Note 6 - Investments Schedule of Unrealized Losses (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security	Dec. 31, 2015 USD ($)
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	332	362
Fair value available-for-sale securities	$ 1,338,867	$ 1,566,440
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 9,827	$ 37,941
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	72	21
Fair value available-for-sale securities	$ 262,531	$ 58,451
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 6,607	$ 842
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	404	383
Fair value available-for-sale securities	$ 1,601,398	$ 1,624,891
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	16,434	38,783
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net	1,420	526	$ 0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Period Increase (Decrease)		0	0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Credit Losses on Debt Securities Held	0	$ 0	$ 0
Convertible Debt Securities [Member]
Continuous Loss Position, Total
Other than Temporary Impairment Losses, Investments	$ 500
US government and agency securities
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	6	7
Fair value available-for-sale securities	$ 23,309	$ 73,160
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 129	$ 3,459
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	3	0
Fair value available-for-sale securities	$ 9,799	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 238	$ 0
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	9	7
Fair value available-for-sale securities	$ 33,108	$ 73,160
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	367	$ 3,459
Debt Securities [Member]
Continuous Loss Position, Total
Other than Temporary Impairment Losses, Investments	$ 400
State and municipal obligations
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	21	7
Fair value available-for-sale securities	$ 65,898	$ 30,901
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 699	$ 821
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	0	0
Fair value available-for-sale securities	$ 0	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 0	$ 0
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	21	7
Fair value available-for-sale securities	$ 65,898	$ 30,901
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 699	$ 821
Corporate bonds and notes
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	152	185
Fair value available-for-sale securities	$ 672,318	$ 788,876
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 4,601	$ 22,135
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	32	2
Fair value available-for-sale securities	$ 139,105	$ 4,582
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 3,451	$ 277
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	184	187
Fair value available-for-sale securities	$ 811,423	$ 793,458
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 8,052	$ 22,412
Residential Mortgage Backed Securities [Member]
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	8	56
Fair value available-for-sale securities	$ 19,943	$ 311,031
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 204	$ 7,822
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	26	1
Fair value available-for-sale securities	$ 101,812	$ 1,398
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 2,658	$ 9
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	34	57
Fair value available-for-sale securities	$ 121,755	$ 312,429
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 2,862	$ 7,831
CMBS
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	35	37
Fair value available-for-sale securities	$ 139,353	$ 218,170
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 1,395	$ 2,909
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	4	2
Fair value available-for-sale securities	$ 3,518	$ 6,585
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 215	$ 114
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	39	39
Fair value available-for-sale securities	$ 142,871	$ 224,755
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 1,610	$ 3,023
Other ABS
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	92	58
Fair value available-for-sale securities	$ 260,864	$ 131,268
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 777	$ 470
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	7	16
Fair value available-for-sale securities	$ 8,297	$ 45,886
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 45	$ 442
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	99	74
Fair value available-for-sale securities	$ 269,161	$ 177,154
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 822	$ 912
Foreign Government Debt Securities [Member]
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	5	12
Fair value available-for-sale securities	$ 7,397	$ 13,034
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 33	$ 325
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	0	0
Fair value available-for-sale securities	$ 0	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 0	$ 0
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	5	12
Fair value available-for-sale securities	$ 7,397	$ 13,034
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 33	$ 325
Equity securities
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	13
Fair value available-for-sale securities	$ 149,785
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 1,989
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	0
Fair value available-for-sale securities	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate 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, Aggregate Loss	1,989
Other than Temporary Impairment Losses, Investments	$ 500