CONSOLIDATED BALANCE SHEETS (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Preferred shares, shares authorized	100,000,000	100,000,000
Common shares, par value (in dollars per share)	$ 0.01	$ 0.01
Common shares, shares authorized	200,000,000	200,000,000
Common shares, shares issued	123,637,424	123,521,535
Common shares, shares outstanding	123,637,424	123,521,535
Series B
Preferred shares, dividend yield (as a percent)	8.875%	8.875%
Preferred shares, shares issued	0	3,450,000
Preferred shares, shares outstanding	0	3,450,000
Preferred shares, aggregate liquidation preference (in dollars)	$ 0	$ 86,250
Series C
Preferred shares, dividend yield (as a percent)	7.00%	7.00%
Preferred shares, shares issued	6,700,000	12,700,000
Preferred shares, shares outstanding	6,700,000	12,700,000
Preferred shares, aggregate liquidation preference (in dollars)	167,500	317,500
Series D
Preferred shares, dividend yield (as a percent)	7.125%	7.125%
Preferred shares, shares issued	11,600,000	0
Preferred shares, shares outstanding	11,600,000	0
Preferred shares, aggregate liquidation preference (in dollars)	$ 290,000	$ 0