Note 5 - Investments Gain (Loss) on Investments (Details) $ in Thousands		3 Months Ended		12 Months Ended
		Mar. 31, 2018 USD ($) security	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) security
Gain (Loss) on Investments [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security		612		332
Other gains		$ 12	$ 18
Net realized losses on investments		(3,442)	(8,179)
Other than Temporary Impairment Losses, Investments			0
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net		844		$ (1,400)
Unrealized gains on trading securities	[1]		5,226
Other Securities, Change in Unrealized Holding Gain (Loss)	[1]	(12,804)
Total net gains on investments		(17,090)	(2,953)
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		2,417,708		1,338,867
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		$ 46,167		$ 9,827
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security		68		72
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		$ 223,335		$ 262,531
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		$ 11,699		$ 6,607
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security		680		404
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		$ 2,641,043		$ 1,601,398
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		57,866		$ 16,434
Net gains (losses) on other financial instruments		(1,797)	102
Net Gains (Losses) on Investments and Other Financial Instruments		(18,887)	(2,851)
Fixed Maturities [Member]
Gain (Loss) on Investments [Line Items]
Available-for-sale Securities, Gross Realized (Loss) Gain, Excluding Other than Temporary Impairments		(3,120)	(2,509)
Equity securities
Gain (Loss) on Investments [Line Items]
Equity Securities, Realized Gain (Loss)		142	0
Trading Securities [Member]
Gain (Loss) on Investments [Line Items]
Trading securities		(538)	(5,694)
Short-term Investments [Member]
Gain (Loss) on Investments [Line Items]
Short-term investments		0	6
Other investments
Gain (Loss) on Investments [Line Items]
Cost-method Investments, Realized Gain (Loss), Excluding Other than Temporary Impairments		$ 62	$ 0
US States and Political Subdivisions Debt Securities [Member]
Gain (Loss) on Investments [Line Items]
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | security		28		21
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than Twelve Months, Fair Value		$ 79,897		$ 65,898
Available-for-sale Securities, Continuous Unrealized Loss Position, Less than 12 Months, Accumulated Loss		$ 1,729		$ 699
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | security		0		0
Available-for-sale Securities, Continuous Unrealized Loss Position, Twelve Months or Longer, Fair Value		$ 0		$ 0
Available-for-sale Securities, Continuous Unrealized Loss Position, 12 Months or Longer, Accumulated Loss		$ 0		$ 0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | security		28		21
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value		$ 79,897		$ 65,898
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss		$ 1,729		$ 699

[1] These amounts include unrealized gains (losses) on investment securities other than securities available for sale. For the three months ended March 31, 2017, the amount excludes the net change in unrealized gains and losses on equity securities. Prior to the implementation of the update to the standard for the accounting of financial instruments effective January 1, 2018, the unrealized losses associated with equity securities were classified in accumulated other comprehensive income.