Stock Options, Restricted Stock and Employee Stock Purchase Plan - Non-vested Restricted Stock Activity (Details) - Restricted Stock - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of Shares
Beginning balance, non-vested (in Shares)	416	253	337
Granted (in Shares)	666	303	0
Vested (in Shares)	(137)	(118)	(84)
Canceled (in Shares)	(57)	(22)	0
Ending balance, non-vested (in Shares)	888	416	253
Weighted Average Per Share Grant Date Fair Value
Beginning balance, non-vested (in Dollars per share)	$ 31.52	$ 35.31	$ 35.31
Granted (in Dollars per share)	33.10	29.50	0.00
Vested (in Dollars per share)	32.55	34.95	35.31
Canceled (in Dollars per share)	31.34	28.85	0.00
Ending balance, non-vested (in Dollars per share)	$ 32.55	$ 31.52	$ 35.31