Common and Preferred Shares Preferred Shares (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 21, 2017
Schedule of Preferred Stock [Line Items]
Proceeds from Issuance of Preferred Stock and Preference Stock	$ 144,490	$ 0	$ 0
Preferred Share Redemption Costs	$ 4,457	$ 0	$ 0
Series C Preferred Shares [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Shares Issued	5,399,050	5,399,050
Preferred share dividend percentage	5.75%
Preferred share dividend rate (in dollars per share)	$ 1.4375
Per share liquidation preference	25
Preferred shares conversion rate	0.3857
Preferred shares conversion price	64.82
Common shares quarterly dividend per share threshold, minimum	$ 0.6875
Common share closing price percent of preferred share prevailing conversion price, minimum	135.00%
Preferred Shares declared dividends per share	$ 1.4375	$ 1.4375
Preferred Shares, Convertible, Conversion Adjustment	$ 0.4918	$ 0.4394
Series E Preferred Shares [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Shares Issued	3,449,115	3,450,000
Preferred share dividend percentage	9.00%
Preferred share dividend rate (in dollars per share)	$ 2.25
Per share liquidation preference	25
Preferred shares conversion rate	0.4616
Preferred shares conversion price	54.16
Common shares quarterly dividend per share threshold, minimum	$ 0.84
Common share closing price percent of preferred share prevailing conversion price, minimum	150.00%
Preferred Shares declared dividends per share	$ 2.25	$ 2.25
Preferred Shares, Convertible, Conversion Adjustment	$ 0.2619	$ 0.2139
Series F Preferred Stock [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Shares Issued	0	5,000,000		5,000,000
Preferred share dividend percentage	6.625%
Preferred Stock, Redemption Price Per Share	$ 25
Per share liquidation preference	$ 25.00
Preferred Stock, Redemption Amount	$ 126,500
Preferred Shares declared dividends per share	$ 1.54123	$ 1.65625
Preferred Stock, Dividends, Per Share, Cash Paid	$ 0.299045
Preferred Share Redemption Costs	$ 4,500
Series G Preferred Stock [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Shares Issued	6,000,000	0
Preferred share dividend percentage	5.75%
Proceeds from Issuance of Preferred Stock and Preference Stock	$ 144,500
Preferred share dividend rate (in dollars per share)	$ 1.44
Per share liquidation preference	25
Preferred Shares declared dividends per share	0.18368
Taxable Ordinary Income [Member] | Series C Preferred Shares [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Dividends, Per Share, Non-cash Distributions	0.3527	$ 0.2850
Preferred Stock, Dividends, Per Share, Cash Paid	1.3462	1.2735
Taxable Ordinary Income [Member] | Series E Preferred Shares [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Dividends, Per Share, Non-cash Distributions	0.1428	0.0883
Preferred Stock, Dividends, Per Share, Cash Paid	2.1070	1.9933
Taxable Ordinary Income [Member] | Series F Preferred Stock [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid	1.8310	1.4673
Return of Capital [Member] | Series C Preferred Shares [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Dividends, Per Share, Non-cash Distributions	0.1152	0.1177
Preferred Stock, Dividends, Per Share, Cash Paid	0.0000	0.0000
Return of Capital [Member] | Series E Preferred Shares [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Dividends, Per Share, Non-cash Distributions	0.1094	0.1142
Preferred Stock, Dividends, Per Share, Cash Paid	0.0000	0.0000
Return of Capital [Member] | Series F Preferred Stock [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid	0.0000	0.0000
Long-term Capital Gain [Member] | Series C Preferred Shares [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Dividends, Per Share, Non-cash Distributions	0.0239	0.0367
Unrecaptured Section 1250 gain	0.0352	0.0426
Dividends, Per Share, Non-cash Distributions, Unrecaptured Section 1250 Gain	0.0092	0.0095
Preferred Stock, Dividends, Per Share, Cash Paid	0.0913	0.1640
Long-term Capital Gain [Member] | Series E Preferred Shares [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Dividends, Per Share, Non-cash Distributions	0.0097	0.0114
Unrecaptured Section 1250 gain	0.0551	0.0668
Dividends, Per Share, Non-cash Distributions, Unrecaptured Section 1250 Gain	0.0037	0.0030
Preferred Stock, Dividends, Per Share, Cash Paid	0.1430	0.2567
Long-term Capital Gain [Member] | Series F Preferred Stock [Member]
Schedule of Preferred Stock [Line Items]
Unrecaptured Section 1250 gain	0.0479	0.0491
Preferred Stock, Dividends, Per Share, Cash Paid	0.1243	0.1889
Total Cash Distribution Per Share [Member] | Series C Preferred Shares [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid	1.4375	1.4375
Total Cash Distribution Per Share [Member] | Series E Preferred Shares [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid	2.2500	2.2500
Total Cash Distribution Per Share [Member] | Series F Preferred Stock [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid	1.9553	1.6562
Total non-cash Distributions Per Share [Domain] | Series C Preferred Shares [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Dividends, Per Share, Non-cash Distributions	0.4918	0.4394
Total non-cash Distributions Per Share [Domain] | Series E Preferred Shares [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Dividends, Per Share, Non-cash Distributions	$ 0.2619	$ 0.2139