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Common stock Common stock issuance (Details 2) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Class of Stock [Line Items] |  |  |
Aggregate Authorized Issuance of Debt Securities | $ 1,000,000,000.0 |  |
Common stock, shares authorized | 500,000,000 | 500,000,000 |
Common Stock, Shares, Issued | 200,922,790 | 196,564,907 |
Proceeds from Issuance of Common Stock | $ 106,848,000 | $ 0 | $ 0
At-the-market offering |  |  |
Class of Stock [Line Items] |  |  |
Common stock, shares authorized | 10,000,000.0 |  |
Common Stock, Shares, Issued | 3,600,000 |  |
Proceeds from Issuance of Common Stock | $ 94,000,000.0 |  |
Sales Commissions Paid from Issuance of Common Stock through the ATM Program | $ 950,000 |  |
Common Stock, Authorized Shares Remaining | 6,400,000 |  |
K-Plan |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Authorized Shares Remaining | 7,300,000 |  |

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