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Equity Equity - Shares Issued and Gross Proceeds (Details) - USD ($) $ in Thousands | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Class of Stock [Line Items] |  |  |
Stock Redeemed or Called During Period, Shares | (2,791,262) | (790,941) |
Common Stock, Shares, Outstanding | 77,660,266 | 77,175,283 | 70,939,647
Common Class T [Member] |  |  |
Class of Stock [Line Items] |  |  |
Stock Redeemed or Called During Period, Shares | 0 | 0 |
Common Stock, Shares, Outstanding | 226,297 | 4,148 | 0
Common Class S [Member] |  |  |
Class of Stock [Line Items] |  |  |
Stock Redeemed or Called During Period, Shares | 0 | 0 |
Common Stock, Shares, Outstanding | 277 | 268 | 0
Common Class D [Member] |  |  |
Class of Stock [Line Items] |  |  |
Stock Redeemed or Called During Period, Shares | 0 | 0 |
Common Stock, Shares, Outstanding | 19,157 | 268 | 0
Common Class I [Member] |  |  |
Class of Stock [Line Items] |  |  |
Stock Redeemed or Called During Period, Shares | 0 | 0 |
Common Stock, Shares, Outstanding | 803,069 | 267,476 | 0
Common Class A [Member] |  |  |
Class of Stock [Line Items] |  |  |
Stock Redeemed or Called During Period, Shares | (1,351,178) | (557,206) |
Common Stock, Shares, Outstanding | 25,781,325 | 25,995,943 | 25,562,982
Common Class AA [Member] |  |  |
Class of Stock [Line Items] |  |  |
Stock Redeemed or Called During Period, Shares | (1,434,491) | (233,735) |
Common Stock, Shares, Outstanding | 49,840,818 | 49,942,471 | 44,595,631
Common Class AAA [Member] |  |  |
Class of Stock [Line Items] |  |  |
Stock Redeemed or Called During Period, Shares | (5,593) | 0 |
Common Stock, Shares, Outstanding | 989,323 | 964,709 | 781,034
IPO [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Gross Proceeds Received to Date, New Issues | $ 733,987 | $ 726,557 |
Common stock issued (shares) | 73,694,264 | 72,931,727 |
IPO [Member] | Common Class T [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Gross Proceeds Received to Date, New Issues | $ 2,245 | $ 41 |
Common stock issued (shares) | 224,647 | 4,144 |
IPO [Member] | Common Class S [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Gross Proceeds Received to Date, New Issues | $ 3 | $ 3 |
Common stock issued (shares) | 264 | 264 |
IPO [Member] | Common Class D [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Gross Proceeds Received to Date, New Issues | $ 182 | $ 3 |
Common stock issued (shares) | 18,921 | 264 |
IPO [Member] | Common Class I [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Gross Proceeds Received to Date, New Issues | $ 7,538 | $ 2,493 |
Common stock issued (shares) | 786,573 | 263,200 |
IPO [Member] | Common Class A [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Gross Proceeds Received to Date, New Issues | $ 240,780 | $ 240,780 |
Common stock issued (shares) | 24,199,764 | 24,199,760 |
IPO [Member] | Common Class AA [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Gross Proceeds Received to Date, New Issues | $ 474,858 | $ 474,858 |
Common stock issued (shares) | 47,562,870 | 47,562,870 |
IPO [Member] | Common Class AAA [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Gross Proceeds Received to Date, New Issues | $ 8,381 | $ 8,379 |
Common stock issued (shares) | 901,225 | 901,225 |
Distribution Reinvestment Plan |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Gross Proceeds Received to Date, New Issues | $ 58,376 | $ 41,954 |
Common stock issued (shares) | 6,152,779 | 4,440,514 |
Distribution Reinvestment Plan | Common Class T [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Gross Proceeds Received to Date, New Issues | $ 16 | $ 0 |
Common stock issued (shares) | 1,650 | 4 |
Distribution Reinvestment Plan | Common Class S [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Gross Proceeds Received to Date, New Issues | $ 0 | $ 0 |
Common stock issued (shares) | 13 | 4 |
Distribution Reinvestment Plan | Common Class D [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Gross Proceeds Received to Date, New Issues | $ 2 | $ 0 |
Common stock issued (shares) | 236 | 4 |
Distribution Reinvestment Plan | Common Class I [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Gross Proceeds Received to Date, New Issues | $ 158 | $ 41 |
Common stock issued (shares) | 16,496 | 4,276 |
Distribution Reinvestment Plan | Common Class A [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Gross Proceeds Received to Date, New Issues | $ 25,328 | $ 19,773 |
Common stock issued (shares) | 2,669,097 | 2,089,748 |
Distribution Reinvestment Plan | Common Class AA [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Gross Proceeds Received to Date, New Issues | $ 32,369 | $ 21,828 |
Common stock issued (shares) | 3,412,273 | 2,313,170 |
Distribution Reinvestment Plan | Common Class AAA [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common Stock, Gross Proceeds Received to Date, New Issues | $ 503 | $ 312 |
Common stock issued (shares) | 53,014 | 33,308 |
Share Distribution [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common stock issued (shares) | 568,485 | 568,483 |
Share Distribution [Member] | Common Class T [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common stock issued (shares) | 0 | 0 |
Share Distribution [Member] | Common Class S [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common stock issued (shares) | 0 | 0 |
Share Distribution [Member] | Common Class D [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common stock issued (shares) | 0 | 0 |
Share Distribution [Member] | Common Class I [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common stock issued (shares) | 0 | 0 |
Share Distribution [Member] | Common Class A [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common stock issued (shares) | 263,642 | 263,641 |
Share Distribution [Member] | Common Class AA [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common stock issued (shares) | 300,166 | 300,166 |
Share Distribution [Member] | Common Class AAA [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common stock issued (shares) | 4,677 | 4,676 |
Restricted Stock Issuance [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common stock issued (shares) | 36,000 | 25,500 |
Restricted Stock Issuance [Member] | Common Class T [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common stock issued (shares) | 0 | 0 |
Restricted Stock Issuance [Member] | Common Class S [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common stock issued (shares) | 0 | 0 |
Restricted Stock Issuance [Member] | Common Class D [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common stock issued (shares) | 0 | 0 |
Restricted Stock Issuance [Member] | Common Class I [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common stock issued (shares) | 0 | 0 |
Restricted Stock Issuance [Member] | Common Class A [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common stock issued (shares) | 0 | 0 |
Restricted Stock Issuance [Member] | Common Class AA [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common stock issued (shares) | 0 | 0 |
Restricted Stock Issuance [Member] | Common Class AAA [Member] |  |  |
Class of Stock [Line Items] |  |  |
Common stock issued (shares) | 36,000 | 25,500 |

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