Note 6 - Investments Gross Unrealized Losses and Related Fair Values of Available for Sale Securities (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) security	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($)
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	550	332
Fair value available-for-sale securities	$ 2,201,228	$ 1,338,867
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 50,707	$ 9,827
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	237	72
Fair value available-for-sale securities	$ 957,742	$ 262,531
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 37,333	$ 6,607
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	787	404
Fair value available-for-sale securities	$ 3,158,970	$ 1,601,398
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	88,040	16,434
Other than Temporary Impairment Losses, Investments	1,744	1,420	$ 526
Other Than Temporary Impairment Losses, Investments, Credit Losses, Portion Recognized in Earnings	0		0
Other Than Temporary Impairment Losses, Investments, Credit Losses, Portion Recognized in AOCI	$ 0	$ 0	0
US government and agency securities
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	2	6
Fair value available-for-sale securities	$ 27,415	$ 23,309
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 796	$ 129
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	8	3
Fair value available-for-sale securities	$ 23,476	$ 9,799
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 712	$ 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	$ 50,891	$ 33,108
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 1,508	$ 367
State and municipal obligations
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	12	21
Fair value available-for-sale securities	$ 41,263	$ 65,898
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 955	$ 699
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	16	0
Fair value available-for-sale securities	$ 39,982	$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 945	$ 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	$ 81,245	$ 65,898
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 1,900	$ 699
Corporate bonds and notes
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	330	152
Fair value available-for-sale securities	$ 1,208,430	$ 672,318
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 36,284	$ 4,601
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	126	32
Fair value available-for-sale securities	$ 601,533	$ 139,105
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 27,604	$ 3,451
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	456	184
Fair value available-for-sale securities	$ 1,809,963	$ 811,423
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 63,888	$ 8,052
RMBS
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	15	8
Fair value available-for-sale securities	$ 92,315	$ 19,943
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 782	$ 204
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	28	26
Fair value available-for-sale securities	$ 77,395	$ 101,812
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 3,704	$ 2,658
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	43	34
Fair value available-for-sale securities	$ 169,710	$ 121,755
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 4,486	$ 2,862
CMBS
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	62	35
Fair value available-for-sale securities	$ 328,696	$ 139,353
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 3,973	$ 1,395
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	33	4
Fair value available-for-sale securities	$ 125,728	$ 3,518
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 3,385	$ 215
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	95	39
Fair value available-for-sale securities	$ 454,424	$ 142,871
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	$ 7,358	$ 1,610
Other ABS
Continuous Loss Position Less Than Twelve Months
Number of Securities | security	129	92
Fair value available-for-sale securities	$ 503,109	$ 260,864
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss	$ 7,917	$ 777
Continuous Unrealized Loss Position, Twelve Months Or Greater
Number of securities | security	26	7
Fair value available-for-sale securities	$ 89,628	$ 8,297
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Aggregate Loss	$ 983	$ 45
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	155	99
Fair value available-for-sale securities	$ 592,737	$ 269,161
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss	8,900	$ 822
Foreign government and agency securities
Continuous Loss Position Less Than Twelve Months
Number of Securities | security		5
Fair value available-for-sale securities		$ 7,397
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Aggregate Loss		$ 33
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		5
Fair value available-for-sale securities		$ 7,397
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss		$ 33
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, Credit Losses, Portion Recognized in Earnings		500
Convertible Debt Securities
Continuous Loss Position, Total
Other Than Temporary Impairment Losses, Investments, Credit Losses, Portion Recognized in Earnings		500
Debt Securities [Member]
Continuous Loss Position, Total
Other than Temporary Impairment Losses, Investments	$ 1,700		$ 500
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net		400
Debt and Equity Securities [Member]
Continuous Loss Position, Total
Other than Temporary Impairment Losses, Investments		1,400
Other Than Temporary Impairment Losses, Investments, Credit Losses, Portion Recognized in Earnings		$ 1,000