Fair Value Long-term Debt, Other Temporary Investments, Nuclear Trusts (Details) - USD ($) $ in Millions |
12 Months Ended |
|
|
Dec. 31, 2016 |
Dec. 31, 2015 |
Dec. 31, 2014 |
Dec. 31, 2016 |
Dec. 31, 2015 |
Book Values and Fair Values of Long - term Debt |
|
|
|
|
|
|
|
Total Long-term Debt Outstanding |
|
|
|
|
$ 20,256.4
|
$ 19,572.7
|
[1] |
Long-term Debt, Fair Value |
|
|
|
|
22,211.9
|
21,201.3
|
|
Other Temporary Investments |
|
|
|
|
|
|
|
Cost |
|
|
|
|
318.8
|
375.8
|
|
Gross Unrealized Gains |
|
|
|
|
13.9
|
11.7
|
|
Gross Unrealized Losses |
|
|
|
|
(1.0)
|
(0.7)
|
|
Estimated Fair Value |
|
|
|
|
331.7
|
386.8
|
|
Debt and Equity Securities Within Other Temporary Investments [Abstract] |
|
|
|
|
|
|
|
Proceeds From Investment Sales |
|
$ 0.0
|
$ 0.0
|
$ 0.0
|
|
|
|
Purchases of Investments |
|
2.3
|
10.7
|
1.6
|
|
|
|
Gross Realized Gains on Investment Sales |
|
0.0
|
0.0
|
0.0
|
|
|
|
Gross Realized Losses on Investment Sales |
|
0.0
|
0.0
|
0.0
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
2,256.2
|
2,106.4
|
|
2,256.2
|
2,106.4
|
|
Gross Unrealized Gains |
|
|
|
|
707.7
|
611.8
|
|
Other-Than-Temporary Impairments |
|
|
|
|
(87.2)
|
(83.3)
|
|
Securities Activity Within the Decommissioning and SNF Trusts |
|
|
|
|
|
|
|
Proceeds from Investment Sales |
|
2,957.7
|
2,218.4
|
1,031.8
|
|
|
|
Purchases of Investments |
|
3,000.0
|
2,272.0
|
1,086.4
|
|
|
|
Gross Realized Gains on Investment Sales |
|
46.1
|
69.1
|
32.3
|
|
|
|
Gross Realized Losses on Investment Sales |
|
24.4
|
53.0
|
15.4
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
2,256.2
|
2,106.4
|
|
|
|
|
Fair Value Measurements (Textuals) [Abstract] |
|
|
|
|
|
|
|
Adjusted Cost of Debt Securities |
|
|
|
|
938.0
|
771.0
|
|
Adjusted Cost of Domestic Equity Securities |
|
|
|
|
592.0
|
555.0
|
|
Includes Debt Included In Liabilities Held For Sale [Member] |
|
|
|
|
|
|
|
Book Values and Fair Values of Long - term Debt |
|
|
|
|
|
|
|
Total Long-term Debt Outstanding |
[2] |
|
|
|
20,391.2
|
|
|
Appalachian Power Co [Member] |
|
|
|
|
|
|
|
Book Values and Fair Values of Long - term Debt |
|
|
|
|
|
|
|
Total Long-term Debt Outstanding |
|
|
|
|
4,033.9
|
3,930.7
|
|
Long-term Debt, Fair Value |
|
|
|
|
4,613.2
|
4,416.7
|
|
Indiana Michigan Power Co [Member] |
|
|
|
|
|
|
|
Book Values and Fair Values of Long - term Debt |
|
|
|
|
|
|
|
Total Long-term Debt Outstanding |
|
|
|
|
2,471.4
|
2,000.0
|
|
Long-term Debt, Fair Value |
|
|
|
|
2,661.6
|
2,193.6
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
2,256.2
|
2,106.4
|
|
2,256.2
|
2,106.4
|
|
Gross Unrealized Gains |
|
|
|
|
707.7
|
611.8
|
|
Other-Than-Temporary Impairments |
|
|
|
|
(87.2)
|
(83.3)
|
|
Securities Activity Within the Decommissioning and SNF Trusts |
|
|
|
|
|
|
|
Proceeds from Investment Sales |
|
2,957.7
|
2,218.4
|
1,031.8
|
|
|
|
Purchases of Investments |
|
3,000.0
|
2,272.0
|
1,086.4
|
|
|
|
Gross Realized Gains on Investment Sales |
|
46.1
|
69.1
|
32.3
|
|
|
|
Gross Realized Losses on Investment Sales |
|
24.4
|
53.0
|
$ 15.4
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
2,256.2
|
2,106.4
|
|
|
|
|
Fair Value Measurements (Textuals) [Abstract] |
|
|
|
|
|
|
|
Adjusted Cost of Debt Securities |
|
|
|
|
938.0
|
771.0
|
|
Adjusted Cost of Domestic Equity Securities |
|
|
|
|
592.0
|
555.0
|
|
Ohio Power Co [Member] |
|
|
|
|
|
|
|
Book Values and Fair Values of Long - term Debt |
|
|
|
|
|
|
|
