Note 6 - Investments - Gross Unrealized Losses and Fair Value of Available for Sale Securities (Details) $ in Thousands	Sep. 30, 2020 USD ($) security	Dec. 31, 2019 USD ($) security
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	309	154
Fair Value, Less Than 12 Months	$ 930,550	$ 660,883
Unrealized Losses, Less Than 12 Months	$ (16,906)	$ (4,183)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	37	70
Fair Value, 12 Months or Greater	$ 160,200	$ 254,728
Unrealized Losses, 12 Months or Greater	$ (3,590)	$ (4,145)
Continuous Loss Position, Total
Number of Securities, Total | security	346	224
Fair Value, Total	$ 1,090,750	$ 915,611
Unrealized Losses, Total	$ (20,496)	$ (8,328)
U.S. government and agencies securities
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	4	2
Fair Value, Less Than 12 Months	$ 99,250	$ 26,142
Unrealized Losses, Less Than 12 Months	$ (1,661)	$ (731)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	0	2
Fair Value, 12 Months or Greater	$ 0	$ 2,529
Unrealized Losses, 12 Months or Greater	$ 0	$ (2)
Continuous Loss Position, Total
Number of Securities, Total | security	4	4
Fair Value, Total	$ 99,250	$ 28,671
Unrealized Losses, Total	$ (1,661)	$ (733)
State and municipal obligations
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	10	1
Fair Value, Less Than 12 Months	$ 27,543	$ 3,959
Unrealized Losses, Less Than 12 Months	$ (193)	$ (41)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	0	0
Fair Value, 12 Months or Greater	$ 0	$ 0
Unrealized Losses, 12 Months or Greater	$ 0	$ 0
Continuous Loss Position, Total
Number of Securities, Total | security	10	1
Fair Value, Total	$ 27,543	$ 3,959
Unrealized Losses, Total	$ (193)	$ (41)
Corporate bonds and notes
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	130	25
Fair Value, Less Than 12 Months	$ 306,915	$ 110,871
Unrealized Losses, Less Than 12 Months	$ (7,471)	$ (1,728)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	0	0
Fair Value, 12 Months or Greater	$ 0	$ 0
Unrealized Losses, 12 Months or Greater	$ 0	$ 0
Continuous Loss Position, Total
Number of Securities, Total | security	130	25
Fair Value, Total	$ 306,915	$ 110,871
Unrealized Losses, Total	$ (7,471)	$ (1,728)
RMBS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	4	27
Fair Value, Less Than 12 Months	$ 11,723	$ 184,378
Unrealized Losses, Less Than 12 Months	$ (19)	$ (535)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	2	16
Fair Value, 12 Months or Greater	$ 956	$ 36,192
Unrealized Losses, 12 Months or Greater	$ (11)	$ (992)
Continuous Loss Position, Total
Number of Securities, Total | security	6	43
Fair Value, Total	$ 12,679	$ 220,570
Unrealized Losses, Total	$ (30)	$ (1,527)
CMBS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	60	36
Fair Value, Less Than 12 Months	$ 176,482	$ 109,589
Unrealized Losses, Less Than 12 Months	$ (4,537)	$ (478)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	6	8
Fair Value, 12 Months or Greater	$ 7,146	$ 6,346
Unrealized Losses, 12 Months or Greater	$ (254)	$ (147)
Continuous Loss Position, Total
Number of Securities, Total | security	66	44
Fair Value, Total	$ 183,628	$ 115,935
Unrealized Losses, Total	$ (4,791)	$ (625)
Other ABS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	101	63
Fair Value, Less Than 12 Months	$ 308,637	$ 225,944
Unrealized Losses, Less Than 12 Months	$ (3,025)	$ (670)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	29	44
Fair Value, 12 Months or Greater	$ 152,098	$ 209,661
Unrealized Losses, 12 Months or Greater	$ (3,325)	$ (3,004)
Continuous Loss Position, Total
Number of Securities, Total | security	130	107
Fair Value, Total	$ 460,735	$ 435,605
Unrealized Losses, Total	$ (6,350)	$ (3,674)