Income Taxes - Summary of Average Taxable Nature of Dividends (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Common [Member]
Income Tax Contingency [Line Items]
Total dividends per share, common	$ 0.67	$ 0.6	$ 0.525
Taxable percentage allocation on dividends	100.00%	100.00%	100.00%
Common [Member] | Ordinary Income [Member]
Income Tax Contingency [Line Items]
Taxable percentage allocation on dividends	49.44%	35.53%	95.68%
Common [Member] | Fifteen Percent Rate, Qualifying Dividend [Member]
Income Tax Contingency [Line Items]
Taxable percentage allocation on dividends	0.05%	4.11%	0.99%
Common [Member] | Fifteen Percent Rate Gain [Member]
Income Tax Contingency [Line Items]
Taxable percentage allocation on dividends	0.00%	2.09%	0.00%
Common [Member] | Return of Capital [Member]
Income Tax Contingency [Line Items]
Taxable percentage allocation on dividends	50.51%	58.27%	3.33%
Series B Cumulative Redeemable Preferred Shares [Member]
Income Tax Contingency [Line Items]
Total dividends per share, preferred	$ 0	$ 0	$ 1.341667
Taxable percentage allocation on dividends	0.00%	0.00%	100.00%
Series B Cumulative Redeemable Preferred Shares [Member] | Ordinary Income [Member]
Income Tax Contingency [Line Items]
Taxable percentage allocation on dividends	0.00%	0.00%	98.98%
Series B Cumulative Redeemable Preferred Shares [Member] | Fifteen Percent Rate, Qualifying Dividend [Member]
Income Tax Contingency [Line Items]
Taxable percentage allocation on dividends	0.00%	0.00%	1.02%
Series B Cumulative Redeemable Preferred Shares [Member] | Fifteen Percent Rate Gain [Member]
Income Tax Contingency [Line Items]
Taxable percentage allocation on dividends	0.00%	0.00%	0.00%
Series B Cumulative Redeemable Preferred Shares [Member] | Return of Capital [Member]
Income Tax Contingency [Line Items]
Taxable percentage allocation on dividends	0.00%	0.00%	0.00%
Series C Cumulative Redeemable Preferred Shares [Member]
Income Tax Contingency [Line Items]
Total dividends per share, preferred	$ 3.25	$ 3.25	$ 3.25
Taxable percentage allocation on dividends	100.00%	100.00%	100.00%
Series C Cumulative Redeemable Preferred Shares [Member] | Ordinary Income [Member]
Income Tax Contingency [Line Items]
Taxable percentage allocation on dividends	99.90%	85.14%	98.98%
Series C Cumulative Redeemable Preferred Shares [Member] | Fifteen Percent Rate, Qualifying Dividend [Member]
Income Tax Contingency [Line Items]
Taxable percentage allocation on dividends	0.10%	9.85%	1.02%
Series C Cumulative Redeemable Preferred Shares [Member] | Fifteen Percent Rate Gain [Member]
Income Tax Contingency [Line Items]
Taxable percentage allocation on dividends	0.00%	5.01%	0.00%
Series C Cumulative Redeemable Preferred Shares [Member] | Return of Capital [Member]
Income Tax Contingency [Line Items]
Taxable percentage allocation on dividends	0.00%	0.00%	0.00%
Series D Cumulative Redeemable Preferred Shares [Member]
Income Tax Contingency [Line Items]
Total dividends per share, preferred	$ 0	$ 1.043368	$ 1.8875
Taxable percentage allocation on dividends	0.00%	100.00%	100.00%
Series D Cumulative Redeemable Preferred Shares [Member] | Ordinary Income [Member]
Income Tax Contingency [Line Items]
Taxable percentage allocation on dividends	0.00%	85.14%	98.98%
Series D Cumulative Redeemable Preferred Shares [Member] | Fifteen Percent Rate, Qualifying Dividend [Member]
Income Tax Contingency [Line Items]
Taxable percentage allocation on dividends	0.00%	9.85%	1.02%
Series D Cumulative Redeemable Preferred Shares [Member] | Fifteen Percent Rate Gain [Member]
Income Tax Contingency [Line Items]
Taxable percentage allocation on dividends	0.00%	5.01%	0.00%
Series D Cumulative Redeemable Preferred Shares [Member] | Return of Capital [Member]
Income Tax Contingency [Line Items]
Taxable percentage allocation on dividends	0.00%	0.00%	0.00%