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Notes Payable (Schedule of Long-term Debt) (Details) - USD ($) $ in Thousands | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017 | Mar. 08, 2017
Debt Instrument [Line Items] |  |  |
Interest rate |  |  | 4.40%
Total principal amount | $ 375,000 | $ 375,000 |
Less unamortized discount and debt issuance costs(3) | 3,365 | 3,665 |
Notes payable | 371,635 | 371,335 |
Unsecured debt [Member] | Borrowings [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Notes payable | $ 371,635 | 371,335 |
Unsecured debt [Member] | Borrowings [Member] | Level 2 [Member] | Unsecured Notes One [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate | 4.40% |  |
Unsecured debt | $ 375,000 | 375,000 |
Unsecured debt [Member] | Borrowings [Member] | Level 2 [Member] | Unsecured Notes Two [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Unsecured debt | $ 0 | $ 0 |
LIBOR [Member] | Minimum [Member] | Unsecured debt [Member] | Borrowings [Member] | Level 2 [Member] | Unsecured Notes Two [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Basis spread on variable rate | 0.1125% |  |
LIBOR [Member] | Maximum [Member] | Unsecured debt [Member] | Borrowings [Member] | Level 2 [Member] | Unsecured Notes Two [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Basis spread on variable rate | 0.1625% |  |

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