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Subsequent Events - Monthly Distributions Declared by Company's Board of Directors (Detail) - $ / shares |  | 9 Months Ended
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Oct. 10, 2017 | Sep. 30, 2017
October 31, 2017 | Senior Common Stock |  |
Dividends Payable [Line Items] |  |
Distributions Payable To Holders Of Records Months |  | October
November 30, 2017 | Senior Common Stock |  |
Dividends Payable [Line Items] |  |
Distributions Payable To Holders Of Records Months |  | November
December 29, 2017 | Senior Common Stock |  |
Dividends Payable [Line Items] |  |
Distributions Payable To Holders Of Records Months |  | December
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 | October 31, 2017 |  |
Dividends Payable [Line Items] |  |
Record Date | Oct. 20, 2017 |
Payment Date | Oct. 31, 2017 |
Subsequent event | October 31, 2017 | Series A Preferred Stock |  |
Dividends Payable [Line Items] |  |
Distribution per Share (in dollars per share) | $ 0.1614583 |
Subsequent event | October 31, 2017 | Series B Preferred Stock |  |
Dividends Payable [Line Items] |  |
Distribution per Share (in dollars per share) | 0.15625 |
Subsequent event | October 31, 2017 | Series D Preferred Stock |  |
Dividends Payable [Line Items] |  |
Distribution per Share (in dollars per share) | $ 0.1458333 |
Subsequent event | October 31, 2017 | Senior Common Stock |  |
Dividends Payable [Line Items] |  |
Payment Date | Nov. 07, 2017 |
Distribution per Share (in dollars per share) | $ 0.0875 |
Subsequent event | October 31, 2017 | Common Stock |  |
Dividends Payable [Line Items] |  |
Distribution per Share (in dollars per share) | $ 0.125 |
Subsequent event | November 30, 2017 |  |
Dividends Payable [Line Items] |  |
Record Date | Nov. 20, 2017 |
Payment Date | Nov. 30, 2017 |
Subsequent event | November 30, 2017 | Series A Preferred Stock |  |
Dividends Payable [Line Items] |  |
Distribution per Share (in dollars per share) | $ 0.1614583 |
Subsequent event | November 30, 2017 | Series B Preferred Stock |  |
Dividends Payable [Line Items] |  |
Distribution per Share (in dollars per share) | 0.15625 |
Subsequent event | November 30, 2017 | Series D Preferred Stock |  |
Dividends Payable [Line Items] |  |
Distribution per Share (in dollars per share) | $ 0.1458333 |
Subsequent event | November 30, 2017 | Senior Common Stock |  |
Dividends Payable [Line Items] |  |
Payment Date | Dec. 07, 2017 |
Distribution per Share (in dollars per share) | $ 0.0875 |
Subsequent event | November 30, 2017 | Common Stock |  |
Dividends Payable [Line Items] |  |
Distribution per Share (in dollars per share) | $ 0.125 |
Subsequent event | December 29, 2017 |  |
Dividends Payable [Line Items] |  |
Record Date | Dec. 19, 2017 |
Payment Date | Dec. 29, 2017 |
Subsequent event | December 29, 2017 | Series A Preferred Stock |  |
Dividends Payable [Line Items] |  |
Distribution per Share (in dollars per share) | $ 0.1614583 |
Subsequent event | December 29, 2017 | Series B Preferred Stock |  |
Dividends Payable [Line Items] |  |
Distribution per Share (in dollars per share) | 0.15625 |
Subsequent event | December 29, 2017 | Series D Preferred Stock |  |
Dividends Payable [Line Items] |  |
Distribution per Share (in dollars per share) | $ 0.1458333 |
Subsequent event | December 29, 2017 | Senior Common Stock |  |
Dividends Payable [Line Items] |  |
Payment Date | Jan. 08, 2018 |
Distribution per Share (in dollars per share) | $ 0.0875 |
Subsequent event | December 29, 2017 | Common Stock |  |
Dividends Payable [Line Items] |  |
Distribution per Share (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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- Definition The per share amount of a dividend declared, but not paid, as of the financial reporting date. + 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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