Stock-Based Compensation Stock-Based Compensation - Stock Option Activity (Details) - Stock Options - Fuel Tech Incentive Plan $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2017 USD ($) $ / shares shares
Number of Options
Outstanding, beginning of period (in shares) | shares	1,039,750
Granted (in shares) | shares	0
Exercised (in shares) | shares	0
Expired or forfeited (in shares) | shares	(50,000)
Outstanding, end of period (in shares) | shares	989,750
Number of Shares, exercisable (in shares) | shares	989,750
Weighted- Average Exercise Price
Outstanding, beginning of period (in dollars per share) | $ / shares	$ 8.39
Granted (in dollars per share) | $ / shares	0.00
Exercised (in dollars per share) | $ / shares	0.00
Expired or forfeited (in dollars per share) | $ / shares	26.15
Outstanding, end of period (in dollars per share) | $ / shares	7.49
Weighted average exercise price, exercisable (in dollars per share) | $ / shares	$ 7.49
Weighted- Average Remaining Contractual Term, Outstanding	4 years 3 months 7 days
Weighted- Average Remaining Contractual Term, Exercisable	4 years 3 months 7 days
Aggregate Intrinsic Value, Outstanding | $	$ 0
Aggregate Intrinsic Value, Exercisable | $	$ 0