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Fair Value Measurements Fair Value Measurements, Financial Instruments Measured On a Recurring Basis, Quantitative Information (Details) $ in Thousands | Sep. 30, 2018 USD ($) | Dec. 31, 2017 USD ($) | Sep. 30, 2017 USD ($)
--- | --- | --- | ---
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Available for sale securities | $ 8,072,014 |  | $ 8,321,578 |  | $ 8,383,199 |
Municipal and other tax-exempt securities [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Available for sale securities | 4,349 |  | 27,080 |  | 28,368 |
Other debt securities [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Available for sale securities | 25,447 |  | 25,481 |  | 4,153 |
Fair Value, Measurements, Recurring [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Available for sale securities | 8,072,014 |  | 8,321,578 |  | 8,383,199 |
Residential mortgage loans held for sale | 175,866 |  | 221,378 |  | 275,643 |
Fair Value, Measurements, Recurring [Member] | Municipal and other tax-exempt securities [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Available for sale securities | 4,349 |  | 27,080 |  | 28,368 |
Fair Value, Measurements, Recurring [Member] | Other debt securities [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Available for sale securities | 25,447 |  | 25,481 |  | 4,153 |
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 |  | 5,274 |  | 8,938 |
Residential mortgage loans held for sale | 16,838 |  | 12,299 |  | 12,100 |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Municipal and other tax-exempt securities [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Available for sale securities |  |  | 4,802 |  | 4,785 |
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 |  | $ 4,153 |
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale securities [Member] | Municipal and other tax-exempt securities [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Investment Grade Tax Exempt Securities Yield Spread Over Comparable Securities Minimum |  |  | 372 |  | 352 |
Investment Grade Tax Exempt Securities Yield Spread Over Comparable Securities Maximum | 413 |  | 466 |  | 467 |
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% |  | 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 | 94.94% |  | 94.75% |  | 95.94% |
Interest Rate Spread [Member] | Discounted Cash Flow [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale securities [Member] | Municipal and other tax-exempt securities [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Available for sale securities |  |  | $ 4,802 |  | $ 4,785 |
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 |  | 4,153 |
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 | $ 16,838 |  | $ 12,299 |  | $ 12,100 |
Minimum [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale securities [Member] | Municipal and other tax-exempt securities [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Range of fair value as a percentage of par value | 0.00% | [1] | 92.25% | [2] | 92.25% | [3]
Range of Interest Rates used to Value Securities | 0.00% | [4] | 6.60% | [5] | 6.05% | [6]
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 | 94.36% | [1] | 94.39% | [2] | 94.38% | [3]
Range of Interest Rates used to Value Securities | 6.37% | [4] | 6.85% | [5] | 6.65% | [6]
Maximum [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale securities [Member] | Municipal and other tax-exempt securities [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Range of fair value as a percentage of par value | 0.00% | [1] | 94.76% | [2] | 95.02% | [3]
Range of Interest Rates used to Value Securities | 0.00% | [4] | 6.60% | [5] | 6.05% | [6]
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 | 94.36% | [1] | 94.39% | [2] | 94.38% | [3]
Range of Interest Rates used to Value Securities | 6.37% | [4] | 6.85% | [5] | 6.73% | [6]
Weighted Average [Member] | Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | Available-for-sale securities [Member] | Municipal and other tax-exempt securities [Member] |  |  |  |  |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |  |  |  |  |
Range of fair value as a percentage of par value | 0.00% | [1] | 93.75% | [2] | 93.91% | [3]
Range of Interest Rates used to Value Securities | 0.00% | [4] | 6.60% | [5] | 6.05% | [6]
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 | 94.36% | [1] | 94.39% | [2] | 94.38% | [3]
Range of Interest Rates used to Value Securities | 6.37% | [4] | 6.85% | [5] | 6.72% | [6]
[1] | Represents fair value as a percentage of par value.
[2] | Represents fair value as a percentage of par value.
[3] | Represents fair value as a percentage of par value.
[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.
[5] | 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.
[6] | 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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- Definition Represents a liquidity discount applied to the market value of mortgage loans qualifying for sale to U.S. government agencies when a mortgage loan is ineligible for sale to U.S. government agencies. + References No definition available. + Details
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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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