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Fair Value Measurements Fair Value Measurements, Financial Instruments Measured On a Recurring Basis, Quantitative Information (Details) - Fair Value, Measurements, Recurring [Member] - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Thousands | 6 Months Ended | 12 Months Ended
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Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
Available-for-sale securities [Member] | Municipal and other tax-exempt securities |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |
Fair Value Inputs, Asset, Par Value | $ 2,050 |  | $ 5,095 |  | $ 5,095 |
Fair Value Inputs, Asset, Amortized Cost/Unpaid Principal Balance | 2,033 |  | 5,067 |  | 5,068 |
Fair Value Inputs, Asset, Fair Value | $ 2,030 |  | $ 4,655 |  | $ 4,802 |
Fair Value Measurements, Valuation Techniques | Discounted cash flows | [1] | Discounted cash flows | [2] | Discounted cash flows | [3]
Fair Value Measurements, Significant Unobservable Input | Interest rate spread |  | Interest rate spread |  | Interest rate spread |
Fair Value Inputs, Assets, Discount Rate, Minimum | 6.69% | [4] | 5.98% | [5] | 6.60% | [6]
Fair Value Inputs, Assets, Discount Rate, Maximum | 6.69% | [4] | 5.98% | [5] | 6.60% | [6]
Fair Value Inputs, Assets, Discount Rate, Weighted Average | 6.69% | [4] | 5.98% | [5] | 6.60% | [6]
Fair Value Inputs, Assets, Fair Value As Percentage of Par Value, Minimum | 99.00% | [7] | 90.00% | [8] | 92.25% | [9]
Fair Value Inputs, Assets, Fair Value As Percentage of Par Value, Maximum | 99.00% | [7] | 94.90% | [8] | 94.76% | [9]
Fair Value Inputs, Fair Value As Percentage of Par Value, Weighted Average | 99.00% | [7] | 92.93% | [8] | 93.75% | [9]
Investment Grade Tax Exempt Securities Yield Spread Over Comparable Securities Minimum | 413 |  | 360 |  | 372 |
Investment Grade Tax Exempt Securities Yield Spread Over Comparable Securities Maximum | 413 |  | 446 |  | 466 |
Available-for-sale securities [Member] | Other debt securities |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |
Fair Value Inputs, Asset, Par Value | $ 500 |  | $ 4,400 |  | $ 500 |
Fair Value Inputs, Asset, Amortized Cost/Unpaid Principal Balance | 500 |  | 4,400 |  | 500 |
Fair Value Inputs, Asset, Fair Value | $ 471 |  | $ 4,152 |  | $ 472 |
Fair Value Measurements, Valuation Techniques | Discounted cash flows | [1] | Discounted cash flows | [2] | Discounted cash flows | [3]
Fair Value Measurements, Significant Unobservable Input | Interest rate spread |  | Interest rate spread |  | Interest rate spread |
Fair Value Inputs, Assets, Discount Rate, Minimum | 6.32% | [10] | 5.41% | [11] | 6.85% | [12]
Fair Value Inputs, Assets, Discount Rate, Maximum | 6.32% | [10] | 6.72% | [11] | 6.85% | [12]
Fair Value Inputs, Assets, Discount Rate, Weighted Average | 6.32% | [10] | 6.57% | [11] | 6.85% | [12]
Fair Value Inputs, Assets, Fair Value As Percentage of Par Value, Minimum | 94.36% | [7] | 94.31% | [8] | 94.39% | [9]
Fair Value Inputs, Assets, Fair Value As Percentage of Par Value, Maximum | 94.36% | [7] | 94.38% | [8] | 94.39% | [9]
Fair Value Inputs, Fair Value As Percentage of Par Value, Weighted Average | 94.36% | [7] | 94.37% | [8] | 94.39% | [9]
Average Yields On Comparable Short-term Taxable Securities, Maximum | 3.00% |  | 3.00% |  | 3.00% |
Residential mortgage loans held for sale [Member] |  |  |  |  |  |
Fair Value Inputs, Assets, Quantitative Information [Line Items] |  |  |  |  |  |
Fair Value Inputs, Asset, Amortized Cost/Unpaid Principal Balance | $ 15,025 |  | $ 13,274 |  | $ 12,981 |
Fair Value Inputs, Asset, Fair Value | $ 14,252 |  | $ 12,735 |  | $ 12,299 |
Fair Value Measurements, Valuation Techniques | Quoted prices of loans sold in securitization transactions, with a liquidity discount applied |  | Quoted prices of loans sold in securitization transactions, with a liquidity discount applied |  | Quoted prices of loans sold in securitization transactions, with a liquidity discount applied |
Fair Value Measurements, Significant Unobservable Input | Liquidity discount applied to the market value of mortgage loans qualifying for sale to U.S. government agencies. |  | Liquidity discount applied to the market value of a mortgage loans qualifying for sale to U.S. government agencies. |  | Liquidity discount applied to the market value of mortgage loans qualifying for sale to U.S. government agencies. |
Liquidity Discount on Mortgage Loans Qualifying for Sale to US Govt Agencies | 94.86% |  | 95.94% |  | 94.75% |
[1] | Discounted cash flows developed using discount rates primarily based on reference to interest rate spreads for comparable securities of similar duration and credit rating as determined by the nationally-recognized rating agencies, adjusted for lack of trading volume
[2] | Discounted cash flows developed using discount rates primarily based on reference to interest rate spreads for comparable securities of similar duration and credit rating as determined by the nationally-recognized rating agencies, adjusted for lack of trading volume
[3] | Discounted cash flows developed using discount rates primarily based on reference to interest rate spreads for comparable securities of similar duration and credit rating as determined by the nationally-recognized rating agencies, adjusted for lack of trading volume
[4] | Interest rate yields used to value investment grade tax-exempt securities represent a spread of 413 basis points over average yields for comparable tax-exempt securities.
[5] | Interest rate yields used to value investment grade tax-exempt securities represent a spread of 360 to 446 basis points over average yields for comparable tax-exempt securities.
[6] | Interest rate yields used to value investment grade tax-exempt securities represent a spread of 372 to 466 basis points over average yields for comparable tax-exempt securities.
[7] | Represents fair value as a percentage of par value.
[8] | Represents fair value as a percentage of par value.
[9] | Represents fair value as a percentage of par value.
[10] | Interest rate yields used to value investment grade taxable securities based on comparable short-term taxable securities which are generally yielding less than 3 percent.
[11] | Interest rate yields used to value investment grade taxable securities based on comparable short-term taxable securities which are generally yielding less than 3 percent.
[12] | Interest rate yields used to value investment grade taxable securities based on comparable short-term taxable securities which are generally yielding less than 3 percent.

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