Consolidated Statements Of Capitalization (Parenthetical) - $ / shares	3 Months Ended	12 Months Ended
Mar. 31, 2017	Dec. 31, 2016
Common stock, par value	$ 0.50	$ 0.50
Long-Term Debt Of Subsidiaries 10.00% To 10.99% [Member]
Maturity date	2018	2018
Revolving Credit Agreement, Due 2021 [Member]
Maturity date	2021	2021
Bank notes at 1.921% due 2017 [Member]
Interest rate	1.921%	1.921%
Maturity date	2017	2017
Bank notes at 1.975% due 2018 [Member]
Interest rate	1.975%	1.975%
Maturity date	2018	2018
Notes at 3.01% due 2027 [Member]
Interest rate	3.01%	3.01%
Maturity date	2027	2027
Notes at 3.59% due 2041 [Member]
Interest rate	3.59%	3.59%
Maturity date	2041	2041
Minimum [Member] \| Long-Term Debt Of Subsidiaries 0.00% To 0.99% [Member]
Interest rate	0.00%	0.00%
Maturity date	2023	2023
Minimum [Member] \| Long-Term Debt Of Subsidiaries 1.00% To 1.99% [Member]
Interest rate	1.00%	1.00%
Maturity date	2019	2019
Minimum [Member] \| Long-Term Debt Of Subsidiaries 2.00% To 2.99% [Member]
Interest rate	2.00%	2.00%
Maturity date	2024	2024
Minimum [Member] \| Long-Term Debt Of Subsidiaries 3.00% To 3.99% [Member]
Interest rate	3.00%	3.00%
Maturity date	2019	2019
Minimum [Member] \| Long-Term Debt Of Subsidiaries 4.00% To 4.99% [Member]
Interest rate	4.00%	4.00%
Maturity date	2020	2020
Minimum [Member] \| Long-Term Debt Of Subsidiaries 5.00% To 5.99% [Member]
Interest rate	5.00%	5.00%
Maturity date	2017	2017
Minimum [Member] \| Long-Term Debt Of Subsidiaries 6.00% To 6.99% [Member]
Interest rate	6.00%	6.00%
Maturity date	2017	2017
Minimum [Member] \| Long-Term Debt Of Subsidiaries 7.00% To 7.99% [Member]
Interest rate	7.00%	7.00%
Maturity date	2022	2022
Minimum [Member] \| Long-Term Debt Of Subsidiaries 8.00% To 8.99% [Member]
Interest rate	8.00%	8.00%
Maturity date	2021	2021
Minimum [Member] \| Long-Term Debt Of Subsidiaries 9.00% To 9.99% [Member]
Interest rate	9.00%	9.00%
Maturity date	2018	2018
Minimum [Member] \| Long-Term Debt Of Subsidiaries 10.00% To 10.99% [Member]
Interest rate	10.00%	10.00%
Minimum [Member] \| Notes Ranging From 4.62% To 4.87%, due 2017 through 2024 [Member]
Interest rate	4.62%	4.62%
Maturity date	2017	2017
Minimum [Member] \| Notes Ranging From 5.20% To 5.95%, due 2017 through 2037 [Member]
Interest rate	5.20%	5.20%
Maturity date	2017	2017
Maximum [Member] \| Long-Term Debt Of Subsidiaries 0.00% To 0.99% [Member]
Interest rate	0.99%	0.99%
Maturity date	2033	2033
Maximum [Member] \| Long-Term Debt Of Subsidiaries 1.00% To 1.99% [Member]
Interest rate	1.99%	1.99%
Maturity date	2035	2035
Maximum [Member] \| Long-Term Debt Of Subsidiaries 2.00% To 2.99% [Member]
Interest rate	2.99%	2.99%
Maturity date	2031	2031
Maximum [Member] \| Long-Term Debt Of Subsidiaries 3.00% To 3.99% [Member]
Interest rate	3.99%	3.99%
Maturity date	2056	2056
Maximum [Member] \| Long-Term Debt Of Subsidiaries 4.00% To 4.99% [Member]
Interest rate	4.99%	4.99%
Maturity date	2054	2054
Maximum [Member] \| Long-Term Debt Of Subsidiaries 5.00% To 5.99% [Member]
Interest rate	5.99%	5.99%
Maturity date	2043	2043
Maximum [Member] \| Long-Term Debt Of Subsidiaries 6.00% To 6.99% [Member]
Interest rate	6.99%	6.99%
Maturity date	2036	2036
Maximum [Member] \| Long-Term Debt Of Subsidiaries 7.00% To 7.99% [Member]
Interest rate	7.99%	7.99%
Maturity date	2027	2027
Maximum [Member] \| Long-Term Debt Of Subsidiaries 8.00% To 8.99% [Member]
Interest rate	8.99%	8.99%
Maturity date	2025	2025
Maximum [Member] \| Long-Term Debt Of Subsidiaries 9.00% To 9.99% [Member]
Interest rate	9.99%	9.99%
Maturity date	2026	2026
Maximum [Member] \| Long-Term Debt Of Subsidiaries 10.00% To 10.99% [Member]
Interest rate	10.99%	10.99%
Maximum [Member] \| Notes Ranging From 4.62% To 4.87%, due 2017 through 2024 [Member]
Interest rate	4.87%	4.87%
Maturity date	2024	2024
Maximum [Member] \| Notes Ranging From 5.20% To 5.95%, due 2017 through 2037 [Member]
Interest rate	5.95%	5.95%
Maturity date	2037	2037