Shareholders' Equity (Details 3) (USD $)	12 Months Ended			12 Months Ended
	Dec. 31, 2011 Series C	Dec. 31, 2012 Series C	Aug. 31, 2012 Series C	Dec. 31, 2012 Series D	Dec. 31, 2011 Series D	Dec. 31, 2012 Series E	Dec. 31, 2011 Series E
Shareholders' Equity
Number of Preferred shares redeemed	6,000,000
Preferred shares outstanding	6,000,000	0		15,180,000	15,180,000	11,000,000	11,000,000
Preferred shares dividend (in dollars per share)	$ 1.78125			$ 1.625		$ 1.8125
Preferred shares, dividend yield (as a percent)	7.125%		7.125%	6.50%	6.50%	7.25%	7.25%
Liquidation preference (in dollars per share)	$ 25.00			$ 25.00		$ 25.00
Redemption price per share (in dollars per share)			$ 25.00			$ 25.00
Initial conversion rate				0.480775
Initial conversion price per share (in dollars per share)				$ 52.00
Additional common shares				7,298,165
Percentage of market price of the entity's common stock by which liquidation preference plus accrued and unpaid distributions are divided				98.00%
Purchase price as a percentage of liquidation preference				100.00%