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Summary of Significant Accounting Policies - Narrative (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2018 | Dec. 31, 2018 | Jan. 01, 2019 | Jan. 01, 2018 | Dec. 31, 2017
Change in Accounting Estimate [Line Items] |  |  |  |  |  |
Performance obligation, description of timing |  | one year or less |  |  |  |
Mortgage loans secured by residential real estate in process of foreclosure | [1] |  | $ 25,290 |  |  | $ 43,382
Deposit Bases |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |
Intangible assets, amortization period |  |  | 10 years |  |  |
Minimum | Furniture and Fixtures |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |
Useful life of premises and equipment |  |  | 3 years |  |  |
Minimum | Building |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |
Useful life of premises and equipment |  |  | 7 years |  |  |
Maximum | Furniture and Fixtures |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |
Useful life of premises and equipment |  |  | 15 years |  |  |
Maximum | Building |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |
Useful life of premises and equipment |  |  | 45 years |  |  |
Non-Accruing | Minimum |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |
Impaired commercial loans |  |  | $ 1,000 |  |  |
Government Guaranteed Mortgage Loans upon Foreclosure Receivable |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |
Mortgage loans secured by residential real estate in process of foreclosure |  |  | 2,900 |  |  |
Non-Interest income |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |
Accounts receivable, net |  |  | $ 8,100 |  |  |
Other Assets | Accounting Standards Update 2016-01 |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |
Adjustment to reflect adoption of accounting standard |  |  |  |  | $ 265,900 |
Available-for-sale Securities | Accounting Standards Update 2016-01 |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |
Adjustment to reflect adoption of accounting standard |  |  |  |  | $ (265,900) |
Pro Forma | Accounting Standards Update 2016-02 | Retained Earnings |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |
Adjustment to reflect adoption of accounting standard |  | $ 3,000 |  |  |  |
Subsequent Event | Accounting Standards Update 2016-02 |  |  |  |  |  |
Change in Accounting Estimate [Line Items] |  |  |  |  |  |
Lease assets |  |  |  | $ 196,000 |  |
Lease liabilities |  |  |  | $ 204,000 |  |
[1] | December 31, 2018 and 2017 include $9.7 million and $12.2 million, respectively, of foreclosed residential real estate.

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