Allowance for Losses on Loans - Credit Risk Profile Based on Payment Activity (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 10,306,673		$ 9,823,270
Nonaccrual loans by portfolio segment	54,559		67,810
Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 9,958,694		$ 9,426,177
Total grade as a % of total net loans	99.50%		99.30%
Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 93,329	$ 98,846	$ 128,577
Nonaccrual loans by portfolio segment	$ 54,559	$ 56,748	$ 67,810
Grade as a percentage of total gross loans	0.50%		0.70%
Single-family residential
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 42,395		$ 59,074
Single-family residential | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 5,576,559		$ 5,592,771
Total grade as a % of total net loans	99.20%		99.00%
Single-family residential | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 42,395		$ 59,074
Grade as a percentage of total gross loans	0.80%		1.00%
Construction
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 0		$ 754
Construction | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 785,846		$ 199,755
Total grade as a % of total net loans	100.00%		99.60%
Construction | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 0		$ 754
Grade as a percentage of total gross loans	0.00%		0.40%
Construction - custom
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 67		$ 732
Construction - custom | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 398,730		$ 395,575
Total grade as a % of total net loans	100.00%		99.80%
Construction - custom | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 67		$ 732
Grade as a percentage of total gross loans	0.00%		0.20%
Land - acquisition & development
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 477		$ 0
Land - acquisition & development | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 101,128		$ 94,208
Total grade as a % of total net loans	99.50%		100.00%
Land - acquisition & development | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 477		$ 0
Grade as a percentage of total gross loans	0.50%		0.00%
Land - consumer lot loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 940		$ 1,273
Land - consumer lot loans | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 99,916		$ 102,716
Total grade as a % of total net loans	99.10%		98.80%
Land - consumer lot loans | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 940		$ 1,273
Grade as a percentage of total gross loans	0.90%		1.20%
Multi-family
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 1,520		$ 2,558
Multi-family | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 1,071,702		$ 1,123,165
Total grade as a % of total net loans	99.90%		99.80%
Multi-family | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 1,520		$ 2,558
Grade as a percentage of total gross loans	0.10%		0.20%
Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 7,701		$ 2,176
Commercial real estate | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 825,869		$ 984,093
Total grade as a % of total net loans	99.10%		99.80%
Commercial real estate | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 7,701		$ 2,176
Grade as a percentage of total gross loans	0.90%		0.20%
Commercial & industrial
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 596		$ 0
Commercial & industrial | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 804,676		$ 612,836
Total grade as a % of total net loans	99.90%		100.00%
Commercial & industrial | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 596		$ 0
Grade as a percentage of total gross loans	0.10%		0.00%
HELOC
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 554		$ 563
HELOC | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 129,905		$ 127,083
Total grade as a % of total net loans	99.60%		99.60%
HELOC | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 554		$ 563
Grade as a percentage of total gross loans	0.40%		0.40%
Consumer
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 309		$ 680
Consumer | Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Loans receivable, gross	$ 164,363		$ 193,975
Total grade as a % of total net loans	99.80%		99.70%
Consumer | Non-Performing Loans
Financing Receivable, Recorded Investment [Line Items]
Nonaccrual loans by portfolio segment	$ 309		$ 680
Grade as a percentage of total gross loans	0.20%		0.30%