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Investments - Available-for-Sale (Details)
$ in Millions
Dec. 31, 2017
USD ($)
Security
Dec. 31, 2016
USD ($)
Security
Available-for-sale investments    
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value $ 1,940 $ 841
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses $ 5 $ 4
Number of Positions, 12 Months or Less (in security) | Security 719 580
In a Continuous Loss Position for 12 Months or More, Estimated Fair value $ 227 $ 0
In a Continuous Loss Position for 12 Months or More, Unrealized Losses $ 3 $ 0
Number of Positions, 12 Months or More (in security) | Security 164 0
Corporate debt securities    
Available-for-sale investments    
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value $ 1,297 $ 542
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses $ 3 $ 2
Number of Positions, 12 Months or Less (in security) | Security 561 378
In a Continuous Loss Position for 12 Months or More, Estimated Fair value $ 94 $ 0
In a Continuous Loss Position for 12 Months or More, Unrealized Losses $ 1 $ 0
Number of Positions, 12 Months or More (in security) | Security 69 0
U.S. treasury notes    
Available-for-sale investments    
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value $ 470  
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses $ 1  
Number of Positions, 12 Months or Less (in security) | Security 89  
In a Continuous Loss Position for 12 Months or More, Estimated Fair value $ 0  
In a Continuous Loss Position for 12 Months or More, Unrealized Losses $ 0  
Number of Positions, 12 Months or More (in security) | Security 0  
GSEs    
Available-for-sale investments    
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value $ 173 $ 198
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses $ 1 $ 1
Number of Positions, 12 Months or Less (in security) | Security 69 73
In a Continuous Loss Position for 12 Months or More, Estimated Fair value $ 95 $ 0
In a Continuous Loss Position for 12 Months or More, Unrealized Losses $ 1 $ 0
Number of Positions, 12 Months or More (in security) | Security 47 0
Municipal securities    
Available-for-sale investments    
In a Continuous Loss Position for Less than 12 Months, Estimated Fair Value $ 0 $ 101
In a Continuous Loss Position for Less than 12 Months, Unrealized Losses $ 0 $ 1
Number of Positions, 12 Months or Less (in security) | Security 0 129
In a Continuous Loss Position for 12 Months or More, Estimated Fair value $ 38 $ 0
In a Continuous Loss Position for 12 Months or More, Unrealized Losses $ 1 $ 0
Number of Positions, 12 Months or More (in security) | Security 48 0