Note 12 - Share-based Compensation - Summary of Significant Assumptions for Share-based Compensation Awards (Details) - $ / shares		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016		Dec. 31, 2015
Risk free interest rate
Expected award (Year)		5 years
Stock Appreciation Rights (SARs) [Member]
Exercise price
Exercise price (in dollars per share)		$ 16.97	$ 16.35		$ 18.66
Market price
Market price (in dollars per share)		$ 16.97	$ 16.35		$ 18.66
Risk free interest rate
Risk free interest rate	[1]	1.90%	1.30%		1.50%
Expected award (Year)	[2]	5 years
Expected volatility
Expected volatility	[3]	36.60%	36.50%		34.90%
Expected dividend yield
Expected dividend yield	[4]	2.10%	2.00%		1.60%
Employee Stock Option [Member] | Minimum [Member]
Exercise price
Exercise price (in dollars per share)		$ 16.97	$ 16.35		$ 16.78
Market price
Market price (in dollars per share)		$ 16.97	$ 16.35		$ 16.78
Risk free interest rate
Risk free interest rate		1.80%	1.10%	[1]	1.50%	[1]
Expected award (Year)	[2]	5 years
Expected volatility
Expected volatility	[3]	36.60%	36.50%		34.90%
Expected dividend yield
Expected dividend yield	[4]	1.80%	1.80%		1.60%
Employee Stock Option [Member] | Maximum [Member]
Exercise price
Exercise price (in dollars per share)		$ 21.63	$ 18.55		$ 18.66
Market price
Market price (in dollars per share)		$ 21.63	$ 18.55		$ 18.66
Risk free interest rate
Risk free interest rate	[1]	2.40%	1.80%		2.10%
Expected award (Year)	[2]	10 years
Expected volatility
Expected volatility	[3]	41.40%	40.30%		39.00%
Expected dividend yield
Expected dividend yield	[4]	2.10%	2.10%		1.90%

[1] The risk-free interest rate is based on the yield of a U.S. treasury bond with a similar maturity as the expected life of the awards.

[2] The expected life in years for awards granted was based on the historical exercise patterns experienced by the Company when the award is made.

[3] The determination of expected stock price volatility for awards granted in each of the three years ended December 31, 2017, 2016 and 2015 was based on historical Superior common stock prices over a period commensurate with the expected life.

[4] The dividend yield assumption is based on the history and expectation of the Company's dividend payouts.