Loans - Credit risk profile by risk grade of non acquired 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
Non-acquired loans
Loans
Total loans	9,252,831	8,310,613
Non-acquired loans | Pass
Loans
Total loans	9,123,097	8,190,887
Non-acquired loans | Special mention
Loans
Total loans	83,830	66,373
Non-acquired loans | Substandard
Loans
Total loans	45,904	53,353
Non-acquired loans | Other income producing property
Loans
Total loans	218,617	214,235
Non-acquired loans | Other income producing property | Pass
Loans
Total loans	213,291	208,345
Non-acquired loans | Other income producing property | Special mention
Loans
Total loans	3,966	4,250
Non-acquired loans | Other income producing property | Substandard
Loans
Total loans	1,360	1,640
Non-acquired loans | Commercial loans
Loans
Total loans	6,062,991	5,313,632
Non-acquired loans | Commercial loans | Pass
Loans
Total loans	5,971,069	5,236,143
Non-acquired loans | Commercial loans | Special mention
Loans
Total loans	69,268	48,222
Non-acquired loans | Commercial loans | Substandard
Loans
Total loans	22,654	29,267
Non-acquired loans | Commercial loans | Construction and land development
Loans
Total loans	968,360	810,551
Non-acquired loans | Commercial loans | Construction and land development | Pass
Loans
Total loans	959,206	801,949
Non-acquired loans | Commercial loans | Construction and land development | Special mention
Loans
Total loans	7,095	5,303
Non-acquired loans | Commercial loans | Construction and land development | Substandard
Loans
Total loans	2,059	3,299
Non-acquired loans | Commercial loans | Commercial non-owner occupied
Loans
Total loans	1,811,138	1,615,416
Non-acquired loans | Commercial loans | Commercial non-owner occupied | Pass
Loans
Total loans	1,787,306	1,607,034
Non-acquired loans | Commercial loans | Commercial non-owner occupied | Special mention
Loans
Total loans	22,410	7,372
Non-acquired loans | Commercial loans | Commercial non-owner occupied | Substandard
Loans
Total loans	1,422	1,010
Non-acquired loans | Commercial loans | Commercial owner occupied real estate loan
Loans
Total loans	1,784,017	1,601,360
Non-acquired loans | Commercial loans | Commercial owner occupied real estate loan | Pass
Loans
Total loans	1,754,801	1,564,083
Non-acquired loans | Commercial loans | Commercial owner occupied real estate loan | Special mention
Loans
Total loans	19,742	24,241
Non-acquired loans | Commercial loans | Commercial owner occupied real estate loan | Substandard
Loans
Total loans	9,474	13,036
Non-acquired loans | Commercial loans | Commercial and industrial
Loans
Total loans	1,280,859	1,072,070
Non-acquired loans | Commercial loans | Commercial and industrial | Pass
Loans
Total loans	1,256,465	1,054,732
Non-acquired loans | Commercial loans | Commercial and industrial | Special mention
Loans
Total loans	16,055	7,056
Non-acquired loans | Commercial loans | Commercial and industrial | Substandard
Loans
Total loans	8,339	10,282
Non-acquired loans | Commercial loans | Other loans
Loans
Total loans	13,892	18,224
Non-acquired loans | Consumer
Loans
Total loans	3,189,840	2,996,981
Non-acquired loans | Consumer | Pass
Loans
Total loans	3,152,028	2,954,744
Non-acquired loans | Consumer | Special mention
Loans
Total loans	14,562	18,151
Non-acquired loans | Consumer | Substandard
Loans
Total loans	23,250	24,086
Non-acquired loans | Consumer | Consumer Owner Occupied Loans
Loans
Total loans	2,118,839	2,005,314
Non-acquired loans | Consumer | Consumer Owner Occupied Loans | Pass
Loans
Total loans	2,094,080	1,978,122
Non-acquired loans | Consumer | Consumer Owner Occupied Loans | Special mention
Loans
Total loans	9,585	10,769
Non-acquired loans | Consumer | Consumer Owner Occupied Loans | Substandard
Loans
Total loans	15,174	16,423
Non-acquired loans | Consumer | Home equity loans
Loans
Total loans	518,628	508,326
Non-acquired loans | Consumer | Home equity loans | Pass
Loans
Total loans	508,054	495,127
Non-acquired loans | Consumer | Home equity loans | Special mention
Loans
Total loans	4,490	6,915
Non-acquired loans | Consumer | Home equity loans | Substandard
Loans
Total loans	6,084	6,284
Non-acquired loans | Consumer | Consumer
Loans
Total loans	538,481	465,117
Non-acquired loans | Consumer | Consumer | Pass
Loans
Total loans	536,002	463,271
Non-acquired loans | Consumer | Consumer | Special mention
Loans
Total loans	487	467
Non-acquired loans | Consumer | Consumer | Substandard
Loans
Total loans	1,992	1,379
Non-acquired loans | Consumer | Other loans
Loans
Total loans	13,892	18,224
Non-acquired loans | Consumer | Other loans | Pass
Loans
Total loans	$ 13,892	$ 18,224