Stock Option and Compensation Plans - Options Outstanding and Exercisable by Exercise Price and Options Exercised (Details) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares
Options Outstanding
Total Shares Underlying Options Outstanding (in shares) | shares	4,818
Weighted Average Remaining Life (Years)	7 years 10 months 24 days
Weighted Average Exercise Price (in dollars per share)	$ 16.94
Aggregate Intrinsic Value of Options Outstanding | $	$ 73
Options Exercisable
Number of Shares for which Options are Exercisable (in shares) | shares	2,099
Weighted Average Exercise Price (in dollars per share)	$ 29.15
Aggregate Intrinsic Value of Options Exercisable | $	$ 71
$0.12-$10.03
Options Outstanding
Total Shares Underlying Options Outstanding (in shares) | shares	3,078
Weighted Average Remaining Life (Years)	8 years 10 months 24 days
Weighted Average Exercise Price (in dollars per share)	$ 3.64
Options Exercisable
Number of Shares for which Options are Exercisable (in shares) | shares	577
Weighted Average Exercise Price (in dollars per share)	$ 5.19
Exercise prices, low end of range (in dollars per share)	0.12
Exercise prices, high end of range (in dollars per share)	$ 10.03
$10.04-$20.00
Options Outstanding
Total Shares Underlying Options Outstanding (in shares) | shares	133
Weighted Average Remaining Life (Years)	5 years
Weighted Average Exercise Price (in dollars per share)	$ 13.68
Options Exercisable
Number of Shares for which Options are Exercisable (in shares) | shares	133
Weighted Average Exercise Price (in dollars per share)	$ 13.68
Exercise prices, low end of range (in dollars per share)	10.04
Exercise prices, high end of range (in dollars per share)	$ 20.00
$20.01-$30.00
Options Outstanding
Total Shares Underlying Options Outstanding (in shares) | shares	114
Weighted Average Remaining Life (Years)	5 years 4 months 24 days
Weighted Average Exercise Price (in dollars per share)	$ 24.69
Options Exercisable
Number of Shares for which Options are Exercisable (in shares) | shares	114
Weighted Average Exercise Price (in dollars per share)	$ 24.69
Exercise prices, low end of range (in dollars per share)	20.01
Exercise prices, high end of range (in dollars per share)	$ 30.00
$30.01-$40.00
Options Outstanding
Total Shares Underlying Options Outstanding (in shares) | shares	731
Weighted Average Remaining Life (Years)	5 years
Weighted Average Exercise Price (in dollars per share)	$ 33.00
Options Exercisable
Number of Shares for which Options are Exercisable (in shares) | shares	731
Weighted Average Exercise Price (in dollars per share)	$ 33.00
Exercise prices, low end of range (in dollars per share)	30.01
Exercise prices, high end of range (in dollars per share)	$ 40.00
$40.01-$55.00
Options Outstanding
Total Shares Underlying Options Outstanding (in shares) | shares	522
Weighted Average Remaining Life (Years)	7 years 1 month 6 days
Weighted Average Exercise Price (in dollars per share)	$ 46.49
Options Exercisable
Number of Shares for which Options are Exercisable (in shares) | shares	378
Weighted Average Exercise Price (in dollars per share)	$ 46.29
Exercise prices, low end of range (in dollars per share)	40.01
Exercise prices, high end of range (in dollars per share)	$ 55.00
$55.01-$73.22
Options Outstanding
Total Shares Underlying Options Outstanding (in shares) | shares	240
Weighted Average Remaining Life (Years)	7 years 7 months 6 days
Weighted Average Exercise Price (in dollars per share)	$ 72.33
Options Exercisable
Number of Shares for which Options are Exercisable (in shares) | shares	166
Weighted Average Exercise Price (in dollars per share)	$ 71.94
Exercise prices, low end of range (in dollars per share)	55.01
Exercise prices, high end of range (in dollars per share)	$ 73.22