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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