Summary Of Significant Accounting Policies Schedule of Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes (Details) - $ / shares	12 Months Ended
	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.