Note 15 - Selected Quarterly Financial Information (Unaudited) - Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ 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
Total revenues	$ 833,131	$ 819,568	$ 846,132	$ 827,288	$ 791,623	$ 612,196	$ 616,793	$ 606,118	$ 3,326,119	$ 2,626,730	$ 2,400,819
Operating income	114,859	113,391	126,692	117,626	94,684	69,568	96,258	94,774	472,568	355,284	343,801
Net income	$ 24,296	$ 39,405	$ 48,484	$ 45,451	$ 22,867	$ 11,837	$ 38,945	$ 41,399	$ 157,636	$ 115,048	$ 189,390
Basic net income per common share (in dollars per share)	$ 0.21	$ 0.35	$ 0.43	$ 0.40	$ 0.21	$ 0.10	$ 0.34	$ 0.36	$ 1.39	$ 1.01	$ 1.65
Diluted net income per common share (in dollars per share)	$ 0.20	$ 0.35	$ 0.43	$ 0.40	$ 0.20	$ 0.10	$ 0.34	$ 0.36	$ 1.38	$ 1.00	$ 1.64
Income from continuing operations, net of tax					$ 22,867	$ 11,837	$ 38,598	$ 41,399		$ 114,701
Income from discontinued operations, net of tax					$ 0	$ 0	$ 347	$ 0	$ 0	$ 347	$ 21,392
Continuing operations (in dollars per share)					$ 0.21	$ 0.10	$ 0.34	$ 0.36	$ 1.39	$ 1.01	$ 1.46
Discontinued operations (in dollars per share)					0	0	0	0		0
Continuing operations (in dollars per share)					0.20	0.10	0.34	0.36	$ 1.38	1.00	$ 1.45
Discontinued operations (in dollars per share)					$ 0	$ 0	$ 0	$ 0		$ 0