Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements - Investments in Unrealized Loss Position (Details)

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Fair Value Measurements - Investments in Unrealized Loss Position (Details) - Debt Securities [Member] - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Balance, beginning of period $ 152 $ 125 $ 151 $ 58
Purchases of investments 3 1 6 67
Payments on investments (1) (1) (1) (1)
Unrealized gains (losses) on investments recorded in AOCI 0 1 (2) 2
Balance, end of period $ 154 $ 126 $ 154 $ 126