Note 6 - Investments - Gross Unrealized Losses and Fair Value of Available for Sale Securities (Details) $ in Thousands	Jun. 30, 2021 USD ($) security	Dec. 31, 2020 USD ($) security
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	360	194
Fair Value, Less Than 12 Months	$ 1,366,295	$ 679,631
Unrealized Losses, Less Than 12 Months	$ (25,428)	$ (12,101)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	36	36
Fair Value, 12 Months or Greater	$ 116,528	$ 158,852
Unrealized Losses, 12 Months or Greater	$ (1,586)	$ (2,056)
Continuous Loss Position, Total
Number of Securities, Total | security	396	230
Fair Value, Total	$ 1,482,823	$ 838,483
Unrealized Losses, Total	$ (27,014)	$ (14,157)
U.S. government and agencies securities
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	6	4
Fair Value, Less Than 12 Months	$ 59,708	$ 90,591
Unrealized Losses, Less Than 12 Months	$ (902)	$ (3,611)
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	6	4
Fair Value, Total	$ 59,708	$ 90,591
Unrealized Losses, Total	$ (902)	$ (3,611)
State and municipal obligations
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	8	4
Fair Value, Less Than 12 Months	$ 10,034	$ 9,626
Unrealized Losses, Less Than 12 Months	$ (122)	$ (100)
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	8	4
Fair Value, Total	$ 10,034	$ 9,626
Unrealized Losses, Total	$ (122)	$ (100)
Corporate bonds and notes
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	174	60
Fair Value, Less Than 12 Months	$ 684,401	$ 174,848
Unrealized Losses, Less Than 12 Months	$ (19,380)	$ (3,758)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	1	0
Fair Value, 12 Months or Greater	$ 2,626	$ 0
Unrealized Losses, 12 Months or Greater	$ (106)	$ 0
Continuous Loss Position, Total
Number of Securities, Total | security	175	60
Fair Value, Total	$ 687,027	$ 174,848
Unrealized Losses, Total	$ (19,486)	$ (3,758)
RMBS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	35	5
Fair Value, Less Than 12 Months	$ 260,712	$ 42,003
Unrealized Losses, Less Than 12 Months	$ (3,292)	$ (305)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	2	2
Fair Value, 12 Months or Greater	$ 803	$ 915
Unrealized Losses, 12 Months or Greater	$ (10)	$ (9)
Continuous Loss Position, Total
Number of Securities, Total | security	37	7
Fair Value, Total	$ 261,515	$ 42,918
Unrealized Losses, Total	$ (3,302)	$ (314)
CMBS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	49	43
Fair Value, Less Than 12 Months	$ 106,289	$ 118,345
Unrealized Losses, Less Than 12 Months	$ (1,280)	$ (3,035)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	16	6
Fair Value, 12 Months or Greater	$ 35,577	$ 8,312
Unrealized Losses, 12 Months or Greater	$ (1,081)	$ (266)
Continuous Loss Position, Total
Number of Securities, Total | security	65	49
Fair Value, Total	$ 141,866	$ 126,657
Unrealized Losses, Total	$ (2,361)	$ (3,301)
CLO
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	43	52
Fair Value, Less Than 12 Months	$ 151,791	$ 173,459
Unrealized Losses, Less Than 12 Months	$ (242)	$ (970)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	16	25
Fair Value, 12 Months or Greater	$ 76,845	$ 137,506
Unrealized Losses, 12 Months or Greater	$ (365)	$ (1,671)
Continuous Loss Position, Total
Number of Securities, Total | security	59	77
Fair Value, Total	$ 228,636	$ 310,965
Unrealized Losses, Total	$ (607)	$ (2,641)
Other ABS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	45	26
Fair Value, Less Than 12 Months	$ 93,360	$ 70,759
Unrealized Losses, Less Than 12 Months	$ (210)	$ (322)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	1	3
Fair Value, 12 Months or Greater	$ 677	$ 12,119
Unrealized Losses, 12 Months or Greater	$ (24)	$ (110)
Continuous Loss Position, Total
Number of Securities, Total | security	46	29
Fair Value, Total	$ 94,037	$ 82,878
Unrealized Losses, Total	$ (234)	$ (432)