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Shareholders' Equity - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Jan. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jan. 10, 2017
Class of Stock [Line Items] |  |  |  |  |
Common stock, shares authorized |  | 240,000,000 | 240,000,000 |  |
Preferred stock, shares authorized |  | 5,000,000 |  |  |
Preferred stock, par value per share (in dollars per share) |  | $ 0.01 |  |  |
Net proceeds from issuance of common stock |  | $ 219,504 | $ 96,547 | $ 0 |
Common stock, no par, 240,000 shares authorized, 147,740 and 120,630 shares issued, respectively |  | $ 1,284,274 | 1,055,934 |  |
Underwritten Public Equity Offering [Member] | Common Stock [Member] |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Shares issued |  |  |  |  | 26,450,000
Price per share issued (in dollars per share) |  |  |  |  | $ 8.65
Net proceeds from issuance of common stock | $ 220,000 |  |  |  |
ATM Program [Member] |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Net proceeds from issuance of common stock |  |  | 96,500 |  |
Common stock, no par, 240,000 shares authorized, 147,740 and 120,630 shares issued, respectively |  |  | 100,000 |  |
Payments for commissions |  |  | $ 2,300 |  |
ATM Program [Member] | Common Stock [Member] |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |
Shares issued |  |  | 13,018,732 |  |

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