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Fair Value Measurements (Available for Sale Securities) (Details) - USD ($) $ in Millions | 1 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jan. 31, 2018 | Sep. 30, 2018 | Sep. 30, 2017 | Sep. 30, 2018 | Sep. 30, 2017 | Dec. 31, 2017
Debt Securities [Member] |  |  |  |  |  |
Marketable Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Realized Gains |  | $ 5 |  | $ 32 |  |
Realized Losses |  | 9 |  | 49 |  |
Proceeds from sale or maturity of securities |  | 576 |  | 1,889 |  |
debt and equity securities [Member] |  |  |  |  |  |
Marketable Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Realized Gains |  |  | $ 29 |  | $ 106 |
Realized Losses |  |  | 15 |  | 58 |
Proceeds from sale or maturity of securities |  |  | 518 |  | 1,772 |
Available for sale securities: Special Use Funds - Debt Securities [Member] |  |  |  |  |  |
Marketable Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Unrealized gains |  |  |  | 8 |  | $ 37
Available-for-sale Securities, Gross Unrealized Loss |  |  |  | 47 |  | 12
Fair value of available for sale securities in an unrealized loss position |  | 1,626 |  | 1,626 |  | 918
FPL [Member] | Debt Securities [Member] |  |  |  |  |  |
Marketable Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Realized Gains |  | 2 |  | 22 |  |
Realized Losses |  | 5 |  | 34 |  |
Proceeds from sale or maturity of securities |  | 487 |  | 1,529 |  |
FPL [Member] | debt and equity securities [Member] |  |  |  |  |  |
Marketable Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Realized Gains |  |  | 9 |  | 35 |
Realized Losses |  |  | 8 |  | 34 |
Proceeds from sale or maturity of securities |  |  | $ 329 |  | $ 1,166 |
FPL [Member] | Available for sale securities: Special Use Funds - Debt Securities [Member] |  |  |  |  |  |
Marketable Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Unrealized gains |  |  |  | 6 |  | 28
Available-for-sale Securities, Gross Unrealized Loss |  |  |  | 37 |  | 9
Fair value of available for sale securities in an unrealized loss position |  | $ 1,253 |  | $ 1,253 |  | $ 670
AOCI Including Portion Attributable to Noncontrolling Interest [Member] | Accounting Standards Update 2016-01 [Member] |  |  |  |  |  |
Marketable Securities, Available-for-sale [Line Items] |  |  |  |  |  |
Net reduction to retained earnings | $ 312 |  |  |  |  |

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