Shareholders' Equity - Preferred Units (Details) - USD ($)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Preferred Units Description and Other Disclosures [Abstract]
Preferred Stock Redemption Premium	$ 1,236,000
Preferred Units, Total Annual Cash Distributions	1,600,000.0
Authorized but Unissued Preferred Shares		16,013,000
Preferred Units, Outstanding		0	0
Series E Preferred Stock [Member]
Preferred Units Description and Other Disclosures [Abstract]
Stock Redeemed or Called During Period, Value	$ 20,000,000
Preferred Stock Dividends Rate Percentage Or Per Dollar Amount	7.00%
Preferred Units, Annual Cash Distributions	$ 1.21
Series F Preferred Stock [Member]
Preferred Units Description and Other Disclosures [Abstract]
Stock Redeemed or Called During Period, Value	$ 17,500,000
Preferred Stock Dividends Rate Percentage Or Per Dollar Amount	6.65%
Preferred Units, Annual Cash Distributions	$ 1.32
Series G Preferred Stock [Member]
Preferred Units Description and Other Disclosures [Abstract]
Stock Redeemed or Called During Period, Value	$ 27,000,000
Preferred Stock Dividends Rate Percentage Or Per Dollar Amount	6.70%
Preferred Units, Annual Cash Distributions	$ 1.33