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Equity Distribution Program (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Class of Stock [Line Items] |  |  |
Proceeds from issuance of common stock | $ 9,700,000 | $ 41,900,000 | $ 465,800,000
Equity Distribution Program |  |  |
Class of Stock [Line Items] |  |  |
Stock program, maximum amount authorized |  |  | $ 500,000,000
Proceeds from issuance of common stock | $ 0 |  |

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- Definition The cash inflow from the additional capital contribution to the entity. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 14 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3255-108585 + Details
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