Note 6 - Investments - Gross Unrealized Losses and Fair Value of Available for Sale Securities (Details) $ in Thousands	Mar. 31, 2021 USD ($) security	Dec. 31, 2020 USD ($) security
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	411	194
Fair Value, Less Than 12 Months	$ 1,498,256	$ 679,631
Unrealized Losses, Less Than 12 Months	$ (65,865)	$ (12,101)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	47	36
Fair Value, 12 Months or Greater	$ 146,430	$ 158,852
Unrealized Losses, 12 Months or Greater	$ (1,991)	$ (2,056)
Continuous Loss Position, Total
Number of Securities, Total | security	458	230
Fair Value, Total	$ 1,644,686	$ 838,483
Unrealized Losses, Total	$ (67,856)	$ (14,157)
U.S. government and agencies securities
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	7	4
Fair Value, Less Than 12 Months	$ 87,663	$ 90,591
Unrealized Losses, Less Than 12 Months	$ (8,681)	$ (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	7	4
Fair Value, Total	$ 87,663	$ 90,591
Unrealized Losses, Total	$ (8,681)	$ (3,611)
State and municipal obligations
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	29	4
Fair Value, Less Than 12 Months	$ 37,553	$ 9,626
Unrealized Losses, Less Than 12 Months	$ (1,510)	$ (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	29	4
Fair Value, Total	$ 37,553	$ 9,626
Unrealized Losses, Total	$ (1,510)	$ (100)
Corporate bonds and notes
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	225	60
Fair Value, Less Than 12 Months	$ 819,835	$ 174,848
Unrealized Losses, Less Than 12 Months	$ (47,419)	$ (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	$ 2,725	$ 0
Unrealized Losses, 12 Months or Greater	$ (181)	$ 0
Continuous Loss Position, Total
Number of Securities, Total | security	226	60
Fair Value, Total	$ 822,560	$ 174,848
Unrealized Losses, Total	$ (47,600)	$ (3,758)
RMBS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	32	5
Fair Value, Less Than 12 Months	$ 239,012	$ 42,003
Unrealized Losses, Less Than 12 Months	$ (3,654)	$ (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	$ 869	$ 915
Unrealized Losses, 12 Months or Greater	$ (12)	$ (9)
Continuous Loss Position, Total
Number of Securities, Total | security	34	7
Fair Value, Total	$ 239,881	$ 42,918
Unrealized Losses, Total	$ (3,666)	$ (314)
CMBS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	51	43
Fair Value, Less Than 12 Months	$ 111,105	$ 118,345
Unrealized Losses, Less Than 12 Months	$ (4,146)	$ (3,035)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	17	6
Fair Value, 12 Months or Greater	$ 42,527	$ 8,312
Unrealized Losses, 12 Months or Greater	$ (1,184)	$ (266)
Continuous Loss Position, Total
Number of Securities, Total | security	68	49
Fair Value, Total	$ 153,632	$ 126,657
Unrealized Losses, Total	$ (5,330)	$ (3,301)
CLO
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	33	52
Fair Value, Less Than 12 Months	$ 101,725	$ 173,459
Unrealized Losses, Less Than 12 Months	$ (172)	$ (970)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	24	25
Fair Value, 12 Months or Greater	$ 92,497	$ 137,506
Unrealized Losses, 12 Months or Greater	$ (436)	$ (1,671)
Continuous Loss Position, Total
Number of Securities, Total | security	57	77
Fair Value, Total	$ 194,222	$ 310,965
Unrealized Losses, Total	$ (608)	$ (2,641)
Other ABS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	34	26
Fair Value, Less Than 12 Months	$ 101,363	$ 70,759
Unrealized Losses, Less Than 12 Months	$ (283)	$ (322)
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	3	3
Fair Value, 12 Months or Greater	$ 7,812	$ 12,119
Unrealized Losses, 12 Months or Greater	$ (178)	$ (110)
Continuous Loss Position, Total
Number of Securities, Total | security	37	29
Fair Value, Total	$ 109,175	$ 82,878
Unrealized Losses, Total	$ (461)	$ (432)