Fair Value Measurements

The following is a summary of available-for-sale marketable securities held as of March 31, 2018 and December 31, 2017 (in thousands):



			Gross Unrealized							Classification on Balance Sheet
	Amortized Cost		Gains		Losses			Aggregate Fair Value		Short-Term Marketable Securities		Long-Term Marketable Securities
As of March 31, 2018
Commercial paper	$	5,707	$	—	$	(17	)	$	5,690	$	5,690	$	—
Corporate bonds	734,080		—		(7,253		)	726,827		301,960		424,867
U.S. government agency obligations	219,515		—		(1,887		)	217,628		140,145		77,483
	$	959,302	$	—	$	(9,157	)	$	950,145	$	447,795	$	502,350

As of December 31, 2017
Commercial paper	$	6,951	$	—	$	(9	)	$	6,942	$	6,942	$	—
Corporate bonds	736,902		2		(3,829		)	733,075		289,378		443,697
U.S. government agency obligations	220,014		—		(1,764		)	218,250		102,234		116,016
	$	963,867	$	2	$	(5,602	)	$	958,267	$	398,554	$	559,713

The Company offers certain eligible employees the ability to participate in a non-qualified deferred compensation plan. The mutual funds held by the Company that are associated with this plan are classified as restricted trading securities. These securities are not included in the available-for-sale securities table above but are included in marketable securities in the consolidated balance sheets.

Unrealized gains and unrealized temporary losses on investments classified as available-for-sale are included within accumulated other comprehensive loss in the consolidated balance sheets. Upon realization, those amounts are reclassified from accumulated other comprehensive loss to interest income in the consolidated statements of income. As of March 31, 2018, the Company held for investment corporate bonds with a fair value of $558.7 million, which are classified as available-for-sale marketable securities and have been in a continuous unrealized loss position for more than 12 months. The unrealized losses of $4.9 million related to these corporate bonds are included in accumulated other comprehensive income as of March 31, 2018. The unrealized losses are attributable to changes in interest rates. Based on the evaluation of available evidence, the Company does not believe any unrealized losses represent other than temporary impairments.

The following table details the fair value measurements within the fair value hierarchy of the Company’s financial assets and liabilities as of March 31, 2018 and December 31, 2017 (in thousands):



	Total Fair Value			Fair Value Measurements at Reporting Date Using
	Total Fair Value			Level 1		Level 2		Level 3
As of March 31, 2018
Cash Equivalents and Marketable Securities:
Money market funds	$	7,059		$	7,059	$	—	$	—
Commercial paper	5,690			—		5,690		—
Corporate bonds	726,827			—		726,827		—
U.S. government agency obligations	217,628			—		217,628		—
Mutual funds	9,975			9,975		—		—
	$	967,179		$	17,034	$	950,145	$	—

Liabilities:
Contingent consideration related to a completed acquisition	$	(5,600	)	$	—	$	—	$	(5,600	)

As of December 31, 2017
Cash Equivalents and Marketable Securities:
Money market funds	$	22,649		$	22,649	$	—	$	—
Commercial paper	10,928			—		10,928		—
Corporate bonds	733,075			—		733,075		—
U.S. government agency obligations	218,248			—		218,248		—
Mutual funds	7,879			7,879		—		—
	$	992,779		$	30,528	$	962,251	$	—

Liabilities:
Contingent consideration related to a completed acquisition	$	(8,631	)	$	—	$	—	$	(8,631	)

As of March 31, 2018 and December 31, 2017, the Company grouped money market funds and mutual funds using a Level 1 valuation because market prices for such investments are readily available in active markets. As of March 31, 2018 and December 31, 2017, the Company grouped commercial paper, corporate bonds and U.S. government agency obligations using a Level 2 valuation because quoted prices for identical or similar assets are available in markets that are inactive. The Company did not have any transfers of assets between Level 1, Level 2 or Level 3 of the fair value measurement hierarchy during the three months ended March 31, 2018.

When developing fair value estimates, the Company maximizes the use of observable inputs and minimizes the use of unobservable inputs. When available, the Company uses quoted market prices to measure fair value. The valuation technique used to measure fair value for the Company's Level 1 and Level 2 assets is a market approach, using prices and other relevant information generated by market transactions involving identical or comparable assets. If market prices are not available, the fair value measurement is based on models that use primarily market-based parameters including yield curves, volatilities, credit ratings and currency rates. In certain cases where market rate assumptions are not available, the Company is required to make judgments about assumptions market participants would use to estimate the fair value of a financial instrument.

The valuation technique used to measure fair value of the Company's Level 3 liabilities, which consist of contingent consideration related to the acquisition of Cyberfend, Inc. in 2016, was primarily an income-based approach. The significant unobservable input used in the fair value measurement of the contingent consideration was the likelihood of achieving certain post-closing financial results.

Contractual maturities of the Company’s available-for-sale marketable securities held as of March 31, 2018 and December 31, 2017 were as follows (in thousands):



	March 31, 2018		December 31, 2017
Due in 1 year or less	$	447,795	$	398,554
Due after 1 year through 3 years	502,350		559,713
	$	950,145	$	958,267

The following table reflects the activity for the Company’s major classes of liabilities measured at fair value using Level 3 inputs during the three months ended March 31, 2018 (in thousands):



	Other Liabilities: Contingent Consideration Obligation
Balance as of January 1, 2018	$	(8,631	)
Fair value adjustment to contingent consideration included in general and administrative expense	(1,135		)
Cash paid upon achievement of milestone	4,166
Balance as of March 31, 2018	$	(5,600	)