Common and Preferred Shares Preferred Shares (Details) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended	12 Months Ended	0 Months Ended	12 Months Ended		0 Months Ended	12 Months Ended		0 Months Ended	12 Months Ended		0 Months Ended	12 Months Ended
	Aug. 31, 2011 Series B Preferred Stock [Member]	Dec. 31, 2011 Series B Preferred Stock [Member]	Dec. 22, 2006 Series C Preferred Shares [Member]	Dec. 31, 2013 Series C Preferred Shares [Member]	Dec. 31, 2012 Series C Preferred Shares [Member]	May 25, 2007 Series D Preferred Stock [Member]	Dec. 31, 2012 Series D Preferred Stock [Member]	Nov. 05, 2012 Series D Preferred Stock [Member]	Apr. 02, 2008 Series E Preferred Shares [Member]	Dec. 31, 2013 Series E Preferred Shares [Member]	Dec. 31, 2012 Series E Preferred Shares [Member]	Oct. 12, 2012 Series F Preferred Stock [Member]	Dec. 31, 2013 Series F Preferred Stock [Member]	Dec. 31, 2012 Series F Preferred Stock [Member]
Schedule of Preferred Stock [Line Items]
Preferred Stock, Dividends, Per Share, Cash Paid							$ 2.0238						$ 1.67006
Preferred Stock, Shares Issued			5,400,000	5,400,000	5,400,000	4,600,000			3,500,000	3,450,000	3,450,000	5,000,000	5,000,000	0
Preferred share dividend percentage	7.75%		5.75%	5.75%		7.375%			9.00%	9.00%		6.625%
Proceeds from issuance of convertible preferred stock			$ 130.8						$ 83.4			$ 120.6
Preferred share dividend rate (in dollars per share)				$ 1.4375						$ 2.25		$ 1.65625
Redemption price	$ 25.32							$ 25.18
Per share liquidation preference	$ 25.00			$ 25				$ 25.00		$ 25		$ 25.00
Prorated quarterly dividend rate	$ 0.484375							$ 0.4609375
Aggregate reduction to income		$ 2.8					$ 3.9
Preferred shares conversion rate				$ 0.3655						$ 0.4546
Preferred shares conversion price				$ 68.40						$ 54.99
Common shares quarterly dividend per share threshold, minimum				$ 0.6875						$ 0.84
Common share closing price percent of preferred share prevailing conversion price, minimum				135.00%						150.00%
Preferred Shares declared dividends per share				$ 1.4375	$ 1.4375		$ 1.5628			$ 2.25	$ 2.25		$ 1.65625	$ 0.42787