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Stockholders' Equity (Dividends) (Detail) - USD ($) |  |  | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Jan. 23, 2012 | Jan. 22, 2012 | Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016 | Dec. 31, 2015
Cash Dividends [Abstract] |  |  |  |  |  |
Cash dividends paid, net of reinvested stock dividends |  |  | $ 3,621,000 | $ 3,348,000 | $ 4,517,000 | $ 4,124,000
Annualized dividend rate (in dollars per share) |  |  | $ 0.10 |  | $ 0.40 |
Dividend Reinvestment Plan [Abstract] |  |  |  |  |  |
Registered shares of common stock pursuant to dividend reinvestment plan (in shares) | 3,000,000 |  |  |  |  |
Percentage of purchase price per share (in hundredths) | 95.00% |  |  |  |  |
Former sales price per share (in dollars per share) | $ 9.50 | $ 10.00 |  |  |  |
Payment of sales commission or dealer manager fee | $ 0 |  |  |  |  |
Notice period for amendments to Dividend Reinvestment Plan |  |  | 30 days |  | 30 days |
Common stock issued under dividend reinvestment plan |  |  | $ 17,400,000 |  | $ 16,325,429 |
Common stock issued under dividend reinvestment plan (in shares) |  |  | 1,834,147 |  | 1,718,466 |

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