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Fair Value Measurements Fair Value Measurements, Financial Instruments Measured On a Recurring Basis, Quantitative Information (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
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Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Available for sale securities | $ 9,025,198 |  | $ 8,857,120 |
Other debt securities [Member] |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Available for sale securities | 472 |  | 35,430 |
Fair Value, Measurements, Recurring [Member] |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Available for sale securities | 9,025,198 |  | 8,857,120 |
Residential mortgage loans held for sale | 160,157 |  | 149,221 |
Fair Value, Measurements, Recurring [Member] | Other debt securities [Member] |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Available for sale securities | 472 |  | 35,430 |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Available for sale securities | 472 |  | 472 |
Residential mortgage loans held for sale | 15,776 |  | 15,207 |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Other debt securities [Member] |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Available for sale securities | $ 472 |  | $ 472 |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale securities [Member] | Other debt securities [Member] |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Average Yields On Comparable Short-term Taxable Securities Maximum | 3.00% |  | 3.00% |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Residential mortgage loans held for sale [Member] |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Liquidity Discount on Mortgage Loans Qualifying for Sale to US Govt Agencies | 92.40% |  | 92.38% |
Interest Rate Spread [Member] | Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale securities [Member] | Other debt securities [Member] |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Available for sale securities | $ 472 |  | $ 472 |
Liquidity discount [Member] | Quoted prices of loans sold in securitization transactions, with a liquidity discount applied [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Residential mortgage loans held for sale [Member] |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Residential mortgage loans held for sale | $ 15,766 |  | $ 15,207 |
Minimum [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale securities [Member] | Other debt securities [Member] |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Range of fair value as a percentage of par value | 94.39% | [1] | 94.44% | [2]
Range of Interest Rates used to Value Securities | 6.83% | [3] | 7.88% | [4]
Maximum [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale securities [Member] | Other debt securities [Member] |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Range of fair value as a percentage of par value | 94.39% | [1] | 94.44% | [2]
Range of Interest Rates used to Value Securities | 6.83% | [3] | 7.88% | [4]
Weighted Average [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale securities [Member] | Other debt securities [Member] |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Range of fair value as a percentage of par value | 94.39% | [1] | 94.44% | [2]
Range of Interest Rates used to Value Securities | 6.83% | [3] | 7.88% | [4]
[1] | Represents fair value as a percentage of par value.
[2] | 2 Represents fair value as a percentage of par value.
[3] | 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.
[4] | 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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