Note 5 - Investments Gross Unrealized Losses and Fair Value of Available for Sale Securities (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) security	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) security
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	168		550
Fair Value, Less Than 12 Months	$ 747,531		$ 2,201,228
Unrealized Losses, Less Than 12 Months	$ 6,406		$ 50,707
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	244		237
Fair Value, 12 Months or Greater	$ 994,813		$ 957,742
Unrealized Losses, 12 Months or Greater	$ 18,135		$ 37,333
Continuous Loss Position, Total
Number of Securities, Total | security	412		787
Fair Value, Total	$ 1,742,344		$ 3,158,970
Unrealized Losses, Total	$ 24,541		$ 88,040
US government and agency securities
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	0		2
Fair Value, Less Than 12 Months	$ 0		$ 27,415
Unrealized Losses, Less Than 12 Months	$ 0		$ 796
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	9		8
Fair Value, 12 Months or Greater	$ 38,177		$ 23,476
Unrealized Losses, 12 Months or Greater	$ 528		$ 712
Continuous Loss Position, Total
Number of Securities, Total | security	9		10
Fair Value, Total	$ 38,177		$ 50,891
Unrealized Losses, Total	$ 528		$ 1,508
State and municipal obligations
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	1		12
Fair Value, Less Than 12 Months	$ 6,487		$ 41,263
Unrealized Losses, Less Than 12 Months	$ 27		$ 955
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	3		16
Fair Value, 12 Months or Greater	$ 10,983		$ 39,982
Unrealized Losses, 12 Months or Greater	$ 274		$ 945
Continuous Loss Position, Total
Number of Securities, Total | security	4		28
Fair Value, Total	$ 17,470		$ 81,245
Unrealized Losses, Total	$ 301		$ 1,900
Corporate bonds and notes
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	54		330
Fair Value, Less Than 12 Months	$ 220,831		$ 1,208,430
Unrealized Losses, Less Than 12 Months	$ 2,676		$ 36,284
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	148		126
Fair Value, 12 Months or Greater	$ 656,876		$ 601,533
Unrealized Losses, 12 Months or Greater	$ 11,839		$ 27,604
Continuous Loss Position, Total
Number of Securities, Total | security	202		456
Fair Value, Total	$ 877,707		$ 1,809,963
Unrealized Losses, Total	$ 14,515		$ 63,888
RMBS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	9		15
Fair Value, Less Than 12 Months	$ 60,465		$ 92,315
Unrealized Losses, Less Than 12 Months	$ 124		$ 782
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	24		28
Fair Value, 12 Months or Greater	$ 67,863		$ 77,395
Unrealized Losses, 12 Months or Greater	$ 2,626		$ 3,704
Continuous Loss Position, Total
Number of Securities, Total | security	33		43
Fair Value, Total	$ 128,328		$ 169,710
Unrealized Losses, Total	$ 2,750		$ 4,486
CMBS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	33		62
Fair Value, Less Than 12 Months	$ 138,708		$ 328,696
Unrealized Losses, Less Than 12 Months	$ 621		$ 3,973
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	19		33
Fair Value, 12 Months or Greater	$ 41,516		$ 125,728
Unrealized Losses, 12 Months or Greater	$ 838		$ 3,385
Continuous Loss Position, Total
Number of Securities, Total | security	52		95
Fair Value, Total	$ 180,224		$ 454,424
Unrealized Losses, Total	$ 1,459		$ 7,358
Other ABS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	71		129
Fair Value, Less Than 12 Months	$ 321,040		$ 503,109
Unrealized Losses, Less Than 12 Months	$ 2,958		$ 7,917
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	41		26
Fair Value, 12 Months or Greater	$ 179,398		$ 89,628
Unrealized Losses, 12 Months or Greater	$ 2,030		$ 983
Continuous Loss Position, Total
Number of Securities, Total | security	112		155
Fair Value, Total	$ 500,438		$ 592,737
Unrealized Losses, Total	4,988		$ 8,900
OTTI Recognized in Earnings
Continuous Loss Position, Total
Other-than-temporary impairment losses	0	$ 844
OTTI Recognized in AOCI
Continuous Loss Position, Total
Other-than-temporary impairment losses	$ 0	$ 0