Common and Preferred Shares Common Shares (Details) - USD ($) $ / shares in Units, $ in Thousands		2 Months Ended	12 Months Ended
	Jan. 21, 2016	Feb. 27, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Jan. 01, 2017
Cash Dividends Paid [Line Items]
Common Stock, Shares, Issued	2,250,000		66,263,487	63,195,182
Net proceeds from issuance of common shares	$ 125,000		$ 142,628	$ 190,158	$ 264,158
Common Stock [Member]
Cash Dividends Paid [Line Items]
Common stock declared dividends per share			$ 3.84	$ 3.63
Total Cash Distribution Per Share [Member] | Common Stock [Member]
Cash Dividends Paid [Line Items]
Common Stock, Dividends, Per Share, Cash Paid			3.8225	3.6125
Taxable Ordinary Income [Member] | Common Stock [Member]
Cash Dividends Paid [Line Items]
Common Stock, Dividends, Per Share, Cash Paid			3.1659	3.0674
Return of Capital [Member] | Common Stock [Member]
Cash Dividends Paid [Line Items]
Common Stock, Dividends, Per Share, Cash Paid			0.2489	0.5451
Long-term Capital Gain [Member] | Common Stock [Member]
Cash Dividends Paid [Line Items]
Unrecaptured Section 1250 gain			0.1060
Common Stock, Dividends, Per Share, Cash Paid			$ 0.4077	$ 0.0000
direct share purchase plan [Member]
Cash Dividends Paid [Line Items]
Common Stock, Shares, Issued			258,263	3,530,058		548,288
Net proceeds from issuance of common shares			$ 16,900	$ 190,300
Subsequent Event [Member] | direct share purchase plan [Member]
Cash Dividends Paid [Line Items]
Net proceeds from issuance of common shares		$ 40,800