Loans and Allowance for Credit Losses - Financing receivable credit quality indicators (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Purchase Credit Deteriorated | Pass | Consumer Loan
Financing Receivable Credit Quality Indicators	$ 17,990	$ 10,989	$ 4,580	$ 1,584	$ 797
Purchase Credit Deteriorated | Pass | Commercial Loan
Financing Receivable Credit Quality Indicators	152,172	94,604	21,289	8,517	7,831
Purchase Credit Deteriorated | Pass | Construction Real Estate
Financing Receivable Credit Quality Indicators	73,397	42,331	6,998
Purchase Credit Deteriorated | Watch | Consumer Loan
Financing Receivable Credit Quality Indicators	83
Purchase Credit Deteriorated | Watch | Commercial Loan
Financing Receivable Credit Quality Indicators	1,284	274	1,791	139
Purchase Credit Deteriorated | Substandard | Consumer Loan
Financing Receivable Credit Quality Indicators		26		36	32
Purchase Credit Deteriorated | Substandard | Commercial Loan
Financing Receivable Credit Quality Indicators	449	1,109	321	1	176
Purchase Credit Deteriorated | Substandard | Construction Real Estate
Financing Receivable Credit Quality Indicators	16
Purchase Credit Deteriorated | Residential real estate loans | Pass
Financing Receivable Credit Quality Indicators	247,325	198,404	39,207	36,547	27,142
Purchase Credit Deteriorated | Residential real estate loans | Watch
Financing Receivable Credit Quality Indicators	125	71	10,994		93
Purchase Credit Deteriorated | Residential real estate loans | Substandard
Financing Receivable Credit Quality Indicators	4,652	48	17	52
Purchase Credit Deteriorated | Commercial Real Estate | Pass
Financing Receivable Credit Quality Indicators	264,035	174,218	109,116	105,165	76,865
Purchase Credit Deteriorated | Commercial Real Estate | Watch
Financing Receivable Credit Quality Indicators	4,271	813	10,740	5,393	530
Purchase Credit Deteriorated | Commercial Real Estate | Special Mention
Financing Receivable Credit Quality Indicators		8,806		1,793	12,826
Purchase Credit Deteriorated | Commercial Real Estate | Substandard
Financing Receivable Credit Quality Indicators	8,713	1,162	506	6	50
Purchase Credit Deteriorated | Commercial Real Estate | Doubtful
Financing Receivable Credit Quality Indicators			857
Purchase Credit Deteriorated | Prior to 2017 | Pass | Consumer Loan
Financing Receivable Credit Quality Indicators	689
Purchase Credit Deteriorated | Prior to 2017 | Pass | Commercial Loan
Financing Receivable Credit Quality Indicators	9,686
Purchase Credit Deteriorated | Prior to 2017 | Watch | Commercial Loan
Financing Receivable Credit Quality Indicators	7
Purchase Credit Deteriorated | Prior to 2017 | Substandard | Commercial Loan
Financing Receivable Credit Quality Indicators	4
Purchase Credit Deteriorated | Prior to 2017 | Residential real estate loans | Pass
Financing Receivable Credit Quality Indicators	84,507
Purchase Credit Deteriorated | Prior to 2017 | Residential real estate loans | Watch
Financing Receivable Credit Quality Indicators	817
Purchase Credit Deteriorated | Prior to 2017 | Residential real estate loans | Substandard
Financing Receivable Credit Quality Indicators	414
Purchase Credit Deteriorated | Prior to 2017 | Commercial Real Estate | Pass
Financing Receivable Credit Quality Indicators	84,000
Purchase Credit Deteriorated | Prior to 2017 | Commercial Real Estate | Watch
Financing Receivable Credit Quality Indicators	460
Purchase Credit Deteriorated | Prior to 2017 | Commercial Real Estate | Substandard
Financing Receivable Credit Quality Indicators	101
Purchase Credit Deteriorated | Revolving Credit Facility | Pass | Consumer Loan
Financing Receivable Credit Quality Indicators	39,459
Purchase Credit Deteriorated | Revolving Credit Facility | Pass | Commercial Loan
Financing Receivable Credit Quality Indicators	118,221
Purchase Credit Deteriorated | Revolving Credit Facility | Watch | Consumer Loan
Financing Receivable Credit Quality Indicators	48
