Note 19 - Quarterly Financial Data (Unaudited) - Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended												12 Months Ended
	Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating revenues	$ 469,222		$ 460,227		$ 449,758		$ 425,708		$ 431,233	$ 390,126	$ 370,350	$ 363,676	$ 1,804,915	$ 1,555,385	$ 1,451,205
Operating income(1)	21,142	[1]	22,892	[1]	20,018	[1]	14,854	[1]	12,457	11,534	2,689	1,928	78,906	28,608	27,731
Net income(1) (2)	$ 6,996	[1],[2]	$ 16,129	[1],[2]	$ 615	[1],[2]	$ 1,159	[1],[2]	$ 9,499	$ (675)	$ (8,452)	$ (4,432)	$ 24,899	$ (4,060)	$ (16,524)
Basic earnings per share (in dollars per share)	$ 0.14		$ 0.33		$ 0.04		$ 0.18		$ 1.49	$ (0.11)	$ (1.32)	$ (0.69)	$ 0.84	$ (0.64)	$ (2.59)
Diluted earnings per share (in dollars per share)	$ 0.14		$ 0.33		$ 0.04		$ 0.18		$ 1.49	$ (0.11)	$ (1.32)	$ (0.69)	$ 0.83	$ (0.64)	$ (2.59)

[1] Fourth quarter 2018 results include an impairment charge of $10.7 million related to assets of business held for sale.

[2] Fourth quarter 2018 results include an impairment charge of $1.8 million related to equity method investments.