Summary of Significant Accounting Policies - Basic and diluted earnings per share (Details 2) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2014
Numerator:
Net income per consolidated statements of income	$ 2,726	$ 2,727	$ 3,256	$ 3,670	$ 3,848	$ 3,769	$ 4,330	$ 4,909	$ 12,379	$ 16,856	$ 16,703
Less: Allocation of earnings to participating securities									(170)
Net income allocated to common stock									12,209	16,856	16,703
Basic earnings per share computation:
Net income allocated to common stock									$ 12,209	$ 16,856	$ 16,703
Weighted average common shares outstanding, including shares considered participating securities									7,727	12,013	12,096
Less: Weighted average participating securities outstanding									(105)
Weighted average common shares outstanding for basic earnings per share									7,622	12,013	12,096
Basic earnings per share (in dollars per share)	$ 0.35	$ 0.36	$ 0.43	$ 0.48	$ 0.34	$ 0.31	$ 0.36	$ 0.40	$ 1.60	$ 1.40	$ 1.38
Diluted earnings per share computation:
Net income allocated to common stock									$ 12,209	$ 16,856	$ 16,703
Undistributed earnings re-allocated to participating securities									2
Net income allocated to common stock									$ 12,211	$ 16,856	$ 16,703
Weighted average common shares outstanding for basic earnings per share									7,622	12,013	12,096
Incremental shares from stock options									70	179	229
Weighted average shares and dilutive potential common shares									7,692	12,192	12,325
Diluted earnings per share (in dollars per share)	$ 0.35	$ 0.35	$ 0.42	$ 0.47	$ 0.33	$ 0.30	$ 0.35	$ 0.40	$ 1.59	$ 1.38	$ 1.36