Loans Receivable - Loans Receivable, Analysis of Age of Loans in Past Due Status (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Sep. 30, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 9,715,006	$ 9,346,474
Current	9,627,653	9,263,876
Past due	$ 87,353	$ 82,598
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.90%	0.88%
Ratio of total past due loans to total loans receivable, current	99.10%	99.12%
Ratio of total past due loans to total loans receivable, past due	0.90%	0.88%
Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 26,202	$ 21,212
Ratio of total past due loans to total loans receivable, past due	0.27%	0.23%
Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 10,446	$ 9,198
Ratio of total past due loans to total loans receivable, past due	0.11%	0.10%
Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 50,705	$ 52,188
Ratio of total past due loans to total loans receivable, past due	0.52%	0.56%
Non-Acquired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 9,498,792	$ 9,073,176
Current	9,413,444	8,997,648
Past due	$ 85,348	$ 75,528
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.90%	0.83%
Non-Acquired loans | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 5,624,134	$ 5,655,928
Current	5,558,731	5,590,673
Past due	$ 65,403	$ 65,255
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.16%	1.15%
Non-Acquired loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 396,322	$ 130,121
Current	396,181	130,121
Past due	$ 141	$ 0
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.04%	0.00%
Non-Acquired loans | Construction - custom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 212,067	$ 205,692
Current	209,153	204,168
Past due	$ 2,914	$ 1,524
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.37%	0.74%
Non-Acquired loans | Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 92,467	$ 75,661
Current	90,541	74,737
Past due	$ 1,926	$ 924
Ratio of past due loans to total loans, net of charge-offs and LIPs	2.08%	1.22%
Non-Acquired loans | Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 101,372	$ 104,494
Current	99,379	102,045
Past due	$ 1,993	$ 2,449
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.97%	2.34%
Non-Acquired loans | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 1,077,248	$ 1,068,038
Current	1,075,662	1,065,667
Past due	$ 1,586	$ 2,371
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.15%	0.22%
Non-Acquired loans | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 889,342	$ 893,072
Current	881,896	892,180
Past due	$ 7,446	$ 892
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.84%	0.10%
Non-Acquired loans | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 810,452	$ 617,545
Current	808,825	616,602
Past due	$ 1,627	$ 943
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.20%	0.15%
Non-Acquired loans | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 130,446	$ 127,648
Current	129,456	127,196
Past due	$ 990	$ 452
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.76%	0.35%
Non-Acquired loans | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 164,942	$ 194,977
Current	163,620	194,259
Past due	$ 1,322	$ 718
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.80%	0.37%
Non-Acquired loans | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 25,788	$ 20,341
Non-Acquired loans | Financing Receivables, 1 to 29 Days Past Due | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	18,000	17,305
Non-Acquired loans | Financing Receivables, 1 to 29 Days Past Due | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	0	0
Non-Acquired loans | Financing Receivables, 1 to 29 Days Past Due | Construction - custom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	1,047	791
Non-Acquired loans | Financing Receivables, 1 to 29 Days Past Due | Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	1,445	406
Non-Acquired loans | Financing Receivables, 1 to 29 Days Past Due | Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	563	689
Non-Acquired loans | Financing Receivables, 1 to 29 Days Past Due | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	970	259
Non-Acquired loans | Financing Receivables, 1 to 29 Days Past Due | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	1,766	131
Non-Acquired loans | Financing Receivables, 1 to 29 Days Past Due | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	862	93
Non-Acquired loans | Financing Receivables, 1 to 29 Days Past Due | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	378	174
Non-Acquired loans | Financing Receivables, 1 to 29 Days Past Due | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	757	493
Non-Acquired loans | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	10,398	9,104
Non-Acquired loans | Financing Receivables, 30 to 59 Days Past Due | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	7,785	7,757
Non-Acquired loans | Financing Receivables, 30 to 59 Days Past Due | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	0	0
Non-Acquired loans | Financing Receivables, 30 to 59 Days Past Due | Construction - custom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	1,800	270
Non-Acquired loans | Financing Receivables, 30 to 59 Days Past Due | Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	0	0
Non-Acquired loans | Financing Receivables, 30 to 59 Days Past Due | Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	490	399
Non-Acquired loans | Financing Receivables, 30 to 59 Days Past Due | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	0	454
Non-Acquired loans | Financing Receivables, 30 to 59 Days Past Due | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	53	0
Non-Acquired loans | Financing Receivables, 30 to 59 Days Past Due | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	0	27
Non-Acquired loans | Financing Receivables, 30 to 59 Days Past Due | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	63	27
Non-Acquired loans | Financing Receivables, 30 to 59 Days Past Due | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	207	170
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	49,162	46,083
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	39,618	40,193
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	141	0
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due | Construction - custom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	67	463
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due | Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	481	518
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due | Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	940	1,361
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	616	1,658
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	5,627	761
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	765	823
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	549	251
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	358	55
Acquired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	131,079	57,682
Current	130,030	56,559
Past due	$ 1,049	$ 1,123
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.80%	1.95%
Acquired loans | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 376	$ 356
Acquired loans | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	2	0
Acquired loans | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	671	767
Credit impaired acquired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	50,924	139,726
Current	50,604	138,940
Past due	$ 320	$ 786
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.63%	0.56%
Credit impaired acquired loans | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 0	$ 243
Credit impaired acquired loans | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	44	4
Credit impaired acquired loans | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	276	539
Covered loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	34,211	75,890
Current	33,575	70,729
Past due	$ 636	$ 5,161
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.86%	6.80%
Covered loans | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 38	$ 272
Covered loans | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	2	90
Covered loans | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 596	$ 4,799