Investments

The Company's investments are intended to establish a high-quality portfolio that preserves principal, meets liquidity needs, avoids inappropriate concentrations, and delivers an appropriate yield in relationship to the Company's investment guidelines and market conditions. The following is a summary of available-for-sale securities at September 30, 2017 (amounts in thousands):



	Available-for-sale Securities
	Adjusted Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Government agency bonds	$	612,396	$	12	$	(1,456	)	$	610,952
Municipal bonds - taxable	10,000		16		—			10,016
Corporate bonds and debt	393,630		87		(182		)	393,535
Marketable equity securities	707		2,236		—			2,943
Total	$	1,016,733	$	2,351	$	(1,638	)	$	1,017,446

The following is a summary of available-for-sale securities at March 31, 2017 (amounts in thousands):



	Available-for-sale Securities
	Adjusted Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Government agency bonds	$	227,089	$	3	$	(227	)	$	226,865
Municipal bonds - tax-exempt	55,289		—		(10		)	55,279
Municipal bonds - taxable	10,000		43		—			10,043
Corporate bonds and debt	207,888		53		(169		)	207,772
Marketable equity securities	707		879		—			1,586
Total	$	500,973	$	978	$	(406	)	$	501,545

At September 30, 2017, the Company's available-for-sale securities are presented on the condensed consolidated balance sheets as short-term investments of $488.2 million and long-term investments of $529.2 million. At March 31, 2017, the Company's available-for-sale securities are presented on the condensed consolidated balance sheets as short-term investments of $394.1 million and long-term investments of $107.5 million.

The Company had no proceeds from sales of available-for-sale investments during the six months ended September 30, 2017. The Company sold available-for-sale investments for proceeds of $470.6 million during the six months ended September 30, 2016 to finance a portion of the purchase price of its Atmel acquisition which closed on April 4, 2016. An immaterial amount of available-for-sale investments were sold during the three months ended September 30, 2016. The Company had no material realized gains from the sale of available-for-sale securities during the three and six months ended September 30, 2017 and 2016. The Company determines the cost of available-for-sale debt securities sold on a first-in first-out (FIFO) basis at the individual security level for sales from multiple lots. For sales of marketable equity securities, the Company uses an average cost basis at the individual security level. Gains and losses recognized in earnings are credited or charged to other income (expense) on the consolidated statements of operations.

The following tables show all investments in an unrealized loss position for which an other-than-temporary impairment has not been recognized and the related gross unrealized losses and fair value, aggregated by investment category and the length of time that the individual securities have been in a continuous unrealized loss position (amounts in thousands):



	September 30, 2017
	Less than 12 Months					12 Months or Greater				Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Government agency bonds	$	596,240	$	(1,456	)	$	—	$	—	$	596,240	$	(1,456	)
Corporate bonds and debt	251,785		(182		)	—		—		251,785		(182		)
Total	$	848,025	$	(1,638	)	$	—	$	—	$	848,025	$	(1,638	)



	March 31, 2017
	Less than 12 Months					12 Months or Greater				Total
	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Government agency bonds	$	196,875	$	(227	)	$	—	$	—	$	196,875	$	(227	)
Municipal bonds	55,279		(10		)	—		—		55,279		(10		)
Corporate bonds and debt	132,820		(169		)	—		—		132,820		(169		)
Total	$	384,974	$	(406	)	$	—	$	—	$	384,974	$	(406	)

Management does not believe any of the unrealized losses represent an other-than-temporary impairment based on its evaluation of available evidence as of September 30, 2017 and the Company's intent is to hold these investments until these assets are no longer impaired.

The amortized cost and estimated fair value of the available-for-sale securities at September 30, 2017, by contractual maturity, excluding marketable equity securities of $2.9 million, which have no contractual maturity, are shown below (amounts in thousands). Expected maturities can differ from contractual maturities because the issuers of the securities may have the right to prepay obligations without prepayment penalties, and the Company views its available-for-sale securities as available for current operations.



	Adjusted Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
Available-for-sale
Due in one year or less	$	397,536	$	2	$	(177	)	$	397,361
Due after one year and through five years	618,490		113		(1,461		)	617,142
Due after five years and through ten years	—		—		—			—
Due after ten years	—		—		—			—
Total	$	1,016,026	$	115	$	(1,638	)	$	1,014,503