Income Taxes (Taxability of Cash Distributions Paid on Common Shares) (Details) - Parent Company [Member] - $ / shares	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Investments, Owned, Federal Income Tax Note
Ordinary income (dollars per share)									$ 3.28	$ 3.07	$ 2.76
Ordinary income (percentage per share)									100.00%	99.70%	94.40%
Return of capital (dollars per share)									$ 0	$ 0.00	$ 0
Return of capital (percentage per share)									0.00%	0.00%	6.00%
Un-recaptured Section 1250 gain dividends per share									$ 0.00	$ 0.01	$ 0.00
Un-recaptured Section 1250 gain dividends per share percentage									0.00%	0.00%	0.00%
Total (dollars per share)	$ 0.82	$ 0.82	$ 0.82	$ 0.82	$ 0.77	$ 0.77	$ 0.77	$ 0.77	$ 3.28	$ 3.08	$ 2.92
Total (percentage per share)									100.00%	100.00%	100.00%