Loans - Credit risk profile by risk grade of acquired non-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
Other income producing property
Loans
Total loans	346,554	361,879
Consumer Owner Occupied Loans
Loans
Total loans	2,706,960	2,760,126
Other loans
Loans
Total loans	13,892	1,457
Commercial loans | Commercial non-owner occupied
Loans
Total loans	2,323,967	2,357,730
Commercial loans | Commercial and industrial
Loans
Total loans	1,386,327	1,261,527
Consumer | Home equity loans
Loans
Total loans	758,020	773,363
Consumer | Consumer
Loans
Total loans	663,422	654,883
Acquired non-credit impaired loans
Loans
Total loans	1,760,427	1,965,603
Acquired non-credit impaired loans | Pass
Loans
Total loans	1,697,942	1,897,922
Acquired non-credit impaired loans | Special mention
Loans
Total loans	39,772	31,970
Acquired non-credit impaired loans | Substandard
Loans
Total loans	22,713	35,711
Acquired non-credit impaired loans | Construction and land development
Loans
Total loans		53,302
Acquired non-credit impaired loans | Commercial non-owner occupied
Loans
Total loans		503,443
Acquired non-credit impaired loans | Commercial owner occupied real estate loan
Loans
Total loans		345,040
Acquired non-credit impaired loans | Commercial and industrial
Loans
Total loans		126,092
Acquired non-credit impaired loans | Other income producing property
Loans
Total loans	95,697	103,093
Acquired non-credit impaired loans | Consumer Owner Occupied Loans
Loans
Total loans		543,432
Acquired non-credit impaired loans | Home equity loans
Loans
Total loans		198,112
Acquired non-credit impaired loans | Consumer
Loans
Total loans		93,089
Acquired non-credit impaired loans | Commercial loans
Loans
Total loans	985,780	1,130,970
Acquired non-credit impaired loans | Commercial loans | Pass
Loans
Total loans	950,062	1,090,140
Acquired non-credit impaired loans | Commercial loans | Special mention
Loans
Total loans	27,005	19,186
Acquired non-credit impaired loans | Commercial loans | Substandard
Loans
Total loans	8,713	21,644
Acquired non-credit impaired loans | Commercial loans | Construction and land development
Loans
Total loans	33,569	53,302
Acquired non-credit impaired loans | Commercial loans | Construction and land development | Pass
Loans
Total loans	31,690	51,340
Acquired non-credit impaired loans | Commercial loans | Construction and land development | Special mention
Loans
Total loans	966	736
Acquired non-credit impaired loans | Commercial loans | Construction and land development | Substandard
Loans
Total loans	913	1,226
Acquired non-credit impaired loans | Commercial loans | Commercial non-owner occupied
Loans
Total loans	447,441	503,443
Acquired non-credit impaired loans | Commercial loans | Commercial non-owner occupied | Pass
Loans
Total loans	432,710	492,258
Acquired non-credit impaired loans | Commercial loans | Commercial non-owner occupied | Special mention
Loans
Total loans	14,162	5,035
Acquired non-credit impaired loans | Commercial loans | Commercial non-owner occupied | Substandard
Loans
Total loans	569	6,150
Acquired non-credit impaired loans | Commercial loans | Commercial owner occupied real estate loan
Loans
Total loans	307,193	345,040
Acquired non-credit impaired loans | Commercial loans | Commercial owner occupied real estate loan | Pass
Loans
Total loans	300,678	330,005
Acquired non-credit impaired loans | Commercial loans | Commercial owner occupied real estate loan | Special mention
Loans
Total loans	3,092	3,427
Acquired non-credit impaired loans | Commercial loans | Commercial owner occupied real estate loan | Substandard
Loans
Total loans	3,423	11,608
Acquired non-credit impaired loans | Commercial loans | Commercial and industrial
Loans
Total loans	101,880	126,092
Acquired non-credit impaired loans | Commercial loans | Commercial and industrial | Pass
Loans
Total loans	97,092	120,926
Acquired non-credit impaired loans | Commercial loans | Commercial and industrial | Special mention
Loans
Total loans	2,948	4,051
Acquired non-credit impaired loans | Commercial loans | Commercial and industrial | Substandard
Loans
Total loans	1,840	1,115
Acquired non-credit impaired loans | Commercial loans | Other income producing property
Loans
Total loans	95,697	103,093
Acquired non-credit impaired loans | Commercial loans | Other income producing property | Pass
Loans
Total loans	87,892	95,611
Acquired non-credit impaired loans | Commercial loans | Other income producing property | Special mention
Loans
Total loans	5,837	5,937
Acquired non-credit impaired loans | Commercial loans | Other income producing property | Substandard
Loans
Total loans	1,968	1,545
Acquired non-credit impaired loans | Consumer
Loans
Total loans	774,647	834,633
Acquired non-credit impaired loans | Consumer | Pass
Loans
Total loans	747,880	807,782
Acquired non-credit impaired loans | Consumer | Special mention
Loans
Total loans	12,767	12,784
Acquired non-credit impaired loans | Consumer | Substandard
Loans
Total loans	14,000	14,067
Acquired non-credit impaired loans | Consumer | Consumer Owner Occupied Loans
Loans
Total loans	496,431	543,432
Acquired non-credit impaired loans | Consumer | Consumer Owner Occupied Loans | Pass
Loans
Total loans	486,433	532,760
Acquired non-credit impaired loans | Consumer | Consumer Owner Occupied Loans | Special mention
Loans
Total loans	6,434	6,805
Acquired non-credit impaired loans | Consumer | Consumer Owner Occupied Loans | Substandard
Loans
Total loans	3,564	3,867
Acquired non-credit impaired loans | Consumer | Home equity loans
Loans
Total loans	188,732	198,112
Acquired non-credit impaired loans | Consumer | Home equity loans | Pass
Loans
Total loans	174,912	184,946
Acquired non-credit impaired loans | Consumer | Home equity loans | Special mention
Loans
Total loans	5,679	5,405
Acquired non-credit impaired loans | Consumer | Home equity loans | Substandard
Loans
Total loans	8,141	7,761
Acquired non-credit impaired loans | Consumer | Consumer
Loans
Total loans	89,484	93,089
Acquired non-credit impaired loans | Consumer | Consumer | Pass
Loans
Total loans	86,535	90,076
Acquired non-credit impaired loans | Consumer | Consumer | Special mention
Loans
Total loans	654	574
Acquired non-credit impaired loans | Consumer | Consumer | Substandard
Loans
Total loans	$ 2,295	$ 2,439