Note 1 - Business and Significant Accounting Policies - Effect of Revision on Balance Sheet Line Items (Details) - USD ($)	3 Months Ended										12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016		Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015	Jan. 01, 2015	Dec. 31, 2014
Deferred Tax Assets, Net, Noncurrent	$ 57,337,000										$ 57,337,000
Total assets	2,623,864,000	[1]				2,461,569,000	[1]				2,623,864,000	[1]	2,461,569,000	[1]	$ 2,273,982,000
Deferred Tax Liabilities, Net, Noncurrent	61,961,000					36,411,000					61,961,000		36,411,000		28,989,000
Liability for unrecognized tax benefits	8,890,000					6,444,000					8,890,000		6,444,000
Total liabilities	1,321,748,000					1,288,525,000					1,321,748,000		1,288,525,000
Retained Earnings (Accumulated Deficit), Ending Balance	327,255,000					215,352,000					327,255,000		215,352,000		152,326,000		$ 44,670,000
Accumulated Other Comprehensive Income (Loss), Net of Tax, Total	(4,706,000)					(8,175,000)					(4,706,000)		(8,175,000)		(8,164,000)		(9,221,000)
Stockholders' Equity Attributable to Parent, Ending Balance	1,211,378,000					1,076,690,000					1,211,378,000		1,076,690,000		993,434,000		777,501,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance	1,295,700,000					1,168,272,000					1,295,700,000		1,168,272,000		1,087,307,000		789,324,000
Income tax (provision) benefit	28,329,000		$ (6,224,000)	$ (32,765,000)	$ (11,004,000)	(7,356,000)		$ (12,109,000)	$ (8,515,000)	$ (9,079,000)	(21,664,000)		(37,059,000)		16,057,000
Income from continuing operations											147,109,000		96,294,000		124,148,000
Net Income (Loss) Attributable to Parent, Total	$ 64,620,000		$ 23,960,000	$ 8,640,000	$ 35,194,000	$ 23,324,000		$ 11,959,000	$ 23,724,000	$ 29,701,000	132,414,000		88,708,000		120,372,000
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax, Total											135,000		141,000		147,000
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest, Total											151,415,000		95,876,000		125,205,000
Comprehensive income attributable to the Company’s stockholders											$ 135,883,000		$ 88,697,000		$ 121,429,000
Basic: (in dollars per share)	$ 1.28		$ 0.48	$ 0.17	$ 0.71	$ 0.47		$ 0.24	$ 0.48	$ 0.60	$ 2.64		$ 1.79		$ 2.48
Diluted: (in dollars per share)	$ 1.27		$ 0.47	$ 0.17	$ 0.70	$ 0.46		$ 0.24	$ 0.47	$ 0.60	$ 2.61		$ 1.77		$ 2.45
Total stockholders’ equity attributable to the Company’s stockholders											$ 146,057,000		$ 96,010,000		$ 124,148,000
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Total	$ 68,087,000		$ 27,559,000	$ 11,846,000	$ 39,617,000	$ 26,326,000		$ 14,285,000	$ 24,308,000	$ 31,375,000	147,109,000		96,294,000		124,148,000
Deferred income tax provision											(41,147,000)		23,222,000		(39,962,000)
Liability for unrecognized tax benefits											3,270,000		(4,174,000)		3,107,000
Accounts payable and accrued expenses											51,641,000		(1,375,000)		(939,000)
Net cash provided by operating activities											245,575,000		159,285,000		190,025,000
Previously Reported [Member]
Deferred Tax Assets, Net, Noncurrent	20,135,000										20,135,000
Total assets	2,586,662,000					2,461,569,000					2,586,662,000		2,461,569,000
Deferred Tax Liabilities, Net, Noncurrent						35,382,000							35,382,000
Liability for unrecognized tax benefits	8,890,000					5,738,000					8,890,000		5,738,000
Total liabilities	1,259,787,000					1,286,790,000					1,259,787,000		1,286,790,000
Retained Earnings (Accumulated Deficit), Ending Balance	351,622,000					216,644,000					351,622,000		216,644,000		148,396,000		41,539,000
Accumulated Other Comprehensive Income (Loss), Net of Tax, Total	(4,314,000)					(7,732,000)					(4,314,000)		(7,732,000)		(7,667,000)		(8,668,000)
