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Available-for-Sale Debt Securities - Summary of Available-for-Sale Debt Securities by Contractual Maturity (Details) - USD ($)
$ in Millions
Mar. 31, 2018
Dec. 31, 2017
Amortized Cost    
Within one year $ 16,591  
After one year through five years 8,094  
After five years through ten years 76  
After ten years 27  
Amortized Cost 24,788 $ 29,685
Fair Value    
Within one year 16,550  
After one year through five years 8,003  
After five years through ten years 74  
After ten years 27  
Fair Value $ 24,654 $ 29,587