Financial Instruments

The following table summarizes our financial assets with maturities of less than 90 days from the date of purchase included in cash and cash equivalents in our condensed consolidated balance sheets:



(In millions)	As of March 31, 2019		As of December 31, 2018
Commercial paper	$	446.0	$	231.2
Overnight reverse repurchase agreements	192.7		—
Money market funds	1,074.4		279.5
Short-term debt securities	168.8		194.8
Total	$	1,881.9	$	705.5

The carrying values of our commercial paper, including accrued interest, overnight reverse repurchase agreements, money market funds and short-term debt securities approximate fair value due to their short-term maturities.

Our marketable equity securities gains (losses) are recorded in other income (expense), net in our condensed consolidated statements of income. The following tables summarize our marketable debt and equity securities:



As of March 31, 2019 (In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Corporate debt securities
Current	$	981.0	$	0.5	$	(0.1	)	$	981.4
Non-current	852.0		2.8		(0.5		)	854.3
Government securities
Current	683.6		0.2		(0.1		)	683.7
Non-current	237.5		0.3		(0.2		)	237.6
Mortgage and other asset backed securities
Current	0.7		—		—			0.7
Non-current	280.3		0.8		(0.3		)	280.8
Total marketable debt securities	$	3,035.1	$	4.6	$	(1.2	)	$	3,038.5
Marketable equity securities, non-current	489.3		421.7		(7.0		)	904.0



As of December 31, 2018 (In millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
Corporate debt securities
Current	$	1,608.4	$	—	$	(0.9	)	$	1,607.5
Non-current	854.9		0.7		(3.9		)	851.7
Government securities
Current	706.1		0.1		(0.4		)	705.8
Non-current	264.0		0.1		(0.3		)	263.8
Mortgage and other asset backed securities
Current	0.1		—		—			0.1
Non-current	260.5		0.4		(0.5		)	260.4
Total marketable debt securities	$	3,694.0	$	1.3	$	(6.0	)	$	3,689.3
Marketable equity securities, non-current	496.2		127.7		(8.5		)	615.4

Summary of Contractual Maturities: Available-for-Sale Securities

The estimated fair value and amortized cost of our marketable debt securities available-for-sale by contractual maturity are summarized as follows:



	As of March 31, 2019				As of December 31, 2018
(In millions)	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Due in one year or less	$	1,665.3	$	1,665.8	$	2,314.6	$	2,313.4
Due after one year through five years	1,234.8		1,237.8		1,235.9		1,232.7
Due after five years	135.0		134.9		143.5		143.2
Total available-for-sale securities	$	3,035.1	$	3,038.5	$	3,694.0	$	3,689.3

The average maturity of our marketable debt securities available-for-sale as of March 31, 2019 and December 31, 2018, were approximately 12 months.

Proceeds from Marketable Debt Securities

The proceeds from maturities and sales of marketable debt securities and resulting realized gains and losses are summarized as follows:



	For the Three Months Ended March 31,
(In millions)	2019			2018
Proceeds from maturities and sales	$	1,489.2		$	4,068.9
Realized gains	$	0.6		$	1.8
Realized losses	$	(0.3	)	$	(9.4	)

Strategic Investments

As of March 31, 2019, our strategic investment portfolio was comprised of investments totaling $1,049.7 million, of which $90.1 million was reflected as a component of other current assets in our condensed consolidated balance sheet, with the remaining balance included in investments and other assets. As of December 31, 2018, our strategic investment portfolio was comprised of investments totaling $676.3 million, which is included in investments and other assets in our condensed consolidated balance sheet.

Our strategic investment portfolio includes investments in equity securities of certain biotechnology companies, which are reflected within our disclosures included in Note 7, Fair Value Measurements, to these condensed consolidated financial statements, venture capital funds where the underlying investments are in equity securities of certain biotechnology companies and non-marketable equity securities.

Our investments in equity securities include approximately 11.5 million shares of Ionis' common stock, acquired in June 2018 at a cost of approximately $625.0 million, which is remeasured each reporting period and carried at fair value. This investment is classified as a Level 2 marketable security due to certain holding period restrictions. The remainder of our investments in equity securities of certain publicly-traded biotechnology companies are regularly measured and carried at fair value and classified as Level 1. The effect of the holding period restrictions on our Ionis stock valuation are estimated using an option pricing valuation model. The most significant assumptions within the model are the term of the restrictions and the stock price volatility, which is based upon historical volatility of similar companies. We also use a constant maturity risk-free interest rate to match the remaining term of the restrictions on our investment in Ionis' common stock and a dividend yield of zero based upon the fact that Ionis and similar companies generally have not historically granted cash dividends.

The increase in our strategic investment portfolio primarily reflects an increase in the fair value in our investment in Ionis' common stock as well as an increase in the value of a non-marketable equity security.

For additional information on our June 2018 investment in Ionis' common stock, please read Note 19, Collaborative and Other Relationships, to our consolidated financial statements included in our 2018 Form 10-K.