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Subsequent Events - Monthly Distributions Declared by Company's Board of Directors (Detail) - Subsequent Event - $ / shares |  |  |  |  |  |  |  | 1 Months Ended |
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Feb. 07, 2019 | Apr. 06, 2018 | Mar. 30, 2018 | Mar. 07, 2018 | Feb. 28, 2018 | Jan. 31, 2018 | Jan. 19, 2018 | Mar. 31, 2018 | Feb. 28, 2018 | Jan. 31, 2018 | Feb. 07, 2018
Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  |  |  |  |  | $ 0.4843749 |  |  |  |
Series B Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  |  |  |  |  | 0.46875 |  |  |  |
Series D Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  |  |  |  |  | 0.4374999 |  |  |  |
Senior Common Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  |  |  |  |  | 0.2625 |  |  |  |
Common Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  |  |  |  |  | $ 0.375 |  |  |  |
Dividend Declared for Month of January |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Record date |  |  |  |  |  |  | Jan. 22, 2018 |  |  |  |
Payment date |  |  |  |  |  | Jan. 31, 2018 |  |  |  |  |
Dividend Declared for Month of January | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  |  |  |  | $ 0.1614583 |  |  |  | $ 0.1614583 |
Dividend Declared for Month of January | Series B Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  |  |  |  | 0.15625 |  |  |  | 0.15625 |
Dividend Declared for Month of January | Series D Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  |  |  |  | 0.1458333 |  |  |  | $ 0.1458333 |
Dividend Declared for Month of January | Senior Common Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Payment date | Feb. 07, 2018 |  |  |  |  |  |  |  |  |  |
Month in which distribution is payable to holders of record |  |  |  |  |  |  |  |  |  | January |
Distributions (in dollars per share) |  |  |  |  |  |  |  |  |  |  | $ 0.0875
Dividend Declared for Month of January | Common Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  |  |  |  | $ 0.125 |  |  |  | $ 0.125 |
Dividend Declared for Month of February |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Record date |  |  |  |  |  |  | Feb. 16, 2018 |  |  |  |
Payment date |  |  |  |  | Feb. 28, 2018 |  |  |  |  |  |
Dividend Declared for Month of February | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  |  |  | $ 0.1614583 |  |  |  | $ 0.1614583 |  |
Dividend Declared for Month of February | Series B Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  |  |  | 0.15625 |  |  |  | 0.15625 |  |
Dividend Declared for Month of February | Series D Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  |  |  | 0.1458333 |  |  |  | $ 0.1458333 |  |
Dividend Declared for Month of February | Senior Common Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Payment date |  |  |  | Mar. 07, 2018 |  |  |  |  |  |  |
Month in which distribution is payable to holders of record |  |  |  |  |  |  |  |  | February |  |
Distributions (in dollars per share) |  |  |  | $ 0.0875 |  |  |  |  |  |  |
Dividend Declared for Month of February | Common Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  |  |  | $ 0.125 |  |  |  | $ 0.125 |  |
Dividend Declared for Month of March |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Record date |  |  |  |  |  |  | Mar. 20, 2018 |  |  |  |
Payment date |  |  | Mar. 30, 2018 |  |  |  |  |  |  |  |
Dividend Declared for Month of March | Series A Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  | $ 0.1614583 |  |  |  |  |  |  |  |
Dividend Declared for Month of March | Series B Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  | 0.15625 |  |  |  |  |  |  |  |
Dividend Declared for Month of March | Series D Preferred Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  | 0.1458333 |  |  |  |  |  |  |  |
Dividend Declared for Month of March | Senior Common Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Payment date |  | Apr. 06, 2018 |  |  |  |  |  |  |  |  |
Month in which distribution is payable to holders of record |  |  |  |  |  |  |  | March |  |  |
Distributions (in dollars per share) |  | $ 0.0875 |  |  |  |  |  |  |  |  |
Dividend Declared for Month of March | Common Stock |  |  |  |  |  |  |  |  |  |  |
Dividends Payable [Line Items] |  |  |  |  |  |  |  |  |  |  |
Distributions (in dollars per share) |  |  | $ 0.125 |  |  |  |  |  |  |  |

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- Definition Date the declared dividend will be paid, in CCYY-MM-DD format. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=98513485&loc=d3e4304-108586 + Details
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