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Investments in debt and equity securities (Details 2) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Less than 12 months    
Fair Value $ 587 $ 376
Unrealized Losses 9 10
12 months or more    
Fair Value 163 38
Unrealized Losses 5 3
Total    
Fair Value 750 414
Unrealized Losses 14 13
Corporate bonds    
Less than 12 months    
Fair Value 312 131
Unrealized Losses 2 1
12 months or more    
Fair Value 38 13
Unrealized Losses 0 0
Total    
Fair Value 350 144
Unrealized Losses 2 1
U.S. governmental agency mortgage-backed securities    
Less than 12 months    
Fair Value 129 167
Unrealized Losses 1 2
12 months or more    
Fair Value 110 11
Unrealized Losses 3 0
Total    
Fair Value 239 178
Unrealized Losses 4 2
Large capitalization value    
Less than 12 months    
Fair Value 129 68
Unrealized Losses 5 6
12 months or more    
Fair Value 14 11
Unrealized Losses 2 2
Total    
Fair Value 143 79
Unrealized Losses 7 8
Smaller company growth    
Less than 12 months    
Fair Value 17 10
Unrealized Losses 1 1
12 months or more    
Fair Value 1 3
Unrealized Losses 0 1
Total    
Fair Value 18 13
Unrealized Losses $ 1 $ 2