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Stockholders' Equity (Common and Preferred Stock) (Detail) | 9 Months Ended | 12 Months Ended | 63 Months Ended |
--- | --- | --- | --- | ---
Sep. 30, 2017 Vote $ / shares shares | Dec. 31, 2016 Vote $ / shares shares | Dec. 31, 2011 $ / shares shares | Dec. 31, 2015 $ / shares shares
Private Placement [Member] |  |  |  |
Common Stock [Abstract] |  |  |  |
Common stock shares issued (in shares) |  |  | 20,000,000 |
Sale of Stock, Price Per Share | $ / shares |  |  | $ 10 |
Sale of Stock, Number of Shares Issued in Transaction |  |  | 20,000,000 |
Sale Of Stock Nature Of Consideration Received Per Transaction |  | The Company ceased raising capital under this private placement offering effective December 31, 2011. |  |
Preferred Stock [Member] |  |  |  |
Preferred Stock [Abstract] |  |  |  |
Preferred shares authorized (in shares) | 8,990,000 | 8,990,000 |  |
Convertible Series AA Preferred Stock [Member] |  |  |  |
Preferred Stock [Abstract] |  |  |  |
Preferred shares authorized (in shares) |  | 1,000,000 |  |
Annual cash dividend rate (in hundredths) |  | 7.00% |  |
Liquidation preference per share (in dollars per share) | $ / shares |  | $ 25.00 |  |
Initial conversion price (in dollars per share) | $ / shares |  | $ 25.00 |  |
Total number of common shares issuable upon conversion of outstanding shares of preferred stock (in shares) |  | 2 |  |
Preferred shares issued (in shares) |  | 0 |  |
Common Class A [Member] |  |  |  |
Common Stock [Abstract] |  |  |  |
Common stock, shares authorized (in shares) | 100,000,000 | 100,000,000 |  | 100,000,000
Par value (in dollars per share) | $ / shares | $ 0.01 | $ 0.01 |  | $ 0.01
Common stock shares issued (in shares) | 17,614,160 | 17,502,673 |  | 17,202,228
Number of votes holder is entitled to | Vote | 1 | 1 |  |
Maximum individual common stock ownership (in hundredths) | 9.80% | 9.80% |  |
Common Class B [Member] |  |  |  |
Common Stock [Abstract] |  |  |  |
Common stock, shares authorized (in shares) | 1,000 | 1,000 |  |
Par value (in dollars per share) | $ / shares | $ 0.01 | $ 0.01 |  |
Common stock shares issued (in shares) | 0 | 0 |  |
Number of votes holder is entitled to | Vote | 1 | 1 |  |
Maximum individual common stock ownership (in hundredths) | 9.80% | 9.80% |  |

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