Stock Award Plans and Stock Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Stock Award Plans and Stock Based Compensation

Schedule of estimated weighted-average assumptions used in calculation of fair value of options granted

	Year ended December 31,
	2017	2016	2015
Weighted-average expected volatility	N/A	49.3% — 56.7%	56.2% — 64.7%
Weighted-average expected term	N/A	4.7 years	4.6 — 4.7 years
Risk-free interest rate	N/A	1.1% — 2.0%	1.3% — 1.6%
Expected dividend yield	N/A	0%	0%

Summary of stock option activity

				Weighted
		Weighted		Average
		Average		Remaining	Aggregate
		Exercise		Contractual	Intrinsic
	Options	Price		Term	Value
	(in thousands)			(years)	(in thousands)
Outstanding at December 31, 2016	4,951	$	7.29
Granted	—		—
Exercised	(2,358)		6.58
Canceled	(15)		10.24
Expired	(2)		21.32
Outstanding at December 31, 2017	2,576	$	7.91	3.21	$	53,571
Exercisable at December 31, 2017	2,008	$	7.16	2.92	$	43,236
Options Vested or Expected to Vest at December 31, 2017(1)	2,555	$	7.88	3.25	$	53,189

(1)   In addition to the vested options, the Company expects a portion of the unvested options to vest at some point in the future. Options expected to vest is calculated by applying an estimated forfeiture rate to the unvested options.

Schedule of changes in the Company's non-vested restricted stock units

		Weighted-Average
		Grant Date Fair
	Shares/units	Value per Share
	(in thousands)
Outstanding at December 31, 2016	548	$	9.85
Granted	320		20.72
Vested	(196)		11.32
Forfeited	(55)		10.36
Outstanding at December 31, 2017	617	$	14.93