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Capital Stock - ATM Agreements (Details) - USD ($) $ in Millions |  |  |  |  |  | 9 Months Ended
--- | --- | --- | --- | --- | --- | ---
Aug. 05, 2021 | Jun. 30, 2021 | Apr. 09, 2021 | Dec. 15, 2020 | Dec. 18, 2018 | Sep. 30, 2021
8.625% Series A Cumulative, Perpetual Preferred Stock |  |  |  |  |  |
Sale of stock |  |  |  |  |  |
Shares issued (in shares) |  |  |  | 984,000 |  |
8.375% Series B Cumulative, Perpetual Preferred Stock |  |  |  |  |  |
Sale of stock |  |  |  |  |  |
Depositary share's fractional interest in share of preferred stock |  |  | 0.001 |  |  | 0.001
Depositary Shares |  |  |  |  |  |
Sale of stock |  |  |  |  |  |
Shares issued (in shares) |  |  | 1,600,000 |  |  |
2018 Common Stock ATM Agreement |  |  |  |  |  |
Sale of stock |  |  |  |  |  |
Maximum amount of common shares to be issued |  | $ 50.0 |  |  | $ 30.0 |
2018 Common Stock ATM Agreement | Maximum [Member] |  |  |  |  |  |
Sale of stock |  |  |  |  |  |
Sales commission paid per transaction (as a percent) |  |  |  |  | 3.00% |
2021 Series B Preferred Stock ATM Agreement |  |  |  |  |  |
Sale of stock |  |  |  |  |  |
Maximum amount of shares to be issued | $ 50.0 |  |  |  |  |
2021 Series B Preferred Stock ATM Agreement | 8.375% Series B Cumulative, Perpetual Preferred Stock |  |  |  |  |  |
Sale of stock |  |  |  |  |  |
Depositary share's fractional interest in share of preferred stock | 0.001 |  |  |  |  |
2021 Series B Preferred Stock ATM Agreement | Depositary Shares |  |  |  |  |  |
Sale of stock |  |  |  |  |  |
Shares issued (in shares) |  |  |  |  |  | 0
2021 Series B Preferred Stock ATM Agreement | Maximum [Member] |  |  |  |  |  |
Sale of stock |  |  |  |  |  |
Sales commission paid per transaction (as a percent) | 3.00% |  |  |  |  |
Common Stock | 2018 Common Stock ATM Agreement |  |  |  |  |  |
Sale of stock |  |  |  |  |  |
Shares issued (in shares) |  | 0 |  |  | 0 |

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