Note 6 - Accumulated Other Comprehensive Income - Changes in AOCI (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Balance		$ 1,576,000	$ 1,407,287	$ 1,320,070
Balance		1,782,485	1,576,000	1,407,287
Total ending AOCI		1,576,000	1,407,287	1,407,287
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member] | Equity Securities [Member]
Balance	[1]	0	3,992	7,730
Adoption of accounting standards (Note 2)	[1]	0	(3,992)	0
Other comprehensive income (loss) before reclassifications	[1]	0	0	7,320
Amounts reclassified from AOCI	[1],[2]	0	0	(11,058)
Balance	[1]	0	0	3,992
Total ending AOCI	[1]	0	3,992	7,730
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Parent [Member] | Metropolitan District Bond Securities [Member]
Balance	[1]	0	0	14,341
Other comprehensive income (loss) before reclassifications	[1]	0	0	7,884
Amounts reclassified from AOCI	[1],[2]	0	0	(22,225)
Balance	[1]	0	0	0
Total ending AOCI	[1]	0	0	14,341
AOCI Attributable to Parent [Member]
Balance		0	3,992	22,071
Balance		0	0	3,992
Total ending AOCI		$ 0	$ 0	$ 3,992

[1] All amounts net-of-tax.

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