Our investment policy is consistent with the definition of available-for-sale securities. We do not buy and hold securities principally for the purpose of selling them in the near future nor do we intend to hold securities to maturity. Rather, our policy is focused on the preservation of capital, liquidity and return. From time to time, we may sell certain securities but the objectives are generally not to generate profits on short-term differences in price.

The following table summarizes, by major security type, our assets that are measured at fair value on a recurring basis (in thousands):

	September 30, 2019
	Amortized		Gross Unrealized		Gross Unrealized		Estimated Fair
	Cost		Gains		Losses		Value
Corporate debt securities	$	4,996	$	10	$	—	$	5,006
Government treasury bills		—		—		—		—
	$	4,996	$	10	$	—	$	5,006

	December 31, 2018
	Amortized		Gross Unrealized		Gross Unrealized			Estimated Fair
	Cost		Gains		Losses			Value
Corporate debt securities	$	31,977	$	3	$	(7	)	$	31,973
Government treasury bills		26,662		—		(5	)		26,657
	$	58,639	$	3	$	(12	)	$	58,630

There were $3,000 of gross realized gains from the sale or maturity of marketable securities during the nine months ended September 30, 2019 and $0 for the year ended December 31, 2018, respectively.

During the nine months ended September 30, 2019, we recognized gross interest income on securities of $377,000. Interest income was supplemented by accretion income of $453,000 during the nine months ended September 30, 2019, and reported within interest income in the consolidated statements of operations.

During the nine months ended September 30, 2018, we recognized gross interest income on securities of $499,000. Interest income was inclusive of accretion income of $432,000 and offset by amortization expense on securities of $36,000 during the nine months ended September 30, 2018, and reported net within interest income in the consolidated statements of operations.

The estimated fair value of investments by contractual maturity is as follows (in thousands):

	September 30,		December 31,
	2019		2018
Due within one year	$	5,006	$	58,630
Total	$	5,006	$	58,630