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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, 2020 | Dec. 31, 2019
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Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Available for sale securities | $ 12,694,277 | $ 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 | 12,694,277 | 11,269,643
Residential mortgage loans held for sale | 204,720 | 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 | 9,574 | 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 | 3.00% | 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 | 95.24% | 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 | $ 9,574 | $ 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 | 6.69% | 7.08%
Range of fair value as a percentage of par value | 94.38% | 94.40%
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 | 6.69% | 7.08%
Range of fair value as a percentage of par value | 94.38% | 94.40%
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 | 6.69% | 7.08%
Range of fair value as a percentage of par value | 94.38% | 94.40%

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