Note 2 - Accounting Policies (Details Textual) - USD ($) $ in Thousands	Jun. 30, 2019	Jan. 01, 2019	Dec. 31, 2018
Operating Lease, Liability, Total	$ 20,506		$ 0
Operating Lease, Right-of-Use Asset	20,506		0
Debt Securities, Available-for-sale, Total	2,775,899		2,654,670
Retained Earnings (Accumulated Deficit), Ending Balance	$ 220,173		$ 205,841
Accounting Standards Update 2016-02 [Member]
Operating Lease, Liability, Total		$ 15,300
Operating Lease, Right-of-Use Asset		15,300
Accounting Standards Update 2017-08 [Member]
Debt Securities, Available-for-sale, Total		(3,100)
Retained Earnings (Accumulated Deficit), Ending Balance		$ (2,800)