Note 6 - Share-based Compensation - Summary of Significant Assumptions for Share-based Compensation Awards (Details) - $ / shares		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Expected award life (Year)				5 years
Stock Appreciation Rights (SARs) [Member]
Exercise price (in dollars per share)		$ 23.59	$ 16.97	$ 23.59	$ 16.97
Market price (in dollars per share)				$ 23.59	$ 16.97
Risk free interest rate	[1]			2.60%	1.90%
Expected award life (Year)	[2]			3 years	5 years
Expected volatility	[3]			38.10%	36.60%
Expected dividend yield	[4]			1.60%	2.10%
Employee Stock Option [Member]
Exercise price (in dollars per share)		24.28	18.05	$ 24.28	$ 18.05
Market price (in dollars per share)		$ 24.28	$ 18.05
Risk free interest rate	[1]	2.90%	2.40%
Expected award life (Year)	[2]	10 years	10 years
Expected volatility	[3]	42.10%	41.40%
Expected dividend yield	[4]	1.60%	1.90%
Employee Stock Option [Member] | Minimum [Member]
Exercise price (in dollars per share)		$ 23.59	$ 16.97	23.59	16.97
Market price (in dollars per share)				$ 23.59	$ 16.97
Risk free interest rate	[1]			2.60%	1.90%
Expected award life (Year)	[2]			3 years	5 years
Expected volatility	[3]			38.10%	36.60%
Expected dividend yield	[4]				1.90%
Employee Stock Option [Member] | Maximum [Member]
Exercise price (in dollars per share)		$ 24.28	$ 18.05	$ 24.28	$ 18.05
Market price (in dollars per share)				$ 24.28	$ 18.05
Risk free interest rate	[1]			2.90%	2.40%
Expected award life (Year)	[2]			10 years	10 years
Expected volatility	[3]			42.10%	41.40%
Expected dividend yield	[4]				2.10%

[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 periods ending June 30, 2018 and 2017 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.