Cash, money market funds and certificates of deposit with maturities of three months or less when purchased are included in Cash and cash equivalents in the accompanying condensed consolidated statements of financial position. As of September 30, 2019 and December 31, 2018, marketable securities primarily consisted of treasury bills with maturities of less than one year and are classified as available-for-sale securities. The amortized cost, fair value and gross unrealized gains and losses of our short-term investments are as follows:



			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
(In millions)	Cost		Gains		Losses		Value
Available-for-sale securities, September 30, 2019
Debt securities	$	7	$	—	$	—	$	7
Available-for-sale securities, December 31, 2018
Debt securities	$	9	$	—	$	—	$	9

There were no unrealized losses which had been in a loss position for more than one year as of September 30, 2019 and December 31, 2018.

Gains and losses on sales of investments, as determined on a specific identification basis, are included in investment income in the period they are realized. For the three and nine months ended September 30, 2019 and 2018, there were no gross realized gains or losses resulting from sales of available-for-sale securities. The table below summarizes proceeds and maturities for the periods indicated.



	Three Months Ended				Nine Months Ended
	September 30,				September 30,
(In millions)	2019		2018		2019		2018
Proceeds from sale of securities	$	—	$	16	$	—	$	17
Maturities of securities		5		6		9		15

We periodically review our portfolio of investments to determine whether there has been an other-than-temporary decline in the value of the investments from factors such as deterioration in the financial condition of the issuer or the market(s) in which the issuer competes. There were no impairment charges due to declines in the value of these investments for the three and nine months ended September 30, 2019 and 2018.