Note 6 - Investments Schedule of Unrealized Losses (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) security	Dec. 31, 2015 USD ($) security	Dec. 31, 2014 USD ($)
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	362	305
Fair value available-for-sale securities	$ 1,566,440,000	$ 1,462,577,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 37,941,000	$ 32,079,000
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	21	23
Fair value available-for-sale securities	$ 58,451,000	$ 54,203,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 842,000	$ 2,221,000
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	383	328
Fair value available-for-sale securities	$ 1,624,891,000	$ 1,516,780,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	38,783,000	34,300,000
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net	526,000	0	$ 0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Period Increase (Decrease)	0	0	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
Number of Securities | security	7	1
Fair value available-for-sale securities	$ 73,160,000	$ 5,752,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 3,459,000	$ 21,000
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	7	1
Fair value available-for-sale securities	$ 73,160,000	$ 5,752,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 3,459,000	$ 21,000
State and municipal obligations
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	7	2
Fair value available-for-sale securities	$ 30,901,000	$ 11,674,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 821,000	$ 120,000
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	7	2
Fair value available-for-sale securities	$ 30,901,000	$ 11,674,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 821,000	$ 120,000
Corporate bonds and notes
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	185	117
Fair value available-for-sale securities	$ 788,876,000	$ 510,807,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 22,135,000	$ 16,773,000
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	2	6
Fair value available-for-sale securities	$ 4,582,000	$ 8,700,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 277,000	$ 518,000
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	187	123
Fair value available-for-sale securities	$ 793,458,000	$ 519,507,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 22,412,000	$ 17,291,000
Residential Mortgage Backed Securities [Member]
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	56	12
Fair value available-for-sale securities	$ 311,031,000	$ 168,415,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 7,822,000	$ 2,464,000
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	1	0
Fair value available-for-sale securities	$ 1,398,000	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 9,000	$ 0
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	57	12
Fair value available-for-sale securities	$ 312,429,000	$ 168,415,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 7,831,000	$ 2,464,000
CMBS
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	37	58
Fair value available-for-sale securities	$ 218,170,000	$ 387,268,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 2,909,000	$ 8,939,000
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	2	0
Fair value available-for-sale securities	$ 6,585,000	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 114,000	$ 0
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	39	58
Fair value available-for-sale securities	$ 224,755,000	$ 387,268,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 3,023,000	$ 8,939,000
Other ABS
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	58	96
Fair value available-for-sale securities	$ 131,268,000	$ 284,998,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 470,000	$ 2,559,000
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	16	14
Fair value available-for-sale securities	$ 45,886,000	$ 43,225,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 442,000	$ 1,415,000
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	74	110
Fair value available-for-sale securities	$ 177,154,000	$ 328,223,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 912,000	$ 3,974,000
Foreign government securities
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	12	18
Fair value available-for-sale securities	$ 13,034,000	$ 18,733,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 325,000	$ 1,095,000
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	0	3
Fair value available-for-sale securities	$ 0	$ 2,278,000
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 0	$ 288,000
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	12	21
Fair value available-for-sale securities	$ 13,034,000	$ 21,011,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 325,000	$ 1,383,000
Equity securities
Continuous Loss Position Less Than Twelve Months
Number of Securities | security		1
Fair value available-for-sale securities		$ 74,930,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss		$ 108,000
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		1
Fair value available-for-sale securities		$ 74,930,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss		$ 108,000