Stockholders' Equity Attributable to Parent, Ending Balance	1,236,137,000					1,078,425,000					1,236,137,000		1,078,425,000		990,001,000		774,923,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance	1,320,459,000					1,170,007,000					1,320,459,000		1,170,007,000		1,083,874,000		786,746,000
Income tax (provision) benefit											1,411,000		(31,837,000)		15,258,000
Income from continuing operations											170,184,000		101,516,000		123,349,000
Net Income (Loss) Attributable to Parent, Total											155,489,000		93,930,000		119,573,000
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax, Total											84,000		87,000		91,000
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest, Total											174,439,000		101,044,000		124,350,000
Comprehensive income attributable to the Company’s stockholders											$ 158,907,000		$ 93,865,000		$ 120,574,000
Basic: (in dollars per share)											$ 3.10		$ 1.90		$ 2.46
Diluted: (in dollars per share)											$ 3.06		$ 1.87		$ 2.43
Total stockholders’ equity attributable to the Company’s stockholders											$ 169,132,000		$ 101,232,000		$ 123,349,000
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Total											170,184,000		101,516,000		123,349,000
Deferred income tax provision											(64,104,000)		18,473,000		(39,530,000)
Liability for unrecognized tax benefits											3,152,000		(4,647,000)		2,874,000
Accounts payable and accrued expenses											51,641,000		(1,375,000)		(339,000)
Net cash provided by operating activities											245,575,000		159,285,000		190,025,000
Restatement Adjustment [Member]
Deferred Tax Assets, Net, Noncurrent	37,202,000										37,202,000
Total assets	37,202,000										37,202,000
Deferred Tax Liabilities, Net, Noncurrent	61,961,000					1,029,000					61,961,000		1,029,000
Liability for unrecognized tax benefits						706,000							706,000
Total liabilities	61,961,000					1,735,000					61,961,000		1,735,000
Retained Earnings (Accumulated Deficit), Ending Balance	(24,367,000)					(1,292,000)					(24,367,000)		(1,292,000)		3,930,000	$ 3,100,000	3,131,000
Accumulated Other Comprehensive Income (Loss), Net of Tax, Total	(392,000)					(443,000)					(392,000)		(443,000)		(497,000)		(553,000)
Stockholders' Equity Attributable to Parent, Ending Balance	(24,759,000)					(1,735,000)					(24,759,000)		(1,735,000)		3,433,000		2,578,000
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Ending Balance	(24,759,000)					(1,735,000)					(24,759,000)		(1,735,000)		3,433,000		$ 2,578,000
Income tax (provision) benefit	(1,300,000)					(4,900,000)					(23,075,000)		(5,222,000)		799,000
Income from continuing operations											(23,075,000)		(5,222,000)		799,000
Net Income (Loss) Attributable to Parent, Total	$ (1,300,000)					$ (4,900,000)					(23,075,000)		(5,222,000)		799,000
Other Comprehensive Income (Loss), Derivatives Qualifying as Hedges, Net of Tax, Total											51,000		54,000		56,000
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest, Total											(23,024,000)		(5,168,000)		855,000
Comprehensive income attributable to the Company’s stockholders											$ (23,024,000)		$ (5,168,000)		$ 855,000
Basic: (in dollars per share)											$ (0.46)		$ (0.11)		$ 0.02
Diluted: (in dollars per share)											$ (0.45)		$ (0.10)		$ 0.02
Total stockholders’ equity attributable to the Company’s stockholders											$ (23,075,000)		$ (5,222,000)		$ 799,000
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Total											(23,075,000)		(5,222,000)		799,000
Deferred income tax provision											22,957,000		4,749,000		(432,000)
Liability for unrecognized tax benefits											118,000		473,000		233,000
Accounts payable and accrued expenses															(600,000)
Net cash provided by operating activities

[1] Electricity segment assets include goodwill in the amount of $21.0 million and $6.7 million as of December 31, 2017 and 2016, respectively.