Note 1 - Business and Significant Accounting Policies - Impact of Adoption (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Revenue	[1]									$ 746,044	$ 719,267	$ 692,812
Prepaid expenses and other		$ 12,667				$ 51,441				12,667	51,441		$ 16,056
Deferred financing and lease costs, net		0				3,242				0	3,242		1,583
Property, plant and equipment, net		1,971,415				1,959,578				1,971,415	1,959,578		1,946,723
Operating leases right of use		17,405				0				17,405	0		62,244
Finance leases right of use		14,161				0				14,161	0		13,476
Accounts payable and accrued expenses		141,857				116,362				141,857	116,362		113,502
Current maturity of operating lease liabilities		2,743				0				2,743	0		7,532
Current maturity of finance lease liabilities		3,068				0				3,068	0		2,841
Other long-term liabilities		6,838				16,087				6,838	16,087		6,117
Long term portion of operating lease liabilities		14,008				0				14,008	0		17,668
Long term portion of finance lease liabilities		11,209				0				11,209	0		10,668
Retained earnings		487,873				422,222				487,873	422,222		422,164
Accounting Standards Update 2016-02 [Member]
Prepaid expenses and other													(35,385)
Deferred financing and lease costs, net													(1,659)
Property, plant and equipment, net													(12,855)
Operating leases right of use													62,244
Finance leases right of use													13,476
Accounts payable and accrued expenses													(2,860)
Current maturity of operating lease liabilities													7,532
Current maturity of finance lease liabilities													2,841
Other long-term liabilities													(9,970)
Long term portion of operating lease liabilities													17,668
Long term portion of finance lease liabilities													10,668
Retained earnings													$ (58)
Electricity [Member]
Revenue		$ 144,368	$ 123,978	$ 129,079	$ 142,908	$ 138,320	$ 116,891	$ 122,179	$ 132,489	540,333	509,879	$ 465,593
Electricity [Member] | Accounting Standards Update 2016-02 [Member]
Revenue										479,059	481,619
Electricity and Product Revenue [Member] | Accounting Standards Update 2014-09 [Member]
Revenue										$ 266,985	$ 237,648

[1] Revenues as reported in the geographic area in which they originate.