2. Cash and Investments

Cash and investments consist of the following (in thousands):

	December 31, 2016		December 31, 2015
Cash and cash equivalents:
Cash	$	123,382	$	162,495
Money market funds		439,330		362,587
Commercial paper		235,783		160,151
Corporate bonds		33,985		16,995
U.S. government and government agency debt securities		19,987		39,989
Total cash and cash equivalents	$	852,467	$	742,217
Marketable securities:
U.S. government and government agency debt securities	$	—	$	144,986
Corporate debt securities		—		100,047
Total	$	—	$	245,033

The following tables summarize our amortized cost, gross unrealized gains and losses and fair value of our available-for-sale investments in marketable securities (in thousands) as of December 31, 2015 (we had no available-for-sale investments in marketable securities as of December 31, 2016):

	December 31, 2015
			Gross		Gross
	Amortized		Unrealized		Unrealized			Aggregate
	Cost		Gains		Losses			Fair Value
U.S. government and government agency debt securities	$	145,066	$	—	$	(80	)	$	144,986
Corporate debt securities		100,093		12		(58	)		100,047
Total	$	245,159	$	12	$	(138	)	$	245,033

The estimated fair value of available-for-sale marketable securities as of December 31, 2015, classified by their contractual maturities was as follows (in thousands):

	December 31, 2015
Due within one year	$	245,033
After one year through three years		—
Total	$	245,033

Changes in market interest rates and bond yields cause certain of our investments to fall below their cost basis, resulting in unrealized losses. As of December 31, 2016, we had unrealized losses of $27 thousand related to investments that had a fair value of $34.0 million. As of December 31, 2015, we had unrealized losses of $0.1 million related to investments that had a fair value of $199.1 million. None of these investments were in a material continuous unrealized loss position for more than 12 months.

	December 31, 2016					December 31, 2015
	Fair Value		Unrealized loss			Fair Value		Unrealized loss
U.S. government and government agency debt securities	$	—	$	—		$	137,485	$	(80	)
Corporate debt securities		33,984		(27	)		61,622		(58	)
Total	$	33,984	$	(27	)	$	199,107	$	(138	)

As of December 31, 2016 and 2015, we did not consider any of our marketable securities to be other-than-temporarily impaired. When evaluating our investments for other-than-temporary impairment, we review factors such as the length of time and extent to which fair value has been below its cost basis, the financial condition of the issuer, our ability and intent to hold the security to maturity and whether it is more likely than not that we will be required to sell the investment before recovery of its cost basis.