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Subsequent Events - Monthly Distributions Declared by Company's Board of Directors (Detail) - $ / shares |  |  |  |  |  |  |  |  |  | 1 Months Ended |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Oct. 05, 2018 | Sep. 28, 2018 | Sep. 19, 2018 | Sep. 10, 2018 | Aug. 31, 2018 | Aug. 21, 2018 | Aug. 07, 2018 | Jul. 31, 2018 | Jul. 20, 2018 | Sep. 30, 2018 | Aug. 31, 2018 | Jul. 30, 2018 | Jul. 10, 2018
Subsequent event | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.4843749
Subsequent event | Series B Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  |  |  |  |  |  |  |  |  |  |  |  | 0.46875
Subsequent event | Series D Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  |  |  |  |  |  |  |  |  |  |  |  | 0.4374999
Subsequent event | Senior Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  |  |  |  |  |  |  |  |  |  |  |  | 0.2625
Subsequent event | Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  |  |  |  |  |  |  |  |  |  |  |  | $ 0.375
Subsequent event | April 2018 |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Record Date |  |  |  |  |  |  |  |  | Jul. 20, 2018 |  |  |  |
Payment Date |  |  |  |  |  |  |  | Jul. 31, 2018 |  |  |  |  |
Subsequent event | April 2018 | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  |  |  |  |  |  |  | $ 0.1614583 |  |  |  |  |
Subsequent event | April 2018 | Series B Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  |  |  |  |  |  |  | 0.15625 |  |  |  |  |
Subsequent event | April 2018 | Series D Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  |  |  |  |  |  |  | 0.1458333 |  |  |  |  |
Subsequent event | April 2018 | Senior Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Payable to the Holders of Record During the Month of: |  |  |  |  |  |  |  |  |  |  |  | July |
Payment Date |  |  |  |  |  |  | Aug. 07, 2018 |  |  |  |  |  |
Distribution per share (in dollars per share) |  |  |  |  |  |  | $ 0.0875 |  |  |  |  |  |
Subsequent event | April 2018 | Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  |  |  |  |  |  |  | $ 0.125 |  |  |  |  |
Forecast | May 2018 |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Record Date |  |  |  |  |  | Aug. 21, 2018 |  |  |  |  |  |  |
Payment Date |  |  |  |  | Aug. 31, 2018 |  |  |  |  |  |  |  |
Forecast | May 2018 | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  |  |  |  | $ 0.1614583 |  |  |  |  |  | $ 0.1614583 |  |
Forecast | May 2018 | Series B Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  |  |  |  | 0.15625 |  |  |  |  |  | 0.15625 |  |
Forecast | May 2018 | Series D Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  |  |  |  | 0.1458333 |  |  |  |  |  | $ 0.1458333 |  |
Forecast | May 2018 | Senior Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Payable to the Holders of Record During the Month of: |  |  |  |  |  |  |  |  |  |  | August |  |
Payment Date |  |  |  | Sep. 10, 2018 |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  |  |  | $ 0.0875 |  |  |  |  |  |  |  |  |
Forecast | May 2018 | Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  |  |  |  | $ 0.125 |  |  |  |  |  | $ 0.125 |  |
Forecast | June 2018 |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Record Date |  |  | Sep. 19, 2018 |  |  |  |  |  |  |  |  |  |
Payment Date |  | Sep. 28, 2018 |  |  |  |  |  |  |  |  |  |  |
Forecast | June 2018 | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  | $ 0.1614583 |  |  |  |  |  |  |  |  |  |  |
Forecast | June 2018 | Series B Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  | 0.15625 |  |  |  |  |  |  |  |  |  |  |
Forecast | June 2018 | Series D Preferred Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  | 0.1458333 |  |  |  |  |  |  |  |  |  |  |
Forecast | June 2018 | Senior Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Payable to the Holders of Record During the Month of: |  |  |  |  |  |  |  |  |  | September |  |  |
Payment Date | Oct. 05, 2018 |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) | $ 0.0875 |  |  |  |  |  |  |  |  |  |  |  |
Forecast | June 2018 | Common Stock |  |  |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Distribution per share (in dollars per share) |  | $ 0.125 |  |  |  |  |  |  |  |  |  |  |

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