Common and Preferred Shares Common Shares (Details) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Apr. 06, 2017	Jan. 21, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash Dividends Paid [Line Items]
Common Stock, Shares, Issued		2,250,000	76,858,632	66,263,487
Net proceeds from issuance of common shares		$ 125,000	$ 99,069	$ 142,628	$ 190,158
Common Stock [Member]
Cash Dividends Paid [Line Items]
Common stock declared dividends per share			$ 4.08	$ 3.84
Total Cash Distribution Per Share [Member] | Common Stock [Member]
Cash Dividends Paid [Line Items]
Common Stock, Dividends, Per Share, Cash Paid			4.0600	3.8225
Taxable Ordinary Income [Member] | Common Stock [Member]
Cash Dividends Paid [Line Items]
Common Stock, Dividends, Per Share, Cash Paid			3.5434	3.1659
Return of Capital [Member] | Common Stock [Member]
Cash Dividends Paid [Line Items]
Common Stock, Dividends, Per Share, Cash Paid			0.2762	0.2489
Long-term Capital Gain [Member] | Common Stock [Member]
Cash Dividends Paid [Line Items]
Unrecaptured Section 1250 gain			0.0972	0.1060
Common Stock, Dividends, Per Share, Cash Paid			$ 0.2404	$ 0.4077
direct share purchase plan [Member]
Cash Dividends Paid [Line Items]
Common Stock, Shares, Issued			1,382,730	258,263
Net proceeds from issuance of common shares			$ 98,200	$ 16,900
Common Stock [Member]
Cash Dividends Paid [Line Items]
Stock Issued During Period, Shares, Acquisitions	8,851,264		8,851,264