Allowance for Losses on Loans - Internally Assigned Grade (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Sep. 30, 2015
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 10,306,673	$ 9,823,270
Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 10,080,249	$ 9,605,919
Total grade as a % of total net loans	97.80%	97.80%
Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 23,369	$ 6,721
Total grade as a % of total net loans	0.20%	0.10%
Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 203,055	$ 210,630
Total grade as a % of total net loans	2.00%	2.10%
Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 0	$ 0
Total grade as a % of total net loans	0.00%	0.00%
Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 0	$ 0
Total grade as a % of total net loans	0.00%	0.00%
Non-Acquired loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 10,013,253	$ 9,493,987
Non-Acquired loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	9,829,737	9,333,233
Non-Acquired loans | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	23,369	6,721
Non-Acquired loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	160,147	154,033
Non-Acquired loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Single-family residential
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	5,618,954	5,651,845
Non-Acquired loans | Single-family residential | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	5,547,231	5,558,700
Non-Acquired loans | Single-family residential | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Single-family residential | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	71,723	93,145
Non-Acquired loans | Single-family residential | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Single-family residential | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Construction
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	785,846	200,509
Non-Acquired loans | Construction | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	774,540	197,935
Non-Acquired loans | Construction | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	7,500	0
Non-Acquired loans | Construction | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	3,806	2,574
Non-Acquired loans | Construction | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Construction | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Construction - custom
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	398,797	396,307
Non-Acquired loans | Construction - custom | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	395,010	396,307
Non-Acquired loans | Construction - custom | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Construction - custom | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	3,787	0
Non-Acquired loans | Construction - custom | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Construction - custom | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Land - acquisition & development
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	101,605	94,208
Non-Acquired loans | Land - acquisition & development | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	93,668	89,656
Non-Acquired loans | Land - acquisition & development | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Land - acquisition & development | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	7,937	4,552
Non-Acquired loans | Land - acquisition & development | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Land - acquisition & development | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Land - consumer lot loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	100,856	103,989
Non-Acquired loans | Land - consumer lot loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	99,081	103,569
Non-Acquired loans | Land - consumer lot loans | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Land - consumer lot loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,775	420
Non-Acquired loans | Land - consumer lot loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Land - consumer lot loans | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Multi-family
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,073,222	1,125,722
Non-Acquired loans | Multi-family | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	1,065,161	1,118,673
Non-Acquired loans | Multi-family | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	3,613	865
Non-Acquired loans | Multi-family | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	4,448	6,184
Non-Acquired loans | Multi-family | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Multi-family | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	833,570	986,270
Non-Acquired loans | Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	813,405	971,510
Non-Acquired loans | Commercial real estate | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	8,281	4,360
Non-Acquired loans | Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	11,884	10,400
Non-Acquired loans | Commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Commercial real estate | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Commercial & industrial
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	805,272	612,836
Non-Acquired loans | Commercial & industrial | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	747,693	575,034
Non-Acquired loans | Commercial & industrial | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	3,975	1,496
Non-Acquired loans | Commercial & industrial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	53,604	36,306
Non-Acquired loans | Commercial & industrial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Commercial & industrial | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | HELOC
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	130,459	127,646
Non-Acquired loans | HELOC | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	129,656	127,398
Non-Acquired loans | HELOC | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | HELOC | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	803	248
Non-Acquired loans | HELOC | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | HELOC | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Consumer
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	164,672	194,655
Non-Acquired loans | Consumer | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	164,292	194,451
Non-Acquired loans | Consumer | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Consumer | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	380	204
Non-Acquired loans | Consumer | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-Acquired loans | Consumer | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-impaired acquired loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	152,572	166,293
Non-impaired acquired loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	143,164	149,891
Non-impaired acquired loans | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-impaired acquired loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	9,408	16,402
Non-impaired acquired loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Non-impaired acquired loans | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Credit impaired acquired loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	106,637	87,081
Credit impaired acquired loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	73,699	61,019
Credit impaired acquired loans | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Credit impaired acquired loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	32,938	26,062
Credit impaired acquired loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Credit impaired acquired loans | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Covered loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	34,211	75,909
Covered loans | Pass
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	33,649	61,776
Covered loans | Special mention
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Covered loans | Substandard
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	562	14,133
Covered loans | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	0	0
Covered loans | Loss
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 0	$ 0