Investments (Tables)

12 Months Ended

Dec. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Schedule of maturities for short-term investments

The estimated fair values of short-term investments, which approximate cost at December 31, 2017, are shown below by contractual maturity. Actual maturities may differ from contractual maturities because issuers of the securities may have the right to retire our investments without prepayment penalties.

(in millions)	Available- For-Sale
Due in one year or less	$	323
Due after one year through three years	465
Due after three years through five years	19
Due after five years	18
Total	$	825