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Allowance for Losses on Loans - Loans Evaluated for Impairment (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Sep. 30, 2017 | Dec. 31, 2016 | Sep. 30, 2016
--- | --- | --- | --- | ---
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 127,155 | $ 123,073 | $ 118,456 | $ 113,494
Loans receivable, gross | 12,456,216 | 12,201,387 |  |
Loans Collectively Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | 126,520 | 122,947 |  |
Loans receivable, gross | $ 11,193,552 | $ 11,000,212 |  |
Ratio | 1.10% | 1.10% |  |
Loans Individually Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 635 | $ 126 |  |
Loans receivable, gross | $ 66,888 | $ 28,553 |  |
Ratio | 0.90% | 0.40% |  |
Single-family residential |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 35,928 | $ 36,892 | 38,206 | 37,796
Loans receivable, gross | 5,693,318 | 5,711,004 |  |
Single-family residential | Loans Collectively Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | 35,928 | 36,893 |  |
Loans receivable, gross | $ 5,676,565 | $ 5,713,576 |  |
Ratio | 0.60% | 0.70% |  |
Single-family residential | Loans Individually Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 0 | $ 0 |  |
Loans receivable, gross | $ 24,083 | $ 5,552 |  |
Ratio | 0.00% | 0.00% |  |
Construction |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 25,214 | $ 24,556 | 21,934 | 19,838
Loans receivable, gross | 1,710,418 | 1,597,996 |  |
Construction | Loans Collectively Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | 25,214 | 24,556 |  |
Loans receivable, gross | $ 851,800 | $ 793,958 |  |
Ratio | 3.00% | 3.10% |  |
Construction | Loans Individually Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 0 | $ 0 |  |
Loans receivable, gross | $ 364 | $ 0 |  |
Ratio | 0.00% | 0.00% |  |
Construction - custom |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 2,052 | $ 1,944 | 1,110 | 1,080
Loans receivable, gross | 583,580 | 602,631 |  |
Construction - custom | Loans Collectively Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | 2,052 | 1,944 |  |
Loans receivable, gross | $ 292,255 | $ 277,495 |  |
Ratio | 0.70% | 0.70% |  |
Construction - custom | Loans Individually Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 0 | $ 0 |  |
Loans receivable, gross | $ 0 | $ 105 |  |
Ratio | 0.00% | 0.00% |  |
Land - acquisition & development |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 7,355 | $ 6,829 | 6,665 | 6,023
Loans receivable, gross | 136,938 | 124,308 |  |
Land - acquisition & development | Loans Collectively Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | 7,355 | 6,828 |  |
Loans receivable, gross | $ 110,884 | $ 104,767 |  |
Ratio | 6.60% | 6.50% |  |
Land - acquisition & development | Loans Individually Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 0 | $ 1 |  |
Loans receivable, gross | $ 1,376 | $ 89 |  |
Ratio | 0.00% | 1.00% |  |
Land - consumer lot loans |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 2,906 | $ 2,649 | 2,501 | 2,535
Loans receivable, gross | 105,086 | 104,405 |  |
Land - consumer lot loans | Loans Collectively Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | 2,906 | 2,649 |  |
Loans receivable, gross | $ 97,291 | $ 96,337 |  |
Ratio | 3.00% | 2.80% |  |
Land - consumer lot loans | Loans Individually Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 0 | $ 0 |  |
Loans receivable, gross | $ 416 | $ 171 |  |
Ratio | 0.00% | 0.00% |  |
Multi-family |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 7,904 | $ 7,862 | 7,629 | 6,925
Loans receivable, gross | 1,312,695 | 1,303,148 |  |
Multi-family | Loans Collectively Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | 7,899 | 7,857 |  |
Loans receivable, gross | $ 1,311,943 | $ 1,302,625 |  |
Ratio | 0.60% | 0.60% |  |
Multi-family | Loans Individually Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 5 | $ 5 |  |
Loans receivable, gross | $ 730 | $ 493 |  |
Ratio | 0.70% | 1.00% |  |
Commercial real estate |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 11,625 | $ 11,818 | 10,168 | 8,588
Loans receivable, gross | 1,436,508 | 1,434,610 |  |
Commercial real estate | Loans Collectively Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | 11,519 | 11,697 |  |
Loans receivable, gross | $ 1,389,749 | $ 1,391,668 |  |
Ratio | 0.80% | 0.80% |  |
Commercial real estate | Loans Individually Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 106 | $ 120 |  |
Loans receivable, gross | $ 27,535 | $ 21,765 |  |
Ratio | 0.40% | 0.60% |  |
Commercial & industrial |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 29,268 | $ 28,524 | 27,736 | 28,008
Loans receivable, gross | 1,120,707 | 1,093,360 |  |
Commercial & industrial | Loans Collectively Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | 28,744 | 28,524 |  |
Loans receivable, gross | $ 1,108,961 | $ 1,093,210 |  |
Ratio | 2.60% | 2.60% |  |
Commercial & industrial | Loans Individually Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 524 | $ 0 |  |
Loans receivable, gross | $ 11,694 | $ 81 |  |
Ratio | 4.50% | 0.00% |  |
HELOC |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 808 | $ 855 | 832 | 813
Loans receivable, gross | 136,995 | 144,850 |  |
HELOC | Loans Collectively Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | 808 | 855 |  |
Loans receivable, gross | $ 134,320 | $ 141,689 |  |
Ratio | 0.60% | 0.60% |  |
HELOC | Loans Individually Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 0 | $ 0 |  |
Loans receivable, gross | $ 620 | $ 215 |  |
Ratio | 0.00% | 0.00% |  |
Consumer |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 4,095 | $ 1,144 | $ 1,675 | $ 1,888
Loans receivable, gross | 219,971 | 85,075 |  |
Consumer | Loans Collectively Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | 4,095 | 1,144 |  |
Loans receivable, gross | $ 219,784 | $ 84,887 |  |
Ratio | 1.90% | 1.40% |  |
Consumer | Loans Individually Evaluated for Impairment |  |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |  |
Allowance for loan losses | $ 0 | $ 0 |  |
Loans receivable, gross | $ 70 | $ 82 |  |
Ratio | 0.00% | 0.00% |  |

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