Purchase Credit Deteriorated | Revolving Credit Facility | Watch | Commercial Loan
Financing Receivable Credit Quality Indicators	1,991
Purchase Credit Deteriorated | Revolving Credit Facility | Substandard | Consumer Loan
Financing Receivable Credit Quality Indicators	34
Purchase Credit Deteriorated | Revolving Credit Facility | Substandard | Commercial Loan
Financing Receivable Credit Quality Indicators	1,959
Purchase Credit Deteriorated | Revolving Credit Facility | Residential real estate loans | Pass
Financing Receivable Credit Quality Indicators	5,385
Purchase Credit Deteriorated | Revolving Credit Facility | Commercial Real Estate | Pass
Financing Receivable Credit Quality Indicators	25,836
Purchase Credit Deteriorated | Revolving Credit Facility | Commercial Real Estate | Watch
Financing Receivable Credit Quality Indicators	819
Purchase Credit Deteriorated | Revolving Credit Facility | Commercial Real Estate | Special Mention
Financing Receivable Credit Quality Indicators	300
Purchase Credit Deteriorated | Revolving Credit Facility | Commercial Real Estate | Substandard
Financing Receivable Credit Quality Indicators	69
Purchase Credit Deteriorated | Total Credit Quality Indicator for Years | Pass | Consumer Loan
Financing Receivable Credit Quality Indicators	76,088
Purchase Credit Deteriorated | Total Credit Quality Indicator for Years | Pass | Commercial Loan
Financing Receivable Credit Quality Indicators	412,320
Purchase Credit Deteriorated | Total Credit Quality Indicator for Years | Pass | Construction Real Estate
Financing Receivable Credit Quality Indicators	122,726
Purchase Credit Deteriorated | Total Credit Quality Indicator for Years | Watch | Consumer Loan
Financing Receivable Credit Quality Indicators	131
Purchase Credit Deteriorated | Total Credit Quality Indicator for Years | Watch | Commercial Loan
Financing Receivable Credit Quality Indicators	5,486
Purchase Credit Deteriorated | Total Credit Quality Indicator for Years | Substandard | Consumer Loan
Financing Receivable Credit Quality Indicators	128
Purchase Credit Deteriorated | Total Credit Quality Indicator for Years | Substandard | Commercial Loan
Financing Receivable Credit Quality Indicators	4,019
Purchase Credit Deteriorated | Total Credit Quality Indicator for Years | Substandard | Construction Real Estate
Financing Receivable Credit Quality Indicators	16
Purchase Credit Deteriorated | Total Credit Quality Indicator for Years | Residential real estate loans | Pass
Financing Receivable Credit Quality Indicators	638,517
Purchase Credit Deteriorated | Total Credit Quality Indicator for Years | Residential real estate loans | Watch
Financing Receivable Credit Quality Indicators	12,100
Purchase Credit Deteriorated | Total Credit Quality Indicator for Years | Residential real estate loans | Substandard
Financing Receivable Credit Quality Indicators	5,183
Purchase Credit Deteriorated | Total Credit Quality Indicator for Years | Commercial Real Estate | Pass
Financing Receivable Credit Quality Indicators	839,235
Purchase Credit Deteriorated | Total Credit Quality Indicator for Years | Commercial Real Estate | Watch
Financing Receivable Credit Quality Indicators	23,026
Purchase Credit Deteriorated | Total Credit Quality Indicator for Years | Commercial Real Estate | Special Mention
Financing Receivable Credit Quality Indicators	23,725
Purchase Credit Deteriorated | Total Credit Quality Indicator for Years | Commercial Real Estate | Substandard
Financing Receivable Credit Quality Indicators	10,607
Purchase Credit Deteriorated | Total Credit Quality Indicator for Years | Commercial Real Estate | Doubtful
Financing Receivable Credit Quality Indicators	857
Total Loans
Financing Receivable Credit Quality Indicators	774,512	532,855	206,416	159,233	126,342
Total Loans | Consumer Loan
Financing Receivable Credit Quality Indicators	18,073	11,015	4,580	1,620	829
Total Loans | Commercial Loan
Financing Receivable Credit Quality Indicators	153,905	95,987	23,401	8,657	8,007
