Marketable Securities (Tables)

12 Months Ended

Dec. 31, 2017

Available-for-sale Securities [Abstract]

Cash equivalents and marketable securities classified as available-for-sale

Total cash, cash equivalents and marketable securities are summarized as follows:

	As of December 31, 2017
(Dollars in thousands)	Fair Value		Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Weighted Rate of Return
Money market funds	$	10,915	$	10,915	$	—	$	—		1.16	%
U.S. Government bonds and notes	55,220		55,221		—		(1		)	1.12	%
Corporate notes, bonds, commercial paper and other	195,073		195,204		—		(131		)	1.39	%
Total cash equivalents and marketable securities	261,208		261,340		—		(132		)
Cash	68,168		68,168		—		—
Total cash, cash equivalents and marketable securities	$	329,376	$	329,508	$	—	$	(132	)

	As of December 31, 2016
(Dollars in thousands)	Fair Value		Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Weighted Rate of Return
Money market funds	$	10,681	$	10,681	$	—	$	—		0.41	%
U.S. Government bonds and notes	48,292		48,291		1		—			0.39	%
Corporate notes, bonds, commercial paper and other	62,178		62,199		—		(21		)	0.66	%
Total cash equivalents and marketable securities	121,151		121,171		1		(21		)
Cash	51,031		51,031		—		—
Total cash, cash equivalents and marketable securities	$	172,182	$	172,202	$	1	$	(21	)

Available-for-sale securities reported at fair value

Available-for-sale securities are reported at fair value on the balance sheets and classified as follows:

	As of
	December 31, 2017		December 31, 2016
	(Dollars in thousands)
Cash equivalents	$	157,676	$	84,263
Short term marketable securities	103,532		36,888
Total cash equivalents and marketable securities	261,208		121,151
Cash	68,168		51,031
Total cash, cash equivalents and marketable securities	$	329,376	$	172,182

Estimated fair value of cash equivalents and marketable securities classified by date of contractual maturity and the length of time that the securities have been in a continuous unrealized loss position

The estimated fair value of cash equivalents and marketable securities classified by the length of time that the securities have been in a continuous unrealized loss position at December 31, 2017 and 2016 are as follows:

	Fair Value				Gross Unrealized Loss
	December 31, 2017		December 31, 2016		December 31, 2017			December 31, 2016
	(In thousands)
Less than one year
U.S. Government bonds and notes	$	42,581	$	18,395	$	(1	)	$	—
Corporate notes, bonds and commercial paper	194,015		54,377		(131		)	(21		)
Total Corporate notes, bonds, and commercial paper and U.S. Government bonds and notes	$	236,596	$	72,772	$	(132	)	$	(21	)