Total Long-term Debt Outstanding |
|
|
|
|
1,763.9
|
2,157.7
|
|
Long-term Debt, Fair Value |
|
|
|
|
2,092.5
|
2,472.7
|
|
Public Service Co Of Oklahoma [Member] |
|
|
|
|
|
|
|
Book Values and Fair Values of Long - term Debt |
|
|
|
|
|
|
|
Total Long-term Debt Outstanding |
|
|
|
|
1,286.0
|
1,286.1
|
|
Long-term Debt, Fair Value |
|
|
|
|
1,419.0
|
1,402.9
|
|
Southwestern Electric Power Co [Member] |
|
|
|
|
|
|
|
Book Values and Fair Values of Long - term Debt |
|
|
|
|
|
|
|
Total Long-term Debt Outstanding |
|
|
|
|
2,679.1
|
2,273.5
|
|
Long-term Debt, Fair Value |
|
|
|
|
2,814.3
|
2,417.2
|
|
Cash [Member] |
|
|
|
|
|
|
|
Other Temporary Investments |
|
|
|
|
|
|
|
Cost |
[3] |
|
|
|
211.7
|
271.0
|
|
Gross Unrealized Gains |
[3] |
|
|
|
0.0
|
0.0
|
|
Gross Unrealized Losses |
[3] |
|
|
|
0.0
|
0.0
|
|
Estimated Fair Value |
[3],[4] |
|
|
|
211.7
|
271.0
|
|
Fixed Income Funds [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
967.4
|
811.2
|
|
967.4
|
811.2
|
|
Gross Unrealized Gains |
|
|
|
|
29.8
|
40.2
|
|
Other-Than-Temporary Impairments |
|
|
|
|
(7.6)
|
(4.0)
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
967.4
|
811.2
|
|
|
|
|
Fixed Income Funds [Member] | Indiana Michigan Power Co [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
967.4
|
811.2
|
|
967.4
|
811.2
|
|
Gross Unrealized Gains |
|
|
|
|
29.8
|
40.2
|
|
Other-Than-Temporary Impairments |
|
|
|
|
(7.6)
|
(4.0)
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
967.4
|
811.2
|
|
|
|
|
Mutual Funds Fixed Income [Member] |
|
|
|
|
|
|
|
Other Temporary Investments |
|
|
|
|
|
|
|
Cost |
[5] |
|
|
|
92.7
|
91.1
|
|
Gross Unrealized Gains |
[5] |
|
|
|
0.0
|
0.0
|
|
Gross Unrealized Losses |
[5] |
|
|
|
(1.0)
|
(0.7)
|
|
Estimated Fair Value |
[5] |
|
|
|
91.7
|
90.4
|
|
Domestic [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
[6] |
1,270.1
|
1,126.9
|
|
1,270.1
|
1,126.9
|
|
Gross Unrealized Gains |
|
|
|
|
677.9
|
571.6
|
|
Other-Than-Temporary Impairments |
|
|
|
|
(79.6)
|
(79.3)
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
[6] |
1,270.1
|
1,126.9
|
|
|
|
|
Domestic [Member] | Indiana Michigan Power Co [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
[6] |
1,270.1
|
1,126.9
|
|
1,270.1
|
1,126.9
|
|
Gross Unrealized Gains |
|
|
|
|
677.9
|
571.6
|
|
Other-Than-Temporary Impairments |
|
|
|
|
(79.6)
|
(79.3)
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
[6] |
1,270.1
|
1,126.9
|
|
|
|
|
Mutual Funds Equity [Member] |
|
|
|
|
|
|
|
Other Temporary Investments |
|
|
|
|
|
|
|
Cost |
|
|
|
|
14.4
|
13.7
|
|
Gross Unrealized Gains |
|
|
|
|
13.9
|
11.7
|
|
Gross Unrealized Losses |
|
|
|
|
0.0
|
0.0
|
|
Estimated Fair Value |
|
|
|
|
28.3
|
25.4
|
|
Cash and Cash Equivalents [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
[7] |
18.7
|
168.3
|
|
18.7
|
168.3
|
|
Gross Unrealized Gains |
|
|
|
|
0.0
|
0.0
|
|
Other-Than-Temporary Impairments |
|
|
|
|
0.0
|
0.0
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
[7] |
18.7
|
168.3
|
|
|
|
|
Cash and Cash Equivalents [Member] | Indiana Michigan Power Co [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
[7] |
18.7
|
168.3
|
|
18.7
|
168.3
|
|
Gross Unrealized Gains |
|
|
|
|
0.0
|
0.0
|
|
Other-Than-Temporary Impairments |
|
|
|
|
0.0
|
0.0
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
[7] |
18.7
|
168.3
|
|
|
|
|
US Government Agencies Debt Securities [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
