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Common Stock Common Stock (Details) - shares | 6 Months Ended |
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017
Class of Stock [Line Items] |  |
Sales Agency Agreements Common Stock, Shares Authorized | 200,000,000 | 200,000,000
Document Period End Date | Jun. 30, 2018 |
Sales Agency Agreement [Member] |  |
Class of Stock [Line Items] |  |
Shares issued from sales agency agreements | 0 |
Sales Agency Agreements Common Stock, Shares Authorized | 3,800,000 |
Common Stock Shares Authorized Under Sales Agency Agreements Remaining Shares Authorized To Sell | 1,100,000 |

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