Note 12 - Selected Quarterly Financial Data (Unaudited) (Detail) - Selected quarterly financial data, 2012 (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating revenue	$ 126,587	$ 102,879	$ 94,691	$ 80,674	$ 84,670	$ 76,518	$ 76,201	$ 69,742	$ 404,831	$ 307,131	$ 346,058
Operating income	58,511	41,063	33,046	20,821	26,592	18,222	19,028	11,506	153,441	75,348	106,960
Net income (loss)	(2,109)	15,873	10,994	3,371	7,575	1,984	2,559	(3,083)	28,129	9,035	23,163
Net income (loss) available to common stockholders	$ (2,613)	$ 14,640	$ 9,815	$ 2,192	$ 5,869	$ 27	$ 771	$ (4,872)	$ 24,034	$ 1,795	$ 8,581
Basic net income (loss) available to common stockholders per share (in Dollars per share)	$ (0.05)	$ 0.26	$ 0.17	$ 0.04	$ 0.10		$ 0.01	$ (0.09)	$ 0.42	$ 0.03	$ 0.16
Diluted net income (loss) available to common stockholders per share (in Dollars per share)	$ (0.05)	$ 0.26	$ 0.17	$ 0.04	$ 0.10		$ 0.01	$ (0.09)	$ 0.42	$ 0.03	$ 0.16