Shareholders' Equity (Details 3) (USD $) In Thousands, except Share data, unless otherwise specified			12 Months Ended		1 Months Ended
	Dec. 31, 2011 Series C	Dec. 31, 2010 Series C	Dec. 31, 2011 Series D	Dec. 31, 2010 Series D	Jun. 30, 2011 Series E	Dec. 31, 2011 Series E	Dec. 31, 2010 Series E
Shareholders' Equity
Number of preferred shares outstanding	6,000,000	6,000,000	15,180,000	15,180,000		11,000,000	0
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
Cumulative redeemable preferred shares issued	6,000,000	6,000,000	15,180,000	15,180,000	11,000,000	11,000,000	0
Proceeds from cumulative redeemable preferred shares					$ 265,391
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%