Available for Sale Securities (Tables)

12 Months Ended

Dec. 31, 2017

Investments Debt And Equity Securities [Abstract]

Summary of Available for Sale Securities at Fair Value

Available for sale securities at December 31, 2017 and 2016 consist of the following:

			Gross		Gross
			Unrealized		Unrealized
	Amortized Cost		Gains		Losses			Fair Value
	(in thousands)
December 31, 2017
Cash and cash equivalents	$	70,156	$	—	$	—		$	70,156
Available for sale securities:
Certificates of deposit	$	14,117	—		—			$	14,117
U.S. government debt securities		175,155	—			(352	)		174,803
Corporate debt securities		58,806	—			(90	)		58,716
Total available for sale securities	$	248,078	$	—	$	(442	)	$	247,636
Total cash, cash equivalents, and available for sale securities	$	318,234	$	—	$	(442	)	$	317,792

			Gross		Gross
			Unrealized		Unrealized
	Amortized Cost		Gains		Losses			Fair Value
	(in thousands)
December 31, 2016
Cash and cash equivalents	$	187,335	$	—	$	—		$	187,335
Available for sale securities:
Certificates of deposit	$	12,698	—			—		$	12,698
U.S. government debt securities		50,952	—			(32	)		50,920
Corporate debt securities		9,398	—			(8	)		9,390
Total available for sale securities	$	73,048	$	—	$	(40	)	$	73,008
Total cash, cash equivalents, and available for sale securities	$	260,383	$	—	$	(40	)	$	260,343

Estimated Fair Value of Available for Sale Securities at Fair Value by Contractual Maturity

The estimated fair value of the Company’s available for sale securities balance at December 31, 2017, by contractual maturity, is as follows:  

Due in one year or less	$	217,884
Due after one year		29,752
Total available for sale securities	$	247,636