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Summary Of Significant Accounting Policies of Tax Treatment of Common Dividends Per Share for Federal Tax Purposes (Details) - $ / shares | 12 Months Ended |
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 15, 2020
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Dividends Treatment For Federal Tax Purposes, Percentage | 100.00% |  | 100.00% |  | 100.00% |  |
Dividends treatment for federal tax purposes. | $ 3.88 | [1] | $ 3.15 | [2] | $ 3.57 | [3] |
Distributions Payable, Amount Per Unit |  |  |  |  |  |  | $ 0.98
Ordinary income |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Dividends Treatment For Federal Tax Purposes, Percentage | 64.91% |  | 94.84% |  | 78.17% |  |
Dividends treatment for federal tax purposes. | $ 2.52 |  | $ 2.99 |  | $ 2.79 |  |
Capital Gain Income |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Dividends Treatment For Federal Tax Purposes, Percentage | 25.49% |  | 5.16% |  | 21.83% |  |
Dividends treatment for federal tax purposes. | $ 0.99 |  | $ 0.16 |  | $ 0.78 |  |
Return of Capital |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Dividends Treatment For Federal Tax Purposes, Percentage | 9.60% |  | 0.00% |  | 0.00% |  |
Dividends treatment for federal tax purposes. | $ 0.37 |  | $ 0 |  | $ 0 |  |
Dividend Declared |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Distributions Payable, Amount Per Unit | $ 0.98 |  | 0.98 |  | 0.95 |  |
Allocated to 2019 |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Distributions Payable, Amount Per Unit |  |  | 0.04 |  | 0.26 |  |
Allocated to 2020 |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Distributions Payable, Amount Per Unit |  |  | $ 0.94 |  |  |  |
Allocated to 2018 |  |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |  |
Distributions Payable, Amount Per Unit |  |  |  |  | $ 0.69 |  |
[1] | The fourth quarter 2020 regular quarterly dividend was $0.98 per common share, all of which was allocable to 2021.
[2] | The fourth quarter 2019 regular quarterly dividend was $0.98 per common share of which approximately $0.04 per common share was allocable to 2019 and approximately $0.94 per common share was allocable to 2020.
[3] | The fourth quarter 2018 regular quarterly dividend was $0.95 per common share of which approximately $0.69 per common share was allocable to 2018 and approximately $0.26 per common share was allocable to 2019.

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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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=121583591&loc=d3e4304-108586 + Details
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