Note 6 - Derivative Instruments (Details Textual) gal in Millions, bbl in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) $ / gal	Sep. 30, 2016 $ / gal	Sep. 30, 2017 USD ($) bbl gal	Aug. 31, 2015 USD ($)
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ 1.0		$ 3.7
Cash Flow Hedge Derivative Instrument Assets at Fair Value	$ 1.6		1.6
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months			0.8
Maximum [Member]
Underlying, Derivative Volume | $ / gal	1.83	1.52
Minimum [Member]
Underlying, Derivative Volume | $ / gal	1.41	1.2
Interest Rate Swap [Member]
Derivative, Notional Amount				$ 28.0
Derivative, Fixed Interest Rate				4.20%
Derivative Instruments in Hedges, Liabilities, at Fair Value	$ 0.7		0.7
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred			0.2
Interest Rate Swap [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) Reclassified from Accumulated OCI into Income, Effective Portion, Net	$ 0.1		$ 0.3
Fuel Hedge Contracts for 2017 Fuel Requirements [Member]
Derivative, Nonmonetary Notional Amount, Volume | bbl			3
Percent of Projected Fuel Requirements			25.10%
Fuel Hedge Contracts for 2018 Requirements [Member]
Derivative, Nonmonetary Notional Amount, Volume | gal			7.6
Percent of Projected Fuel Requirements			15.70%