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Fair Value Measurements Fair Value Measurements, Financial Instruments Measured On a Recurring Basis, Quantitative Information (Details) - USD ($) $ in Thousands | Jun. 30, 2020 | Dec. 31, 2019
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Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Available for sale securities | $ 12,475,919 |  | $ 11,269,643 |
Other debt securities [Member] |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Available for sale securities | 472 |  | 472 |
Fair Value, Recurring [Member] |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Available for sale securities | 12,475,919 |  | 11,269,643 |
Residential mortgage loans held for sale | 319,357 |  | 182,271 |
Fair Value, Recurring [Member] | Other debt securities [Member] |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |
Available for sale securities | 472 |  | 472 |
Fair Value, 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 | 9,685 |  | 8,313 |
Fair Value, 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, 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 | 1.00% |  | 3.00% |
Fair Value, 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 | 90.24% |  | 95.23% |
Interest Rate Spread [Member] | Discounted Cash Flow [Member] | Fair Value, 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, 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 | $ 9,685 |  | $ 8,313 |
Minimum [Member] | Fair Value, 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 Interest Rates used to Value Securities | 5.23% | [1] | 7.08% | [2]
Range of fair value as a percentage of par value | 94.30% | [3] | 94.40% | [4]
Maximum [Member] | Fair Value, 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 Interest Rates used to Value Securities | 5.23% | [1] | 7.08% | [2]
Range of fair value as a percentage of par value | 94.30% | [3] | 94.40% | [4]
Weighted Average [Member] | Fair Value, 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 Interest Rates used to Value Securities | 5.23% | [1] | 7.08% | [2]
Range of fair value as a percentage of par value | 94.30% | [3] | 94.40% | [4]
[1] | Interest rate yields used to value investment grade taxable securities based on comparable short-term taxable securities which are generally yielding approximately 1 percent.
[2] | 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.
[3] | Represents fair value as a percentage of par value.
[4] | Represents fair value as a percentage of par value.

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