Loans - Credit risk profile by risk grade of acquired credit impaired loans (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Mar. 31, 2019
Loans
Total loans	$ 11,375,104	$ 11,146,122
Construction and land development
Loans
Total loans	1,017,261	946,503
Commercial loans | Commercial and industrial
Loans
Total loans	1,386,327	1,251,719
Consumer | Real estate
Loans
Total loans	3,464,980	3,523,157
Consumer | Consumer
Loans
Total loans	663,422	610,231
Acquired credit impaired loans
Loans
Total loans	361,846	456,772
Acquired credit impaired loans | Pass
Loans
Total loans	218,364	274,474
Acquired credit impaired loans | Special mention
Loans
Total loans	58,281	71,026
Acquired credit impaired loans | Substandard
Loans
Total loans	85,201	111,272
Acquired credit impaired loans | Commercial loans | Real estate
Loans
Total loans	130,938	173,707
Acquired credit impaired loans | Commercial loans | Real estate | Pass
Loans
Total loans	108,762	140,785
Acquired credit impaired loans | Commercial loans | Real estate | Special mention
Loans
Total loans	6,465	13,025
Acquired credit impaired loans | Commercial loans | Real estate | Substandard
Loans
Total loans	15,711	19,897
Acquired credit impaired loans | Commercial loans | Construction and land development
Loans
Total loans	25,032	32,257
Acquired credit impaired loans | Commercial loans | Construction and land development | Pass
Loans
Total loans	17,756	20,950
Acquired credit impaired loans | Commercial loans | Construction and land development | Special mention
Loans
Total loans	2,904	3,500
Acquired credit impaired loans | Commercial loans | Construction and land development | Substandard
Loans
Total loans	4,372	7,807
Acquired credit impaired loans | Commercial loans | Commercial and industrial
Loans
Total loans	7,029	10,925
Acquired credit impaired loans | Commercial loans | Commercial and industrial | Pass
Loans
Total loans	5,160	6,802
Acquired credit impaired loans | Commercial loans | Commercial and industrial | Special mention
Loans
Total loans	286	530
Acquired credit impaired loans | Commercial loans | Commercial and industrial | Substandard
Loans
Total loans	1,583	3,593
Acquired credit impaired loans | Consumer | Real estate
Loans
Total loans	163,359	199,701
Acquired credit impaired loans | Consumer | Real estate | Pass
Loans
Total loans	82,203	100,803
Acquired credit impaired loans | Consumer | Real estate | Special mention
Loans
Total loans	35,968	40,036
Acquired credit impaired loans | Consumer | Real estate | Substandard
Loans
Total loans	45,188	58,862
Acquired credit impaired loans | Consumer | Consumer
Loans
Total loans	35,488	40,182
Acquired credit impaired loans | Consumer | Consumer | Pass
Loans
Total loans	4,483	5,134
Acquired credit impaired loans | Consumer | Consumer | Special mention
Loans
Total loans	12,658	13,935
Acquired credit impaired loans | Consumer | Consumer | Substandard
Loans
Total loans	$ 18,347	$ 21,113