Allowance for Losses on Loans - Loans Evaluated for Impairment (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 109,919	$ 107,901	$ 106,829	$ 108,323	$ 108,700	$ 114,591
Loans receivable, gross	10,306,673		9,823,270
Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	41,828	47,756	47,347	54,762	55,495	62,763
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	15,726	7,014	6,680	5,445	5,451	6,742
Construction - custom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	1,022	1,062	990	968	965	1,695
Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	7,252	6,778	5,781	7,405	6,671	5,592
Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	2,466	3,001	2,946	3,035	3,113	3,077
Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	6,784	5,047	5,304	4,673	4,500	4,248
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	7,783	10,344	8,960	6,734	5,872	7,548
Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	23,824	24,096	24,980	21,146	23,328	16,527
HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	828	820	902	850	892	928
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	2,406	$ 1,983	2,939	$ 3,305	$ 2,413	$ 3,227
Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	108,883		106,554
Loans receivable, gross	$ 9,340,515		$ 8,963,453
Ratio	1.20%		1.20%
Loans Collectively Evaluated for Impairment | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 40,974		$ 47,073
Loans receivable, gross	$ 5,557,463		$ 5,595,752
Ratio	0.70%		0.80%
Loans Collectively Evaluated for Impairment | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 15,726		$ 6,680
Loans receivable, gross	$ 391,616		$ 124,679
Ratio	4.00%		5.40%
Loans Collectively Evaluated for Impairment | Construction - custom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 1,021		$ 990
Loans receivable, gross	$ 210,809		$ 205,692
Ratio	0.50%		0.50%
Loans Collectively Evaluated for Impairment | Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 7,221		$ 5,781
Loans receivable, gross	$ 88,064		$ 72,602
Ratio	8.20%		8.00%
Loans Collectively Evaluated for Impairment | Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 2,466		$ 2,946
Loans receivable, gross	$ 89,741		$ 93,103
Ratio	2.80%		3.20%
Loans Collectively Evaluated for Impairment | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 6,774		$ 5,304
Loans receivable, gross	$ 1,070,178		$ 1,062,194
Ratio	0.60%		0.50%
Loans Collectively Evaluated for Impairment | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 7,643		$ 8,960
Loans receivable, gross	$ 810,343		$ 844,691
Ratio	0.90%		1.10%
Loans Collectively Evaluated for Impairment | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 23,824		$ 24,980
Loans receivable, gross	$ 829,394		$ 643,577
Ratio	2.90%		3.90%
Loans Collectively Evaluated for Impairment | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 828		$ 902
Loans receivable, gross	$ 128,295		$ 126,594
Ratio	0.70%		0.70%
Loans Collectively Evaluated for Impairment | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 2,406		$ 2,938
Loans receivable, gross	$ 164,612		$ 194,569
Ratio	1.50%		1.50%
Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 1,036		$ 275
Loans receivable, gross	$ 45,717		$ 85,513
Ratio	2.30%		0.30%
Loans Individually Evaluated for Impairment | Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 854		$ 275
Loans receivable, gross	$ 28,462		$ 51,718
Ratio	3.00%		0.50%
Loans Individually Evaluated for Impairment | Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 0		$ 0
Loans receivable, gross	$ 0		$ 5,441
Ratio	0.00%		0.00%
Loans Individually Evaluated for Impairment | Construction - custom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 0		$ 0
Loans receivable, gross	$ 171		$ 0
Ratio	0.00%		0.00%
Loans Individually Evaluated for Impairment | Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 31		$ 0
Loans receivable, gross	$ 1,280		$ 2,198
Ratio	2.40%		0.00%
Loans Individually Evaluated for Impairment | Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 0		$ 0
Loans receivable, gross	$ 1,114		$ 10,824
Ratio	0.00%		0.00%
Loans Individually Evaluated for Impairment | Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 11		$ 0
Loans receivable, gross	$ 1,522		$ 5,348
Ratio	0.70%		0.00%
Loans Individually Evaluated for Impairment | Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 140		$ 0
Loans receivable, gross	$ 12,602		$ 8,826
Ratio	1.10%		0.00%
Loans Individually Evaluated for Impairment | Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 0		$ 0
Loans receivable, gross	$ 0		$ 0
Ratio	0.00%		0.00%
Loans Individually Evaluated for Impairment | HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 0		$ 0
Loans receivable, gross	$ 566		$ 1,072
Ratio	0.00%		0.00%
Loans Individually Evaluated for Impairment | Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for probable losses	$ 0		$ 0
Loans receivable, gross	$ 0		$ 86
Ratio	0.00%		0.00%