Investments and Fair Value Measurements (Tables)

3 Months Ended

Mar. 31, 2018

Available-for-Sale Securities and Fair Value Measurements

Schedule of available-for-sale securities

		March 31, 2018
				Gross		Gross
		Amortized		Unrealized		Unrealized		Estimated
(In thousands)		Cost		Gains		Losses		Fair Value
US government securities	Level 1	$	99,833	$	—	$	(372)	$	99,461
US government agency securities	Level 2		37,187		—		(89)		37,098
Corporate notes	Level 2		120,945		1		(383)		120,563
Commercial paper	Level 2		73,523		—		(11)		73,512
Marketable securities			331,488		1		(855)		330,634
Money market funds	Level 1		73,184		—		—		73,184
Total		$	404,672	$	1	$	(855)	$	403,818

		December 31, 2017
				Gross		Gross
		Amortized		Unrealized		Unrealized		Estimated
(In thousands)		Cost		Gains		Losses		Fair Value
US government securities	Level 1	$	89,896	$	—	$	(342)	$	89,554
US government agency securities	Level 2		50,891		—		(113)		50,778
Corporate notes	Level 2		141,226		2		(280)		140,948
Commercial paper	Level 2		19,893		—		—		19,893
Marketable securities			301,906		2		(735)		301,173
Money market funds	Level 1		69,055		—		—		69,055
Total		$	370,961	$	2	$	(735)	$	370,228