Note 5 - Accumulated Other Comprehensive Income - Changes in AOCI (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Beginning balance 1				$ 1,407,287
Ending balance		$ 1,489,841		1,489,841
Total ending AOCI		1,489,841		1,489,841
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] | Equity Securities [Member]
Beginning balance 1	[1]		$ 9,479	3,992	$ 7,730
Adoption of accounting standards (Note 2)	[1]			(3,992)
Other comprehensive income before reclassifications	[1]		2,389		4,423
Amounts reclassified from AOCI	[1],[2]		(692)		(977)
Ending balance	[1]		11,176		11,176
Total ending AOCI	[1]		9,479	3,992	7,730
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] | Metropolitan District Bond Securities [Member]
Beginning balance 1	[1]		14,578		14,341
Other comprehensive income before reclassifications	[1]		247		484
Amounts reclassified from AOCI	[1]
Ending balance	[1]		14,825		14,825
Total ending AOCI	[1]		14,578		14,341
AOCI Attributable to Parent [Member]
Ending balance			26,001		26,001
Total ending AOCI			$ 26,001		$ 26,001

[1] All amounts net-of-tax.

[2] See separate table below for details about these reclassifications.