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Fair Value Measurement (Narrative) (Details) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2016 USD ($) broker | Dec. 31, 2015 USD ($)
Fair Value Measurement [Line Items] |  |
Percentage of portfolio of unadjusted fair values obtained | 99.70% |
Percentage of portfolio of specific unadjusted broker quotes obtained | 0.30% |
Fair value of assets measured on a recurring basis | $ 3,548,227 | $ 3,380,642
Level 3 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 9,068 | 10,431
Level 2 [Member] |  |
Fair Value Measurement [Line Items] |  |
Fair value of assets measured on a recurring basis | 2,854,987 | 2,892,014
Level 2 [Member] | Commercial Mortgage Backed Securities [Member] |  |
Fair Value Measurement [Line Items] |  |
Trading securities, fair value disclosure | $ 30,000 | $ 37,300
Minimum [Member] |  |
Fair Value Measurement [Line Items] |  |
Number of knowledgeable outside security brokers consulted to determine fair value | broker | 1 |

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