Loans - Credit risk profile by risk grade of non acquired 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
Non-acquired loans
Loans
Total loans	9,252,831	8,928,512
Non-acquired loans | Pass
Loans
Total loans	9,123,097	8,808,623
Non-acquired loans | Special mention
Loans
Total loans	83,830	66,929
Non-acquired loans | Substandard
Loans
Total loans	45,904	52,960
Non-acquired loans | Construction and land development
Loans
Total loans		955,318
Non-acquired loans | Commercial non-owner occupied
Loans
Total loans		1,777,327
Non-acquired loans | Commercial owner occupied real estate loan
Loans
Total loans		1,677,695
Non-acquired loans | Commercial and industrial
Loans
Total loans		1,130,847
Non-acquired loans | Other income producing property
Loans
Total loans	218,617	220,957
Non-acquired loans | Consumer Owner Occupied Loans
Loans
Total loans		2,118,127
Non-acquired loans | Home equity loans
Loans
Total loans		521,744
Non-acquired loans | Consumer
Loans
Total loans		525,040
Non-acquired loans | Other loans
Loans
Total loans	13,892	1,457
Non-acquired loans | Commercial loans
Loans
Total loans	6,062,991	5,762,144
Non-acquired loans | Commercial loans | Pass
Loans
Total loans	5,971,069	5,682,855
Non-acquired loans | Commercial loans | Special mention
Loans
Total loans	69,268	52,081
Non-acquired loans | Commercial loans | Substandard
Loans
Total loans	22,654	27,208
Non-acquired loans | Commercial loans | Construction and land development
Loans
Total loans	968,360	955,318
Non-acquired loans | Commercial loans | Construction and land development | Pass
Loans
Total loans	959,206	947,230
Non-acquired loans | Commercial loans | Construction and land development | Special mention
Loans
Total loans	7,095	5,601
Non-acquired loans | Commercial loans | Construction and land development | Substandard
Loans
Total loans	2,059	2,487
Non-acquired loans | Commercial loans | Commercial non-owner occupied
Loans
Total loans	1,811,138	1,777,327
Non-acquired loans | Commercial loans | Commercial non-owner occupied | Pass
Loans
Total loans	1,787,306	1,768,013
Non-acquired loans | Commercial loans | Commercial non-owner occupied | Special mention
Loans
Total loans	22,410	7,091
Non-acquired loans | Commercial loans | Commercial non-owner occupied | Substandard
Loans
Total loans	1,422	2,223
Non-acquired loans | Commercial loans | Commercial owner occupied real estate loan
Loans
Total loans	1,784,017	1,677,695
Non-acquired loans | Commercial loans | Commercial owner occupied real estate loan | Pass
Loans
Total loans	1,754,801	1,648,456
Non-acquired loans | Commercial loans | Commercial owner occupied real estate loan | Special mention
Loans
Total loans	19,742	17,319
Non-acquired loans | Commercial loans | Commercial owner occupied real estate loan | Substandard
Loans
Total loans	9,474	11,920
Non-acquired loans | Commercial loans | Commercial and industrial
Loans
Total loans	1,280,859	1,130,847
Non-acquired loans | Commercial loans | Commercial and industrial | Pass
Loans
Total loans	1,256,465	1,103,532
Non-acquired loans | Commercial loans | Commercial and industrial | Special mention
Loans
Total loans	16,055	18,148
Non-acquired loans | Commercial loans | Commercial and industrial | Substandard
Loans
Total loans	8,339	9,167
Non-acquired loans | Commercial loans | Other income producing property
Loans
Total loans	218,617	220,957
Non-acquired loans | Commercial loans | Other income producing property | Pass
Loans
Total loans	213,291	215,624
Non-acquired loans | Commercial loans | Other income producing property | Special mention
Loans
Total loans	3,966	3,922
Non-acquired loans | Commercial loans | Other income producing property | Substandard
Loans
Total loans	1,360	1,411
Non-acquired loans | Consumer
Loans
Total loans	3,189,840	3,166,368
Non-acquired loans | Consumer | Pass
Loans
Total loans	3,152,028	3,125,768
Non-acquired loans | Consumer | Special mention
Loans
Total loans	14,562	14,848
Non-acquired loans | Consumer | Substandard
Loans
Total loans	23,250	25,752
Non-acquired loans | Consumer | Consumer Owner Occupied Loans
Loans
Total loans	2,118,839	2,118,127
Non-acquired loans | Consumer | Consumer Owner Occupied Loans | Pass
Loans
Total loans	2,094,080	2,091,129
Non-acquired loans | Consumer | Consumer Owner Occupied Loans | Special mention
Loans
Total loans	9,585	9,054
Non-acquired loans | Consumer | Consumer Owner Occupied Loans | Substandard
Loans
Total loans	15,174	17,944
Non-acquired loans | Consumer | Home equity loans
Loans
Total loans	518,628	521,744
Non-acquired loans | Consumer | Home equity loans | Pass
Loans
Total loans	508,054	510,508
Non-acquired loans | Consumer | Home equity loans | Special mention
Loans
Total loans	4,490	5,373
Non-acquired loans | Consumer | Home equity loans | Substandard
Loans
Total loans	6,084	5,863
Non-acquired loans | Consumer | Consumer
Loans
Total loans	538,481	525,040
Non-acquired loans | Consumer | Consumer | Pass
Loans
Total loans	536,002	522,674
Non-acquired loans | Consumer | Consumer | Special mention
Loans
Total loans	487	421
Non-acquired loans | Consumer | Consumer | Substandard
Loans
Total loans	1,992	1,945
Non-acquired loans | Consumer | Other loans
Loans
Total loans	13,892	1,457
Non-acquired loans | Consumer | Other loans | Pass
Loans
Total loans	$ 13,892	$ 1,457