Note 5 - Investments Schedule of Unrealized Losses (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) security	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) security	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment Losses, Investments	$ 0	$ 1,000	$ 844	$ 1,000
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	691		691		332
Fair value available-for-sale securities	$ 2,736,310		$ 2,736,310		$ 1,338,867
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 69,520		$ 69,520		$ 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	77		77		72
Fair value available-for-sale securities	$ 228,326		$ 228,326		$ 262,531
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 13,205		$ 13,205		$ 6,607
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	768		768		404
Fair value available-for-sale securities	$ 2,964,636		$ 2,964,636		$ 1,601,398
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	82,725		82,725		16,434
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]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security	8		8		6
Fair value available-for-sale securities	$ 41,506		$ 41,506		$ 23,309
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 1,253		$ 1,253		$ 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		3
Fair value available-for-sale securities	$ 9,502		$ 9,502		$ 9,799
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 514		$ 514		$ 238
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	11		11		9
Fair value available-for-sale securities	$ 51,008		$ 51,008		$ 33,108
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 1,767		$ 1,767		$ 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	27		27		21
Fair value available-for-sale securities	$ 78,350		$ 78,350		$ 65,898
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 1,907		$ 1,907		$ 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		0
Fair value available-for-sale securities	$ 0		$ 0		$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 0		$ 0		$ 0
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	27		27		21
Fair value available-for-sale securities	$ 78,350		$ 78,350		$ 65,898
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 1,907		$ 1,907		$ 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	406		406		152
Fair value available-for-sale securities	$ 1,548,096		$ 1,548,096		$ 672,318
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 50,572		$ 50,572		$ 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	29		29		32
Fair value available-for-sale securities	$ 119,977		$ 119,977		$ 139,105
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 8,445		$ 8,445		$ 3,451
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	435		435		184
Fair value available-for-sale securities	$ 1,668,073		$ 1,668,073		$ 811,423
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 59,017		$ 59,017		$ 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	24		24		8
Fair value available-for-sale securities	$ 168,364		$ 168,364		$ 19,943
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 2,175		$ 2,175		$ 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		27		26
Fair value available-for-sale securities	$ 75,189		$ 75,189		$ 101,812
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 3,744		$ 3,744		$ 2,658
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	51		51		34
Fair value available-for-sale securities	$ 243,553		$ 243,553		$ 121,755
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 5,919		$ 5,919		$ 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	76		76		35
Fair value available-for-sale securities	$ 436,659		$ 436,659		$ 139,353
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 9,898		$ 9,898		$ 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	8		8		4
Fair value available-for-sale securities	$ 8,222		$ 8,222		$ 3,518
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 384		$ 384		$ 215
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	84		84		39
Fair value available-for-sale securities	$ 444,881		$ 444,881		$ 142,871
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 10,282		$ 10,282		$ 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	124		124		92
Fair value available-for-sale securities	$ 423,815		$ 423,815		$ 260,864
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 3,063		$ 3,063		$ 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	10		10		7
Fair value available-for-sale securities	$ 15,436		$ 15,436		$ 8,297
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 118		$ 118		$ 45
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	134		134		99
Fair value available-for-sale securities	$ 439,251		$ 439,251		$ 269,161
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 3,181		$ 3,181		$ 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	26		26		5
Fair value available-for-sale securities	$ 39,520		$ 39,520		$ 7,397
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss	$ 652		$ 652		$ 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		0
Fair value available-for-sale securities	$ 0		$ 0		$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss	$ 0		$ 0		$ 0
Continuous Loss Position, Total
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security	26		26		5
Fair value available-for-sale securities	$ 39,520		$ 39,520		$ 7,397
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss	$ 652		$ 652		$ 33
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
Convertible Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net				$ 500