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Notes Payable (Narrative) (Details) $ in Thousands |  |  | 9 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | ---
Mar. 29, 2017 USD ($) | Jul. 02, 2013 USD ($) | Sep. 30, 2017 | Dec. 31, 2017 USD ($) | Mar. 08, 2017 USD ($) | Dec. 31, 2016 USD ($) | Dec. 03, 2014 USD ($)
Debt Instrument [Line Items] |  |  |  |  |  |  |
Revolving credit facility term |  | 5 years |  |  |  |  |
Unamortized discount and debt issuance costs |  |  |  | $ 3,665 |  | $ 0 |
Revolving Credit Facility [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Loan maximum borrowing capacity |  | $ 200,000 |  |  |  |  | $ 250,000
Revolving credit facility term | 5 years |  |  |  |  |  |
Revolving Credit Facility [Member] | Minimum [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Commitment fee on undrawn portion of facility | 0.125% |  |  |  |  |  |
Debt to total capital ratio | 0.15 |  |  |  |  |  |
Revolving Credit Facility [Member] | Maximum [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Commitment fee on undrawn portion of facility | 0.225% |  |  |  |  |  |
Debt to total capital ratio | 0.25 |  |  |  |  |  |
Revolving Credit Facility [Member] | LIBOR [Member] | Minimum [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Basis spread on variable rate | 1.125% |  |  |  |  |  |
Debt to total capital ratio | 0.15 |  |  |  |  |  |
Revolving Credit Facility [Member] | LIBOR [Member] | Maximum [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Basis spread on variable rate | 1.625% |  |  |  |  |  |
Debt to total capital ratio | 0.25 |  |  |  |  |  |
Secured Notes Two [Member] | Revolving Credit Facility [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Loan maximum borrowing capacity | $ 50,000 |  |  |  |  |  |
Secured Notes Two [Member] | Revolving Credit Facility [Member] | Minimum [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Commitment fee on undrawn portion of facility |  |  |  | 0.15% |  |  |
Unsecured Notes Three [Member] | Revolving Credit Facility [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Unamortized debt issuance expense | $ 200 |  |  |  |  |  |
Unsecured Notes Three [Member] | Revolving Credit Facility [Member] | Maximum [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt to total capital ratio |  |  |  | 0.176 |  |  |
Level 2 [Member] | Secured Notes Two [Member] | LIBOR [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Basis spread on variable rate |  |  | 0.40% |  |  |  |
Unsecured Debt [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Interest rate, stated percentage |  |  |  |  | 4.40% |  |
Debt issuance costs |  |  |  |  | $ 3,400 |  |
Discount percent |  |  |  |  | 99.847% |  |
Interest rate, effective percentage |  |  |  |  | 4.45% |  |
Unsecured Debt [Member] | Borrowings [Member] | Level 2 [Member] | Unsecured Notes Two [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Unsecured debt |  |  |  | $ 375,000 |  | $ 0 |
Interest rate, stated percentage |  |  | 4.40% |  |  |  |
Unsecured Debt [Member] | Borrowings [Member] | Level 2 [Member] | Unsecured Notes Three [Member] | LIBOR [Member] | Minimum [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Basis spread on variable rate |  |  | 0.1125% |  |  |  |
Unsecured Debt [Member] | Borrowings [Member] | Level 2 [Member] | Unsecured Notes Three [Member] | LIBOR [Member] | Maximum [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Basis spread on variable rate |  |  | 0.1625% |  |  |  |

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