Note 2 - Mergers and Acquisitions - Pro Forma Combined Operating Results (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Guaranty Bank and Trust Company [Member]
Net interest income	$ 122,923	$ 102,902
Noninterest income	32,703	34,238
Net income	$ 38,728	$ 27,103
Basic (in dollars per share)	$ 2.80	$ 2.05
Diluted (in dollars per share)	$ 2.73	$ 2.02
Community State Bank [Member]
Net interest income			$ 110,035
Noninterest income			34,773
Net income			$ 34,137
Basic (in dollars per share)			$ 2.62
Diluted (in dollars per share)			$ 2.58
Springfield Bancshares
Net interest income	$ 153,229	$ 136,190
Noninterest income	42,538	32,395
Net income	$ 49,542	$ 42,316
Basic (in dollars per share)	$ 3.17	$ 2.82
Diluted (in dollars per share)	$ 3.11	$ 2.75
Bates Companies [Member]
Net interest income	$ 142,368	$ 116,029
Noninterest income	44,455	33,044
Net income	$ 44,032	$ 35,627
Basic (in dollars per share)	$ 2.98	$ 2.67
Diluted (in dollars per share)	$ 2.92	$ 2.60