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Summary of Significant Accounting Policies - Business Combinations (Details) $ in Thousands | Sep. 30, 2016 USD ($) | Sep. 30, 2015 USD ($) branch | Sep. 30, 2014 USD ($) | May 03, 2014 branch | May 02, 2014 branch | Dec. 07, 2013 branch | Dec. 06, 2013 branch | Nov. 01, 2013 branch | Oct. 31, 2013 branch | Nov. 01, 2012 USD ($)
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Intangible assets, goodwill | $ 291,503 | $ 291,503 | $ 291,503 |  |  |  |  |  |  |
Bank of America, National Association |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Number of branches acquired (in branches) | branch |  | 74 |  |  |  |  |  |  |  |
Net loans |  | $ 13,000 |  |  |  |  |  |  |  |
Branch properties |  | $ 25,000 |  |  |  |  |  |  |  |
Premium on deposits |  | 1.99% |  |  |  |  |  |  |  |
Cash and equivalents |  | $ 1,800,000 |  |  |  |  |  |  |  |
Intangibles |  | 11,000 |  |  |  |  |  |  |  |
Intangible assets, goodwill |  | $ 31,000 |  |  |  |  |  |  |  |
Bank of America, National Association | New Mexico |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Number of branches acquired (in branches) | branch |  |  |  |  |  |  |  | 11 | 11 |
Bank of America, National Association | Washington, Oregon, and Idaho |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Number of branches acquired (in branches) | branch |  |  |  |  |  | 40 | 40 |  |  |
Bank of America, National Association | Arizona and Nevada |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Number of branches acquired (in branches) | branch |  |  |  | 23 | 23 |  |  |  |  |
South Valley Bancorp, Inc. |  |  |  |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |  |  |  |
Deposit accounts |  |  |  |  |  |  |  |  |  | $ 1,900,000

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