Available for Sale Securities (Tables)

9 Months Ended

Sep. 30, 2018

Investments Debt And Equity Securities [Abstract]

Summary of Available for Sale Securities at Fair Value

Available for sale securities at September 30, 2018 and December 31, 2017 consist of the following:

			Gross		Gross
			Unrealized		Unrealized
	Amortized Cost		Gains		Losses			Fair Value
	(in thousands)
September 30, 2018
Cash and cash equivalents	$	162,430	$	—	$	—		$	162,430
Available for sale securities:
Certificates of deposit	$	445	$	—	$	—		$	445
U.S. government debt securities		177,181		—		(363	)		176,818
Corporate debt securities		50,497		—		(46	)		50,451
Total available for sale securities	$	228,123	$	—	$	(409	)	$	227,714
Total cash, cash equivalents, and available for sale securities	$	390,553	$	—	$	(409	)	$	390,144

			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

Estimated Fair Value of Available for Sale Securities by Contractual Maturity

The estimated fair value of the Company’s available for sale securities balance at September 30, 2018, by contractual maturity, is as follows:

Due in one year or less	$	212,725
Due after one year		14,989
Total available for sale securities	$	227,714

Summary of Available for Sale Securities that were in Continuous Unrealized Loss Position

The following table summarizes the Company’s available for sale securities that were in a continuous unrealized loss position, but were not deemed to be other-than temporarily impaired, as of September 30, 2018 and December 31, 2017:

	Unrealized Loss for Less Than 12 Months					Unrealized Loss for 12 Months or More					Total
	Gross					Gross					Gross
	Unrealized			Estimated		Unrealized			Estimated		Unrealized			Estimated
	Losses			Fair Value		Losses			Fair Value		Losses			Fair Value
	(in thousands)
September 30, 2018
Available for sale securities:
U.S. government debt securities	$	(342	)	$	158,845	$	(21	)	$	14,981	$	(363	)	$	173,826
Corporate debt securities		(38	)		42,009		(8	)		7,470		(46	)		49,479
Total	$	(380	)	$	200,854	$	(29	)	$	22,451	$	(409	)	$	223,305

	Unrealized Loss for Less Than 12 Months					Unrealized Loss for 12 Months or More					Total
	Gross					Gross					Gross
	Unrealized			Estimated		Unrealized			Estimated		Unrealized			Estimated
	Losses			Fair Value		Losses			Fair Value		Losses			Fair Value
	(in thousands)
December 31, 2017
Available for sale securities:
U.S. government debt securities	$	(343	)	$	170,812	$	(9	)	$	3,991	$	(352	)	$	174,803
Corporate debt securities		(90	)		58,716		—			—		(90	)		58,716
Total	$	(433	)	$	229,528	$	(9	)	$	3,991	$	(442	)	$	233,519