Loans - Credit risk profile by risk grade of acquired non-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
Other income producing property
Loans
Total loans	346,554	380,177
Consumer Owner Occupied Loans
Loans
Total loans	2,706,960	2,731,499
Commercial loans | Commercial non-owner occupied
Loans
Total loans	2,323,967	2,329,449
Commercial loans | Commercial owner occupied real estate loan
Loans
Total loans	2,158,701	2,086,662
Commercial loans | Commercial and industrial
Loans
Total loans	1,386,327	1,251,719
Commercial loans | Other loans
Loans
Total loans	13,892	18,224
Consumer | Home equity loans
Loans
Total loans	758,020	791,658
Consumer | Consumer
Loans
Total loans	663,422	610,231
Acquired non-credit impaired loans
Loans
Total loans	1,760,427	2,378,737
Acquired non-credit impaired loans | Pass
Loans
Total loans	1,697,942	2,292,765
Acquired non-credit impaired loans | Special mention
Loans
Total loans	39,772	59,811
Acquired non-credit impaired loans | Substandard
Loans
Total loans	22,713	26,161
Acquired non-credit impaired loans | Other income producing property
Loans
Total loans	95,697	120,696
Acquired non-credit impaired loans | Commercial loans
Loans
Total loans	985,780	1,438,160
Acquired non-credit impaired loans | Commercial loans | Pass
Loans
Total loans	950,062	1,383,572
Acquired non-credit impaired loans | Commercial loans | Special mention
Loans
Total loans	27,005	44,197
Acquired non-credit impaired loans | Commercial loans | Substandard
Loans
Total loans	8,713	10,391
Acquired non-credit impaired loans | Commercial loans | Construction and land development
Loans
Total loans	33,569	113,572
Acquired non-credit impaired loans | Commercial loans | Construction and land development | Pass
Loans
Total loans	31,690	111,414
Acquired non-credit impaired loans | Commercial loans | Construction and land development | Special mention
Loans
Total loans	966	845
Acquired non-credit impaired loans | Commercial loans | Construction and land development | Substandard
Loans
Total loans	913	1,313
Acquired non-credit impaired loans | Commercial loans | Commercial non-owner occupied
Loans
Total loans	447,441	629,394
Acquired non-credit impaired loans | Commercial loans | Commercial non-owner occupied | Pass
Loans
Total loans	432,710	615,277
Acquired non-credit impaired loans | Commercial loans | Commercial non-owner occupied | Special mention
Loans
Total loans	14,162	13,682
Acquired non-credit impaired loans | Commercial loans | Commercial non-owner occupied | Substandard
Loans
Total loans	569	435
Acquired non-credit impaired loans | Commercial loans | Commercial owner occupied real estate loan
Loans
Total loans	307,193	400,658
Acquired non-credit impaired loans | Commercial loans | Commercial owner occupied real estate loan | Pass
Loans
Total loans	300,678	380,536
Acquired non-credit impaired loans | Commercial loans | Commercial owner occupied real estate loan | Special mention
Loans
Total loans	3,092	15,440
Acquired non-credit impaired loans | Commercial loans | Commercial owner occupied real estate loan | Substandard
Loans
Total loans	3,423	4,682
Acquired non-credit impaired loans | Commercial loans | Commercial and industrial
Loans
Total loans	101,880	173,840
Acquired non-credit impaired loans | Commercial loans | Commercial and industrial | Pass
Loans
Total loans	97,092	163,298
Acquired non-credit impaired loans | Commercial loans | Commercial and industrial | Special mention
Loans
Total loans	2,948	7,897
Acquired non-credit impaired loans | Commercial loans | Commercial and industrial | Substandard
Loans
Total loans	1,840	2,645
Acquired non-credit impaired loans | Commercial loans | Other income producing property
Loans
Total loans	95,697	120,696
Acquired non-credit impaired loans | Commercial loans | Other income producing property | Pass
Loans
Total loans	87,892	113,047
Acquired non-credit impaired loans | Commercial loans | Other income producing property | Special mention
Loans
Total loans	5,837	6,333
Acquired non-credit impaired loans | Commercial loans | Other income producing property | Substandard
Loans
Total loans	1,968	1,316
Acquired non-credit impaired loans | Consumer
Loans
Total loans	774,647	940,577
Acquired non-credit impaired loans | Consumer | Pass
Loans
Total loans	747,880	909,193
Acquired non-credit impaired loans | Consumer | Special mention
Loans
Total loans	12,767	15,614
Acquired non-credit impaired loans | Consumer | Substandard
Loans
Total loans	14,000	15,770
Acquired non-credit impaired loans | Consumer | Consumer Owner Occupied Loans
Loans
Total loans	496,431	610,376
Acquired non-credit impaired loans | Consumer | Consumer Owner Occupied Loans | Pass
Loans
Total loans	486,433	597,220
Acquired non-credit impaired loans | Consumer | Consumer Owner Occupied Loans | Special mention
Loans
Total loans	6,434	7,325
Acquired non-credit impaired loans | Consumer | Consumer Owner Occupied Loans | Substandard
Loans
Total loans	3,564	5,831
Acquired non-credit impaired loans | Consumer | Home equity loans
Loans
Total loans	188,732	225,278
Acquired non-credit impaired loans | Consumer | Home equity loans | Pass
Loans
Total loans	174,912	210,066
Acquired non-credit impaired loans | Consumer | Home equity loans | Special mention
Loans
Total loans	5,679	7,623
Acquired non-credit impaired loans | Consumer | Home equity loans | Substandard
Loans
Total loans	8,141	7,589
Acquired non-credit impaired loans | Consumer | Consumer
Loans
Total loans	89,484	104,923
Acquired non-credit impaired loans | Consumer | Consumer | Pass
Loans
Total loans	86,535	101,907
Acquired non-credit impaired loans | Consumer | Consumer | Special mention
Loans
Total loans	654	666
Acquired non-credit impaired loans | Consumer | Consumer | Substandard
Loans
Total loans	$ 2,295	$ 2,350