Loans - Credit risk profile by risk grade of acquired credit impaired loans (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019
Loans
Total loans	$ 11,375,104	$ 11,290,147
Construction and land development
Loans
Total loans	1,017,261	1,025,387
Commercial loans | Commercial and industrial
Loans
Total loans	1,386,327	1,261,527
Consumer | Consumer
Loans
Total loans	663,422	654,883
Acquired credit impaired loans
Loans
Total loans	361,846	396,032
Acquired credit impaired loans | Pass
Loans
Total loans	218,364	238,772
Acquired credit impaired loans | Special mention
Loans
Total loans	58,281	62,177
Acquired credit impaired loans | Substandard
Loans
Total loans	85,201	95,083
Acquired credit impaired loans | Construction and land development
Loans
Total loans		26,930
Acquired credit impaired loans | Consumer
Loans
Total loans		36,812
Acquired credit impaired loans | Commercial and industrial
Loans
Total loans		8,112
Acquired credit impaired loans | Commercial loans | Real estate
Loans
Total loans	130,938	149,134
Acquired credit impaired loans | Commercial loans | Real estate | Pass
Loans
Total loans	108,762	121,264
Acquired credit impaired loans | Commercial loans | Real estate | Special mention
Loans
Total loans	6,465	9,064
Acquired credit impaired loans | Commercial loans | Real estate | Substandard
Loans
Total loans	15,711	18,806
Acquired credit impaired loans | Commercial loans | Construction and land development
Loans
Total loans	25,032	26,930
Acquired credit impaired loans | Commercial loans | Construction and land development | Pass
Loans
Total loans	17,756	18,978
Acquired credit impaired loans | Commercial loans | Construction and land development | Special mention
Loans
Total loans	2,904	3,018
Acquired credit impaired loans | Commercial loans | Construction and land development | Substandard
Loans
Total loans	4,372	4,934
Acquired credit impaired loans | Commercial loans | Commercial and industrial
Loans
Total loans	7,029	8,112
Acquired credit impaired loans | Commercial loans | Commercial and industrial | Pass
Loans
Total loans	5,160	5,703
Acquired credit impaired loans | Commercial loans | Commercial and industrial | Special mention
Loans
Total loans	286	471
Acquired credit impaired loans | Commercial loans | Commercial and industrial | Substandard
Loans
Total loans	1,583	1,938
Acquired credit impaired loans | Consumer | Real estate
Loans
Total loans	163,359	175,044
Acquired credit impaired loans | Consumer | Real estate | Pass
Loans
Total loans	82,203	88,131
Acquired credit impaired loans | Consumer | Real estate | Special mention
Loans
Total loans	35,968	36,901
Acquired credit impaired loans | Consumer | Real estate | Substandard
Loans
Total loans	45,188	50,012
Acquired credit impaired loans | Consumer | Consumer
Loans
Total loans	35,488	36,812
Acquired credit impaired loans | Consumer | Consumer | Pass
Loans
Total loans	4,483	4,696
Acquired credit impaired loans | Consumer | Consumer | Special mention
Loans
Total loans	12,658	12,723
Acquired credit impaired loans | Consumer | Consumer | Substandard
Loans
Total loans	$ 18,347	$ 19,393