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Available-for-sale securities - Narrative (Details)
12 Months Ended
Dec. 31, 2017
USD ($)
security
Dec. 31, 2016
USD ($)
security
Investments, Debt and Equity Securities [Abstract]    
Cash Excluded from Available-for-Sale Debt Securities Table $ 2,400,000,000 $ 2,500,000,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions (securities) | security 2,957 2,709
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities $ 0 $ 0