785.4
|
731.1
|
|
785.4
|
731.1
|
|
Gross Unrealized Gains |
|
|
|
|
27.1
|
35.9
|
|
Other-Than-Temporary Impairments |
|
|
|
|
(5.5)
|
(2.6)
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
785.4
|
731.1
|
|
|
|
|
US Government Agencies Debt Securities [Member] | Indiana Michigan Power Co [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
785.4
|
731.1
|
|
785.4
|
731.1
|
|
Gross Unrealized Gains |
|
|
|
|
27.1
|
35.9
|
|
Other-Than-Temporary Impairments |
|
|
|
|
(5.5)
|
(2.6)
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
785.4
|
731.1
|
|
|
|
|
Corporate Debt Securities [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
60.9
|
57.9
|
|
60.9
|
57.9
|
|
Gross Unrealized Gains |
|
|
|
|
2.3
|
3.2
|
|
Other-Than-Temporary Impairments |
|
|
|
|
(1.4)
|
(1.1)
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
60.9
|
57.9
|
|
|
|
|
Corporate Debt Securities [Member] | Indiana Michigan Power Co [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
60.9
|
57.9
|
|
60.9
|
57.9
|
|
Gross Unrealized Gains |
|
|
|
|
2.3
|
3.2
|
|
Other-Than-Temporary Impairments |
|
|
|
|
(1.4)
|
(1.1)
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
60.9
|
57.9
|
|
|
|
|
State and Local Jurisdiction [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
121.1
|
22.2
|
|
121.1
|
22.2
|
|
Gross Unrealized Gains |
|
|
|
|
0.4
|
1.1
|
|
Other-Than-Temporary Impairments |
|
|
|
|
(0.7)
|
(0.3)
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
121.1
|
22.2
|
|
|
|
|
State and Local Jurisdiction [Member] | Indiana Michigan Power Co [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
121.1
|
22.2
|
|
121.1
|
22.2
|
|
Gross Unrealized Gains |
|
|
|
|
0.4
|
1.1
|
|
Other-Than-Temporary Impairments |
|
|
|
|
(0.7)
|
$ (0.3)
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
121.1
|
$ 22.2
|
|
|
|
|
Within One Year [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
229.5
|
|
|
229.5
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
229.5
|
|
|
|
|
|
Within One Year [Member] | Indiana Michigan Power Co [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
229.5
|
|
|
229.5
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
229.5
|
|
|
|
|
|
One Year To Five Year [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
335.3
|
|
|
335.3
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
335.3
|
|
|
|
|
|
One Year To Five Year [Member] | Indiana Michigan Power Co [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
335.3
|
|
|
335.3
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
335.3
|
|
|
|
|
|
Five Year To Ten Year [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
204.6
|
|
|
204.6
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
204.6
|
|
|
|
|
|
Five Year To Ten Year [Member] | Indiana Michigan Power Co [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
204.6
|
|
|
204.6
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
204.6
|
|
|
|
|
|
After 10 years [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
198.0
|
|
|
198.0
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
198.0
|
|
|
|
|
|
After 10 years [Member] | Indiana Michigan Power Co [Member] |
|
|
|
|
|
|
|
Nuclear Trust Fund Investments |
|
|
|
|
|
|
|
Estimated Fair Value |
|
198.0
|
|
|
$ 198.0
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
|
|
|
|
|
|
Contractual Maturities, Fair Value of Debt Securities |
|
$ 198.0
|
|
|
|
|
|
|
|