Shareholders' Equity (Narrative) (Details) $ / shares in Units, shares in Millions			12 Months Ended
	Oct. 15, 2015 USD ($)	Mar. 14, 2013 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) item $ / shares	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Class of Stock [Line Items]
Redemption/repurchase of preferred stock			$ 75,000,000
Non-cash distributions related to the redemption of preferred stock			2,487,000
Preferred Stock, Value, Outstanding			920,000,000	$ 995,000,000
Distributions paid to preferred shareholders			$ 59,398,000	60,488,000	$ 59,216,000
Number of quarterly dividends in arrearage before preferred shareholders can elect additional board members | item			6
Number of additional board members the preferred shareholders can elect in the case of an excess arrearage of quarterly dividends | item			2
Dividends in arrears			$ 0
Redeemable preferred stock, redemption price per share | $ / shares			$ 25.00
Preferred stock, aggregate deferred issuance costs outstanding			$ 29,300,000	$ 31,800,000
Series R [Member]
Class of Stock [Line Items]
Cumulative preferred stock, dividend rate	6.875%		6.875%	6.875%
Redemption/repurchase of preferred stock	$ 75,000,000
Issuance Date			Oct. 01, 2010	Oct. 01, 2010
Preferred Stock, Value, Outstanding				$ 75,000,000
Redemption date			Oct. 01, 2015	Oct. 01, 2015
Series V [Member]
Class of Stock [Line Items]
Cumulative preferred stock, dividend rate		5.70%	5.70%	5.70%
Issuance Date			Mar. 01, 2013	Mar. 01, 2013
Preferred Stock, Value, Outstanding		$ 110,000,000	$ 110,000,000	$ 110,000,000
Depositary shares issued | shares		4.4
Depository shares conversion ratio to preferred share		0.10%
Depositary shares issued price per share | $ / shares		$ 25.00
Redemption date			Mar. 01, 2018	Mar. 01, 2018