Note 1 - Summary of Significant Accounting Policies - Comprehensive Income (Details) - USD ($)	3 Months Ended								12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Net income	$ 10,183,000	$ 9,064,000	$ 8,760,000	$ 9,276,000	$ 7,503,000	$ 4,823,000	$ 4,899,000	$ 6,594,000	$ 37,283,000	$ 23,819,000	$ 20,567,000
Net change in unrealized gain on short term investments									0	0	54,000
Comprehensive income									$ 37,283,000	$ 23,819,000	$ 20,621,000