3.	CASH, CASH EQUIVALENTS AND MARKETABLE SECURITIES

Cash and cash equivalents primarily consist of deposits held at major banks, Tier-1 commercial paper and other securities with original maturities of 90 days or less. Marketable securities consist of Tier-1 commercial paper debt securities, corporate debt securities and certain other securities.

The amortized principal amounts of our cash, cash equivalents and marketable securities approximated their fair values at May 31, 2018 and 2017. We use the specific identification method to determine any realized gains or losses from the sale of our marketable securities classified as available-for-sale. Such realized gains and losses were insignificant for fiscal 2018, 2017 and 2016. The following table summarizes the components of our cash equivalents and marketable securities held, substantially all of which were classified as available-for-sale:

	May 31,
(in millions)	2018		2017
Corporate debt securities and other	$	44,302	$	41,618
Commercial paper debt securities		1,647		5,053
Money market funds		6,500		3,302
Total investments	$	52,449	$	49,973
Investments classified as cash equivalents	$	6,808	$	5,679
Investments classified as marketable securities	$	45,641	$	44,294

As of May 31, 2018 and 2017, approximately 26% and 32%, respectively, of our marketable securities investments mature within one year and 74% and 68%, respectively, mature within one to five years. Our investment portfolio is subject to market risk due to changes in interest rates. As described above, we limit purchases of marketable debt securities to investment-grade securities, which have high credit ratings and also limit the amount of credit exposure to any one issuer. As stated in our investment policy, we are averse to principal loss and seek to preserve our invested funds by limiting default risk and market risk.

Restricted cash that was included within cash and cash equivalents as presented within our consolidated balance sheets as of May 31, 2018 and 2017 and our consolidated statements of cash flows for the years ended May 31, 2018, 2017 and 2016 was nominal.