Note 6 - Accumulated Other Comprehensive Income - Changes in AOCI (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Balance		$ 1,407,287	$ 1,320,070	$ 1,256,292
Balance		1,576,000	1,407,287	1,320,070
Total ending AOCI		1,576,000	1,407,287	1,256,292
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] | Equity Securities [Member]
Balance	[1]	3,992	7,730	3,657
Adoption of accounting standards (Note 2)	[1]	(3,992)
Other comprehensive income (loss) before reclassifications	[1]		7,320	3,844
Amounts reclassified from AOCI	[1],[2]		(11,058)	229
Balance	[1]		3,992	7,730
Total ending AOCI	[1]	3,992	3,992	7,730
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] | Metropolitan District Bond Securities [Member]
Balance	[1]		14,341	12,058
Other comprehensive income (loss) before reclassifications	[1]		7,884	2,283
Amounts reclassified from AOCI	[1],[2]		(22,225)
Balance	[1]			14,341
Total ending AOCI	[1]			14,341
AOCI Attributable to Parent [Member]
Balance		3,992	22,071	15,715
Balance			3,992	22,071
Total ending AOCI		$ 3,992	$ 22,071	$ 15,715

[1] All amounts net-of-tax.

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