Note 2 - Acquisitions and Dispositions - Unaudited Pro Forma Results (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Acquisitions Completed in 2017 [Member]
Revenue (less agency commissions)	$ 895,081	$ 926,799
Net income	$ 260,909	$ 88,679
Basic net income per share (in dollars per share)	$ 3.57	$ 1.23
Diluted net income per share (in dollars per share)	$ 3.53	$ 1.22
Acquisitions Completed in 2016 [Member]
Revenue (less agency commissions)		$ 825,787	$ 701,550
Net income		$ 57,795	$ 39,360
Basic net income per share (in dollars per share)		$ 0.80	$ 0.58
Diluted net income per share (in dollars per share)		$ 0.79	$ 0.57
Acquisitions Completed in 2015 [Member]
Revenue (less agency commissions)			$ 621,530
Net income			$ 46,181
Basic net income per share (in dollars per share)			$ 0.68
Diluted net income per share (in dollars per share)			$ 0.67