Note 5 - Investments Gross Unrealized Losses and Fair Value of Available for Sale Securities (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	636		636		332
Fair Value, Less Than 12 Months	$ 2,790,951		$ 2,790,951		$ 1,338,867
Unrealized Losses, Less Than 12 Months	$ 64,321		$ 64,321		$ 9,827
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	101		101		72
Fair Value, 12 Months or Greater	$ 314,131		$ 314,131		$ 262,531
Unrealized Losses, 12 Months or Greater	$ 18,449		$ 18,449		$ 6,607
Continuous Loss Position, Total
Number of Securities, Total | security	737		737		404
Fair Value, Total	$ 3,105,082		$ 3,105,082		$ 1,601,398
Unrealized Losses, Total	82,770		82,770		16,434
Other than Temporary Impairment Losses, Investments	900	$ 0	1,744	$ 1,000
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net			0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held	$ 0		$ 0		$ 0
US government and agency securities
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	9		9		6
Fair Value, Less Than 12 Months	$ 39,342		$ 39,342		$ 23,309
Unrealized Losses, Less Than 12 Months	$ 1,944		$ 1,944		$ 129
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	5		5		3
Fair Value, 12 Months or Greater	$ 11,758		$ 11,758		$ 9,799
Unrealized Losses, 12 Months or Greater	$ 780		$ 780		$ 238
Continuous Loss Position, Total
Number of Securities, Total | security	14		14		9
Fair Value, Total	$ 51,100		$ 51,100		$ 33,108
Unrealized Losses, Total	$ 2,724		$ 2,724		$ 367
State and municipal obligations
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	25		25		21
Fair Value, Less Than 12 Months	$ 76,914		$ 76,914		$ 65,898
Unrealized Losses, Less Than 12 Months	$ 2,166		$ 2,166		$ 699
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	6		6		0
Fair Value, 12 Months or Greater	$ 13,172		$ 13,172		$ 0
Unrealized Losses, 12 Months or Greater	$ 584		$ 584		$ 0
Continuous Loss Position, Total
Number of Securities, Total | security	31		31		21
Fair Value, Total	$ 90,086		$ 90,086		$ 65,898
Unrealized Losses, Total	$ 2,750		$ 2,750		$ 699
Corporate bonds and notes
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	383		383		152
Fair Value, Less Than 12 Months	$ 1,619,774		$ 1,619,774		$ 672,318
Unrealized Losses, Less Than 12 Months	$ 43,840		$ 43,840		$ 4,601
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	35		35		32
Fair Value, 12 Months or Greater	$ 157,323		$ 157,323		$ 139,105
Unrealized Losses, 12 Months or Greater	$ 10,634		$ 10,634		$ 3,451
Continuous Loss Position, Total
Number of Securities, Total | security	418		418		184
Fair Value, Total	$ 1,777,097		$ 1,777,097		$ 811,423
Unrealized Losses, Total	$ 54,474		$ 54,474		$ 8,052
RMBS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	23		23		8
Fair Value, Less Than 12 Months	$ 169,259		$ 169,259		$ 19,943
Unrealized Losses, Less Than 12 Months	$ 3,271		$ 3,271		$ 204
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	28		28		26
Fair Value, 12 Months or Greater	$ 80,158		$ 80,158		$ 101,812
Unrealized Losses, 12 Months or Greater	$ 4,829		$ 4,829		$ 2,658
Continuous Loss Position, Total
Number of Securities, Total | security	51		51		34
Fair Value, Total	$ 249,417		$ 249,417		$ 121,755
Unrealized Losses, Total	$ 8,100		$ 8,100		$ 2,862
CMBS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	78		78		35
Fair Value, Less Than 12 Months	$ 423,684		$ 423,684		$ 139,353
Unrealized Losses, Less Than 12 Months	$ 10,098		$ 10,098		$ 1,395
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	11		11		4
Fair Value, 12 Months or Greater	$ 26,830		$ 26,830		$ 3,518
Unrealized Losses, 12 Months or Greater	$ 1,389		$ 1,389		$ 215
Continuous Loss Position, Total
Number of Securities, Total | security	89		89		39
Fair Value, Total	$ 450,514		$ 450,514		$ 142,871
Unrealized Losses, Total	$ 11,487		$ 11,487		$ 1,610
Other ABS
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security	118		118		92
Fair Value, Less Than 12 Months	$ 461,978		$ 461,978		$ 260,864
Unrealized Losses, Less Than 12 Months	$ 3,002		$ 3,002		$ 777
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of Securities, 12 Months or Greater | security	16		16		7
Fair Value, 12 Months or Greater	$ 24,890		$ 24,890		$ 8,297
Unrealized Losses, 12 Months or Greater	$ 233		$ 233		$ 45
Continuous Loss Position, Total
Number of Securities, Total | security	134		134		99
Fair Value, Total	$ 486,868		$ 486,868		$ 269,161
Unrealized Losses, Total	$ 3,235		$ 3,235		$ 822
Foreign government and agency securities
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security					5
Fair Value, Less Than 12 Months					$ 7,397
Unrealized Losses, Less Than 12 Months					$ 33
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					5
Fair Value, Total					$ 7,397
Unrealized Losses, Total					$ 33
Equity securities
Continuous Loss Position Less Than Twelve Months [Abstract]
Number of Securities, Less than 12 Months | security					13
Fair Value, Less Than 12 Months					$ 149,785
Unrealized Losses, Less Than 12 Months					$ 1,989
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					13
Fair Value, Total					$ 149,785
Unrealized Losses, Total					$ 1,989
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net				500
Convertible Debt Securities [Member]
Continuous Loss Position, Total
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net				$ 500