Loans Receivable - Loans Receivable, Analysis of Age of Loans in Past Due Status (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Mar. 31, 2016	Sep. 30, 2015
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 9,791,480		$ 9,346,474
Current	9,719,843		9,263,876
Past due	$ 71,637		$ 82,598
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.73%	0.90%	0.88%
Ratio of total past due loans to total loans receivable, current	99.27%		99.12%
Ratio of total past due loans to total loans receivable, past due	0.73%		0.88%
Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 17,653		$ 21,212
Ratio of total past due loans to total loans receivable, past due	0.18%		0.23%
Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 9,332		$ 9,198
Ratio of total past due loans to total loans receivable, past due	0.10%		0.10%
Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 44,652		$ 52,188
Ratio of total past due loans to total loans receivable, past due	0.46%		0.56%
Non-Acquired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 9,522,429		$ 9,073,176
Current	9,460,103		8,997,648
Past due	$ 62,326		$ 75,528
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.65%		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	$ 5,596,644		$ 5,655,928
Current	5,542,000		5,590,673
Past due	$ 54,644		$ 65,255
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.98%		1.15%
Non-Acquired loans | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 442,810		$ 130,121
Current	442,810		130,121
Past due	$ 0		$ 0
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.00%		0.00%
Non-Acquired loans | Construction - custom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 213,465		$ 205,692
Current	212,690		204,168
Past due	$ 775		$ 1,524
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.36%		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	$ 86,243		$ 75,661
Current	85,775		74,737
Past due	$ 468		$ 924
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.54%		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	$ 102,248		$ 104,494
Current	100,304		102,045
Past due	$ 1,944		$ 2,449
Ratio of past due loans to total loans, net of charge-offs and LIPs	1.90%		2.34%
Non-Acquired loans | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 1,095,174		$ 1,068,038
Current	1,094,284		1,065,667
Past due	$ 956		$ 2,371
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.09%		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	$ 886,552		$ 893,072
Current	884,644		892,180
Past due	$ 1,783		$ 892
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.20%		0.10%
Non-Acquired loans | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 811,502		$ 617,545
Current	811,486		616,602
Past due	$ 75		$ 943
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.01%		0.15%
Non-Acquired loans | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 134,151		$ 127,648
Current	133,236		127,196
Past due	$ 915		$ 452
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.68%		0.35%
Non-Acquired loans | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	$ 153,640		$ 194,977
Current	152,874		194,259
Past due	$ 766		$ 718
Ratio of past due loans to total loans, net of charge-offs and LIPs	0.50%		0.37%
Non-Acquired loans | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 16,971		$ 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	14,268		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	110		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	0		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	738		689
Non-Acquired loans | Financing Receivables, 1 to 29 Days Past Due | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	956		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	217		131
Non-Acquired loans | Financing Receivables, 1 to 29 Days Past Due | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	0		93
Non-Acquired loans | Financing Receivables, 1 to 29 Days Past Due | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	297		174
Non-Acquired loans | Financing Receivables, 1 to 29 Days Past Due | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	385		493
Non-Acquired loans | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	8,801		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	6,679		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	159		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	101		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	1,443		0
Non-Acquired loans | Financing Receivables, 30 to 59 Days Past Due | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	75		27
Non-Acquired loans | Financing Receivables, 30 to 59 Days Past Due | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	70		27
Non-Acquired loans | Financing Receivables, 30 to 59 Days Past Due | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	274		170
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	36,554		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	33,697		40,193
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	0		0
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due | Construction - custom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	506		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	468		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	1,105		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	0		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	123		761
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	0		823
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	548		251
Non-Acquired loans | Financing Receivables, 60 to 89 Days Past Due | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	107		55
Acquired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	140,369		57,682
Current	137,107		56,559
Past due	$ 3,262		$ 1,123
Ratio of past due loans to total loans, net of charge-offs and LIPs	2.32%		1.95%
Acquired loans | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 265		$ 356
Acquired loans | Financing Receivables, 30 to 59 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	529		0
Acquired loans | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	2,468		767
Credit impaired acquired loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	96,491		139,726
Current	91,168		138,940
Past due	$ 5,323		$ 786
Ratio of past due loans to total loans, net of charge-offs and LIPs	5.52%		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	0		4
Credit impaired acquired loans | Financing Receivables, 60 to 89 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	5,323		539
Covered loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Amount of loans, net of charge-offs and LIP	32,191		75,890
Current	31,465		70,729
Past due	$ 726		$ 5,161
Ratio of past due loans to total loans, net of charge-offs and LIPs	2.26%		6.80%
Covered loans | Financing Receivables, 1 to 29 Days Past Due
Accounts, Notes, Loans and Financing Receivable [Line Items]
Past due	$ 417		$ 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	$ 307		$ 4,799