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Fair Value Measurement (Narrative) (Details) | 9 Months Ended |
--- | --- | ---
Sep. 30, 2018 USD ($) broker | Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Percentage of portfolio of unadjusted fair values obtained | 98.20% |
Minimum [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Number of knowledgeable outside security brokers consulted to determine fair value | broker | 1 |
Borrowings [Member] | Unsecured debt [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Unsecured notes | $ 375,000,000 | $ 375,000,000
Commercial Mortgage Back Securities [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Trading securities, fair value disclosure | 25,000,000 | 19,300,000
Fair Value, Measurements, Recurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets, fair value disclosure | 3,909,578,000 | 3,738,293,000
Liabilities, fair value | 1,550,000 | 1,323,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets, fair value disclosure | 3,001,954,000 | 2,921,995,000
Liabilities, fair value | 961,000 | 1,200,000
Fair Value, Measurements, Recurring [Member] | Borrowings [Member] | Unsecured debt [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Liabilities, fair value | 361,118,000 | 385,583,000
Fair Value, Measurements, Recurring [Member] | Borrowings [Member] | Unsecured debt [Member] | Level 2 [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Liabilities, fair value | 361,118,000 | 385,583,000
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets, fair value disclosure | 1,400,000 | $ 1,500,000
Fair Value, Measurements, Nonrecurring [Member] |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |
Assets, fair value disclosure | 0 |
Liabilities, fair value | $ 0 |

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
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- Definition Fair value of financial obligations, including, but not limited to, debt instruments, derivative liabilities, federal funds purchased and sold under agreements to repurchase, securities loaned or sold under agreements to repurchase, financial instruments sold not yet purchased, guarantees, line of credit, loans and notes payable, servicing liability, and trading liabilities. + References No definition available. + Details
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