Note 5 - Investments Gross Unrealized Losses and Fair Value of Available for Sale Securities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) security	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($) security
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	395		154
Fair Value, Less Than 12 Months	$ 1,492,967		$ 660,883
Unrealized Losses, Less Than 12 Months	$ 90,490		$ 4,183
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	38		70
Fair Value, 12 Months or Greater	$ 151,561		$ 254,728
Unrealized Losses, 12 Months or Greater	$ 14,041		$ 4,145
Continuous Loss Position, Total
Number of Securities, Total | security	433		224
Fair Value, Total	$ 1,644,528		$ 915,611
Unrealized Losses, Total	$ 104,531		$ 8,328
US government and agency securities
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security			2
Fair Value, Less Than 12 Months			$ 26,142
Unrealized Losses, Less Than 12 Months			$ 731
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security			2
Fair Value, 12 Months or Greater			$ 2,529
Unrealized Losses, 12 Months or Greater			$ 2
Continuous Loss Position, Total
Number of Securities, Total | security			4
Fair Value, Total			$ 28,671
Unrealized Losses, Total			$ 733
State and municipal obligations
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	6		1
Fair Value, Less Than 12 Months	$ 19,560		$ 3,959
Unrealized Losses, Less Than 12 Months	$ 318		$ 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	6		1
Fair Value, Total	$ 19,560		$ 3,959
Unrealized Losses, Total	$ 318		$ 41
Corporate bonds and notes
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	174		25
Fair Value, Less Than 12 Months	$ 763,279		$ 110,871
Unrealized Losses, Less Than 12 Months	$ 48,577		$ 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	174		25
Fair Value, Total	$ 763,279		$ 110,871
Unrealized Losses, Total	$ 48,577		$ 1,728
RMBS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	3		27
Fair Value, Less Than 12 Months	$ 19,874		$ 184,378
Unrealized Losses, Less Than 12 Months	$ 656		$ 535
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	4		16
Fair Value, 12 Months or Greater	$ 22,323		$ 36,192
Unrealized Losses, 12 Months or Greater	$ 147		$ 992
Continuous Loss Position, Total
Number of Securities, Total | security	7		43
Fair Value, Total	$ 42,197		$ 220,570
Unrealized Losses, Total	$ 803		$ 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	$ 201,504		$ 109,589
Unrealized Losses, Less Than 12 Months	$ 11,534		$ 478
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	8		8
Fair Value, 12 Months or Greater	$ 7,052		$ 6,346
Unrealized Losses, 12 Months or Greater	$ 463		$ 147
Continuous Loss Position, Total
Number of Securities, Total | security	68		44
Fair Value, Total	$ 208,556		$ 115,935
Unrealized Losses, Total	$ 11,997		$ 625
Other ABS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	151		63
Fair Value, Less Than 12 Months	$ 483,803		$ 225,944
Unrealized Losses, Less Than 12 Months	$ 29,259		$ 670
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	26		44
Fair Value, 12 Months or Greater	$ 122,186		$ 209,661
Unrealized Losses, 12 Months or Greater	$ 13,431		$ 3,004
Continuous Loss Position, Total
Number of Securities, Total | security	177		107
Fair Value, Total	$ 605,989		$ 435,605
Unrealized Losses, Total	$ 42,690		$ 3,674
Foreign government and agency securities
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	1
Fair Value, Less Than 12 Months	$ 4,947
Unrealized Losses, Less Than 12 Months	$ 146
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	0
Fair Value, 12 Months or Greater	$ 0
Unrealized Losses, 12 Months or Greater	$ 0
Continuous Loss Position, Total
Number of Securities, Total | security	1
Fair Value, Total	$ 4,947
Unrealized Losses, Total	146
OTTI Recognized in Earnings Due to Intent to Sell
Continuous Loss Position, Total
Other-than-temporary impairment losses	$ 622
OTTI Recognized in Earnings
Continuous Loss Position, Total
Other-than-temporary impairment losses		$ 0