Total Loans | Construction Real Estate
Financing Receivable Credit Quality Indicators	73,413	42,331	6,998
Total Loans | Pass
Financing Receivable Credit Quality Indicators	754,919	520,546	181,190	151,813	112,635
Total Loans | Watch
Financing Receivable Credit Quality Indicators	5,763	1,158	23,525	5,532	623
Total Loans | Special Mention
Financing Receivable Credit Quality Indicators		8,806		1,793	12,826
Total Loans | Substandard
Financing Receivable Credit Quality Indicators	13,830	2,345	844	95	258
Total Loans | Doubtful
Financing Receivable Credit Quality Indicators			857
Total Loans | Residential real estate loans
Financing Receivable Credit Quality Indicators	252,102	198,523	50,218	36,599	27,235
Total Loans | Commercial Real Estate
Financing Receivable Credit Quality Indicators	277,019	$ 184,999	$ 121,219	$ 112,357	$ 90,271
Total Loans | Prior to 2017
Financing Receivable Credit Quality Indicators	180,685
Total Loans | Prior to 2017 | Consumer Loan
Financing Receivable Credit Quality Indicators	689
Total Loans | Prior to 2017 | Commercial Loan
Financing Receivable Credit Quality Indicators	9,697
Total Loans | Prior to 2017 | Pass
Financing Receivable Credit Quality Indicators	178,882
Total Loans | Prior to 2017 | Watch
Financing Receivable Credit Quality Indicators	1,284
Total Loans | Prior to 2017 | Substandard
Financing Receivable Credit Quality Indicators	519
Total Loans | Prior to 2017 | Residential real estate loans
Financing Receivable Credit Quality Indicators	85,738
Total Loans | Prior to 2017 | Commercial Real Estate
Financing Receivable Credit Quality Indicators	84,561
Total Loans | Revolving Credit Facility
Financing Receivable Credit Quality Indicators	194,121
Total Loans | Revolving Credit Facility | Consumer Loan
Financing Receivable Credit Quality Indicators	39,541
Total Loans | Revolving Credit Facility | Commercial Loan
Financing Receivable Credit Quality Indicators	122,171
Total Loans | Revolving Credit Facility | Pass
Financing Receivable Credit Quality Indicators	188,901
Total Loans | Revolving Credit Facility | Watch
Financing Receivable Credit Quality Indicators	2,858
Total Loans | Revolving Credit Facility | Special Mention
Financing Receivable Credit Quality Indicators	300
Total Loans | Revolving Credit Facility | Substandard
Financing Receivable Credit Quality Indicators	2,062
Total Loans | Revolving Credit Facility | Residential real estate loans
Financing Receivable Credit Quality Indicators	5,385
Total Loans | Revolving Credit Facility | Commercial Real Estate
Financing Receivable Credit Quality Indicators	27,024
Total Loans | Total Credit Quality Indicator for Years
Financing Receivable Credit Quality Indicators	2,174,164
Total Loans | Total Credit Quality Indicator for Years | Consumer Loan
Financing Receivable Credit Quality Indicators	76,347
Total Loans | Total Credit Quality Indicator for Years | Commercial Loan
Financing Receivable Credit Quality Indicators	421,825
Total Loans | Total Credit Quality Indicator for Years | Construction Real Estate
Financing Receivable Credit Quality Indicators	122,742
Total Loans | Total Credit Quality Indicator for Years | Pass
Financing Receivable Credit Quality Indicators	2,088,886
Total Loans | Total Credit Quality Indicator for Years | Watch
Financing Receivable Credit Quality Indicators	40,743
Total Loans | Total Credit Quality Indicator for Years | Special Mention
Financing Receivable Credit Quality Indicators	23,725
Total Loans | Total Credit Quality Indicator for Years | Substandard
Financing Receivable Credit Quality Indicators	19,953
Total Loans | Total Credit Quality Indicator for Years | Doubtful
Financing Receivable Credit Quality Indicators	857
Total Loans | Total Credit Quality Indicator for Years | Residential real estate loans
Financing Receivable Credit Quality Indicators	655,800
Total Loans | Total Credit Quality Indicator for Years | Commercial Real Estate
Financing Receivable Credit Quality Indicators	$ 897,450