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, 2018		Dec. 31, 2017		Dec. 31, 2016		Dec. 17, 2018
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items]
Dividends Treatment For Federal Tax Purposes, Percentage	100.00%		100.00%		100.00%
Dividends treatment for federal tax purposes.	$ 3.57	[1]	$ 2.91	[2]	$ 3.05	[3]
Dividends Payable, Amount Per Share							$ 0.95
Dividend Declared
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items]
Dividends Payable, Amount Per Share	0.95		0.80		0.75
Allocated to fiscal 2016
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items]
Special dividend / distribution (per share / common unit amount)					0.56
Allocated to fiscal 2017
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items]
Special dividend / distribution (per share / common unit amount)			0.47
Dividends Payable, Amount Per Share					$ 0.19
Allocated to fiscal 2019 [Domain]
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items]
Dividends Payable, Amount Per Share	0.26
Allocated to fiscal 2018 [Member]
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items]
Special dividend / distribution (per share / common unit amount)	$ 0.69
Dividends Payable, Amount Per Share			$ 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	21.83%		1.71%		9.49%
Dividends treatment for federal tax purposes.	$ 0.78		$ 0.05		$ 0.29
Ordinary Income
Tax Treatment Of Common Dividends Per Share For Federal Tax Purposes [Line Items]
Dividends Treatment For Federal Tax Purposes, Percentage	78.17%		98.29%		90.51%
Dividends treatment for federal tax purposes.	$ 2.79		$ 2.86		$ 2.76

[1] 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 is allocable to 2019

[2] 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.

[3] 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.