Note 5 - Investments Schedule of Unrealized Losses (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net	$ 844	$ (1,400)
Continuous Loss Position Less Than Twelve Months [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	612	332
Fair value available-for-sale securities	$ 2,417,708	$ 1,338,867
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 46,167	$ 9,827
Continuous Unrealized Loss Position, Twelve Months Or Greater
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	68	72
Fair value available-for-sale securities	$ 223,335	$ 262,531
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 11,699	$ 6,607
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	680	404
Fair value available-for-sale securities	$ 2,641,043	$ 1,601,398
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	57,866	16,434
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held	$ 0	$ 0
US government and agency securities
Continuous Loss Position Less Than Twelve Months [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	7	6
Fair value available-for-sale securities	$ 41,246	$ 23,309
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 1,116	$ 129
Continuous Unrealized Loss Position, Twelve Months Or Greater
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	3	3
Fair value available-for-sale securities	$ 9,520	$ 9,799
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 506	$ 238
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	10	9
Fair value available-for-sale securities	$ 50,766	$ 33,108
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 1,622	$ 367
US States and Political Subdivisions Debt Securities [Member]
Continuous Loss Position Less Than Twelve Months [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	28	21
Fair value available-for-sale securities	$ 79,897	$ 65,898
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 1,729	$ 699
Continuous Unrealized Loss Position, Twelve Months Or Greater
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	0	0
Fair value available-for-sale securities	$ 0	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 0	$ 0
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	28	21
Fair value available-for-sale securities	$ 79,897	$ 65,898
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 1,729	$ 699
Corporate bonds and notes
Continuous Loss Position Less Than Twelve Months [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	344	152
Fair value available-for-sale securities	$ 1,348,488	$ 672,318
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 32,623	$ 4,601
Continuous Unrealized Loss Position, Twelve Months Or Greater
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	28	32
Fair value available-for-sale securities	$ 127,265	$ 139,105
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 7,267	$ 3,451
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	372	184
Fair value available-for-sale securities	$ 1,475,753	$ 811,423
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 39,890	$ 8,052
RMBS
Continuous Loss Position Less Than Twelve Months [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	20	8
Fair value available-for-sale securities	$ 112,041	$ 19,943
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 1,606	$ 204
Continuous Unrealized Loss Position, Twelve Months Or Greater
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	27	26
Fair value available-for-sale securities	$ 79,087	$ 101,812
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 3,626	$ 2,658
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	47	34
Fair value available-for-sale securities	$ 191,128	$ 121,755
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 5,232	$ 2,862
CMBS
Continuous Loss Position Less Than Twelve Months [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	67	35
Fair value available-for-sale securities	$ 375,353	$ 139,353
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 6,592	$ 1,395
Continuous Unrealized Loss Position, Twelve Months Or Greater
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	5	4
Fair value available-for-sale securities	$ 2,814	$ 3,518
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 260	$ 215
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	72	39
Fair value available-for-sale securities	$ 378,167	$ 142,871
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 6,852	$ 1,610
Other ABS
Continuous Loss Position Less Than Twelve Months [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	127	92
Fair value available-for-sale securities	$ 439,883	$ 260,864
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 2,228	$ 777
Continuous Unrealized Loss Position, Twelve Months Or Greater
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	5	7
Fair value available-for-sale securities	$ 4,649	$ 8,297
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 40	$ 45
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	132	99
Fair value available-for-sale securities	$ 444,532	$ 269,161
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 2,268	$ 822
Foreign government securities
Continuous Loss Position Less Than Twelve Months [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	19	5
Fair value available-for-sale securities	$ 20,800	$ 7,397
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 273	$ 33
Continuous Unrealized Loss Position, Twelve Months Or Greater
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security	0	0
Fair value available-for-sale securities	$ 0	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 0	$ 0
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	19	5
Fair value available-for-sale securities	$ 20,800	$ 7,397
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 273	33
Debt Securities
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net		$ 400
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net			$ (500)
Continuous Loss Position Less Than Twelve Months [Abstract]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security		13
Fair value available-for-sale securities		$ 149,785
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		$ 1,989
Continuous Unrealized Loss Position, Twelve Months Or Greater
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security		0
Fair value available-for-sale securities		$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		$ 0
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security		13
Fair value available-for-sale securities		$ 149,785
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		1,989
Debt and Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net		1,000
Convertible Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net		$ 500