subsidiaries	431,260	381,944
Long-Term Debt Of Subsidiaries 4.00% To 4.99% [Member]
Long-term debt:
Long-term debt of subsidiaries	439,408	487,318
Long-Term Debt Of Subsidiaries 5.00% To 5.99% [Member]
Long-term debt:
Long-term debt of subsidiaries	212,953	213,078
Long-Term Debt Of Subsidiaries 6.00% To 6.99% [Member]
Long-term debt:
Long-term debt of subsidiaries	52,990	52,985
Long-Term Debt Of Subsidiaries 7.00% To 7.99% [Member]
Long-term debt:
Long-term debt of subsidiaries	32,887	33,066
Long-Term Debt Of Subsidiaries 8.00% To 8.99% [Member]
Long-term debt:
Long-term debt of subsidiaries	6,453	6,565
Long-Term Debt Of Subsidiaries 9.00% To 9.99% [Member]
Long-term debt:
Long-term debt of subsidiaries	26,400	26,400
Long-Term Debt Of Subsidiaries 10.00% To 10.99% [Member]
Long-term debt:
Long-term debt of subsidiaries	6,000	6,000
Revolving Credit Agreement, Due 2021 [Member]
Long-term debt:
Notes payable to bank under revolving credit agreement, variable rate, due 2017	53,000	25,000
Bank notes at 1.921% and 1.975% due 2017 and 2018 [Member]
Long-term debt:
Unsecured notes payable	100,000	100,000
Notes ranging from 3.01% to 3.59% due 2027 through 2041 [Member]
Long-term debt:
Unsecured notes payable	245,000	245,000
Notes Ranging From 4.62% To 4.87%, due 2017 through 2024 [Member]
Long-term debt:
Unsecured notes payable	133,600	133,600
Notes Ranging From 5.20% To 5.95%, due 2017 through 2037 [Member]
Long-term debt:
Unsecured notes payable	$ 159,809	$ 159,809