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Summary Of Significant Accounting Policies Schedule of Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes (Details) - $ / shares | 12 Months Ended |
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 18, 2017
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items] |  |  |  |  |  |  |
Dividends Payable, Amount Per Share |  |  |  |  |  |  | $ 0.80
Special dividend (per share amount) |  |  |  |  | $ 1.25 |  |
Dividends Treatment For Federal Tax Purposes, Percentage | 100.00% |  | 100.00% |  | 100.00% |  |
Dividends treatment for federal tax purposes. | $ 2.91 | [1] | $ 3.05 | [2] | $ 4.04 | [3] |
Dividend Declared |  |  |  |  |  |  |
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items] |  |  |  |  |  |  |
Dividends Payable, Amount Per Share | 0.80 |  | 0.75 |  | 1.90 |  |
non-special dividend |  |  |  |  |  |  |
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items] |  |  |  |  |  |  |
Dividends Payable, Amount Per Share |  |  |  |  | 0.65 |  |
Allocated to fiscal 2015 |  |  |  |  |  |  |
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items] |  |  |  |  |  |  |
Special dividend (per share amount) |  |  |  |  | 1.35 |  |
Allocated to fiscal 2016 |  |  |  |  |  |  |
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items] |  |  |  |  |  |  |
Special dividend (per share amount) |  |  | 0.56 |  | $ 0.55 |  |
Allocated to fiscal 2017 |  |  |  |  |  |  |
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items] |  |  |  |  |  |  |
Special dividend (per share amount) | 0.47 |  | $ 0.19 |  |  |  |
Allocated to fiscal two thousand and eighteen [Member] |  |  |  |  |  |  |
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items] |  |  |  |  |  |  |
Special dividend (per share amount) | $ 0.33 |  |  |  |  |  |
Capital Gains Income |  |  |  |  |  |  |
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items] |  |  |  |  |  |  |
Dividends Treatment For Federal Tax Purposes, Percentage | 1.71% |  | 9.49% |  | 42.03% |  |
Dividends treatment for federal tax purposes. | $ 0.05 |  | $ 0.29 |  | $ 1.70 |  |
Ordinary Income |  |  |  |  |  |  |
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items] |  |  |  |  |  |  |
Dividends Treatment For Federal Tax Purposes, Percentage | 98.29% |  | 90.51% |  | 57.97% |  |
Dividends treatment for federal tax purposes. | $ 2.86 |  | $ 2.76 |  | $ 2.34 |  |
[1] | The fourth quarter 2017 regular quarterly dividend was $0.80 per common share of which approximately $0.47 per common share was allocable to 2017 and approximately $0.33 per common share is allocable to 2018.
[2] | The fourth quarter 2016 regular quarterly dividend was $0.75 per common share of which approximately $0.56 per common share was allocable to 2016 and approximately $0.19 per common share is allocable to 2017.
[3] | The fourth quarter 2015 dividend of $1.90 per common share consists of a $1.25 per common share special dividend and a $0.65 per common share regular quarterly dividend. Approximately $1.35 per common share was allocable to 2015 and approximately $0.55 per common share is allocable to 2016.

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- Definition [Line Items] for Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Table] + References No definition available. + 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=6367179&loc=d3e4304-108586 + Details
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