Note 12 - Derivatives - Natural Gas Contracts (Details) - MMBTU	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Cash Flow Hedging [Member] \| Natural Gas Contracts [Member]
Derivative, nonmonetary notional amount (Millions of British Thermal Unit)	2,460,000	3,150,000

Nominal energy measure used to calculate payments on a derivative instrument.

No definition available.

Name:	us-gaap_DerivativeNonmonetaryNotionalAmountEnergyMeasure
Namespace Prefix:	us-gaap_
Data Type:	num:energyItemType
Balance Type:	na
Period Type:	duration