Allowance for Losses on Loans - Loans Evaluated for Impairment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 133,086	$ 131,165	$ 129,257	$ 127,576	$ 127,155	$ 123,073
Recorded Investment of Loans	13,240,402		12,853,678
Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	132,710		128,740
Recorded Investment of Loans	$ 12,027,366		$ 11,597,492
Ratio	1.10%		1.10%
Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 376		$ 517
Recorded Investment of Loans	$ 50,249		$ 60,680
Ratio	0.70%		0.90%
Single-family residential
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 31,476	31,484	$ 33,033	34,144	35,928	36,892
Recorded Investment of Loans	5,861,404		5,798,966
Single-family residential | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	31,476		33,033
Recorded Investment of Loans	$ 5,848,663		$ 5,782,870
Ratio	0.50%		0.60%
Single-family residential | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 0		$ 0
Recorded Investment of Loans	$ 16,836		$ 21,345
Ratio	0.00%		0.00%
Construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 33,396	31,463	$ 31,317	27,389	25,214	24,556
Recorded Investment of Loans	1,980,274		1,890,668
Construction | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	33,396		31,317
Recorded Investment of Loans	$ 1,138,688		$ 1,060,428
Ratio	2.90%		3.00%
Construction | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 0		$ 0
Recorded Investment of Loans	$ 1,282		$ 2,427
Ratio	0.00%		0.00%
Construction - custom
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 1,976	1,926	$ 1,842	2,081	2,052	1,944
Recorded Investment of Loans	586,515		624,479
Construction - custom | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	1,976		1,842
Recorded Investment of Loans	$ 307,405		$ 289,192
Ratio	0.60%		0.60%
Construction - custom | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 0		$ 0
Recorded Investment of Loans	$ 0		$ 0
Ratio	0.00%		0.00%
Land - acquisition & development
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 9,734	9,156	$ 7,978	7,622	7,355	6,829
Recorded Investment of Loans	194,739		155,204
Land - acquisition & development | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	9,727		7,969
Recorded Investment of Loans	$ 152,304		$ 122,639
Ratio	6.40%		6.50%
Land - acquisition & development | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 7		$ 9
Recorded Investment of Loans	$ 242		$ 920
Ratio	2.90%		1.00%
Land - consumer lot loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 2,076	2,144	$ 2,164	2,853	2,906	2,649
Recorded Investment of Loans	97,152		102,036
Land - consumer lot loans | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	2,076		2,164
Recorded Investment of Loans	$ 92,614		$ 96,583
Ratio	2.20%		2.20%
Land - consumer lot loans | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 0		$ 0
Recorded Investment of Loans	$ 309		$ 507
Ratio	0.00%		0.00%
Multi-family
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 7,394	7,884	$ 8,329	7,982	7,904	7,862
Recorded Investment of Loans	1,423,723		1,385,125
Multi-family | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	7,390		8,325
Recorded Investment of Loans	$ 1,422,864		$ 1,384,655
Ratio	0.50%		0.60%
Multi-family | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 4		$ 4
Recorded Investment of Loans	$ 837		$ 448
Ratio	0.50%		1.00%
Commercial real estate
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 12,448	12,711	$ 11,852	11,588	11,625	11,818
Recorded Investment of Loans	1,570,502		1,452,168
Commercial real estate | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	12,328		11,702
Recorded Investment of Loans	$ 1,552,990		$ 1,432,791
Ratio	0.80%		0.80%
Commercial real estate | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 120		$ 150
Recorded Investment of Loans	$ 17,512		$ 19,378
Ratio	0.70%		0.80%
Commercial & industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 30,574	30,279	$ 28,702	29,330	29,268	28,524
Recorded Investment of Loans	1,230,888		1,140,874
Commercial & industrial | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	30,329		28,348
Recorded Investment of Loans	$ 1,219,921		$ 1,126,438
Ratio	2.50%		2.50%
Commercial & industrial | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 245		$ 354
Recorded Investment of Loans	$ 12,651		$ 14,437
Ratio	1.90%		2.50%
HELOC
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 1,082	1,064	$ 781	802	808	855
Recorded Investment of Loans	139,203		130,852
HELOC | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	1,082		781
Recorded Investment of Loans	$ 137,716		$ 128,715
Ratio	0.80%		0.60%
HELOC | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 0		$ 0
Recorded Investment of Loans	$ 528		$ 1,162
Ratio	0.00%		0.00%
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 2,930	$ 3,054	$ 3,259	$ 3,785	$ 4,095	$ 1,144
Recorded Investment of Loans	156,002		173,306
Consumer | Loans Collectively Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	2,930		3,259
Recorded Investment of Loans	$ 154,201		$ 173,181
Ratio	1.90%		1.90%
Consumer | Loans Individually Evaluated for Impairment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance Allocation	$ 0		$ 0
Recorded Investment of Loans	$ 52		$ 56
Ratio	0.00%		0.00%