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Stockholders' Equity (Equity Securities Offering) (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
1 Months Ended 3 Months Ended 12 Months Ended
Feb. 20, 2018
Apr. 30, 2019
Mar. 31, 2019
Dec. 31, 2018
Jun. 30, 2018
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Forward Contract Indexed to Issuer's Equity [Line Items]                
Maximum value of shares to be issued under ATM program $ 1,000              
Proceeds from the issuance of common stock           $ 13 $ 21  
Stock issued during period, shares, DRIP               2.2
Stock issued during period, value, DRIP               $ 174
ATM [Member]                
Forward Contract Indexed to Issuer's Equity [Line Items]                
Shares, Issued       2.6       2.6
Proceeds from the issuance of common stock       $ 195        
ATM [Member] | Wells Fargo Bank [Member]                
Forward Contract Indexed to Issuer's Equity [Line Items]                
Shares offered         1.3      
ATM [Member] | JPMorgan Chase [Member]                
Forward Contract Indexed to Issuer's Equity [Line Items]                
Shares offered     1.1     1.1    
Forward contract indexed to issuer's equity, forward rate per share     $ 89.83          
ATM [Member] | Citibank [Member]                
Forward Contract Indexed to Issuer's Equity [Line Items]                
Shares offered         1.3      
Equity Forwards [Member]                
Forward Contract Indexed to Issuer's Equity [Line Items]                
Shares offered             21.3  
Settlement alternative, shares             21.3  
Shares, Issued       10.6 10.6     10.6
Proceeds from the issuance of common stock       $ 766 $ 781      
Subsequent Event [Member] | ATM [Member] | Citibank [Member]                
Forward Contract Indexed to Issuer's Equity [Line Items]                
Shares offered   1.1            
Forward contract indexed to issuer's equity, forward rate per share   $ 88.82