Shareholders' Equity (Narrative) (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	1 Months Ended		2 Months Ended	12 Months Ended		2 Months Ended	12 Months Ended	0 Months Ended	12 Months Ended	2 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended	1 Months Ended	12 Months Ended	0 Months Ended	12 Months Ended		12 Months Ended		0 Months Ended	12 Months Ended	0 Months Ended	12 Months Ended	1 Months Ended
	Jul. 31, 2011	Mar. 31, 2009	May 31, 2011	Dec. 31, 2011	Dec. 31, 2010	May 31, 2011 Series Q [Member]	Dec. 31, 2011 Series Q [Member]	Apr. 13, 2010 Series O [Member]	Dec. 31, 2011 Series O [Member]	Jun. 30, 2011 Series I [Member]	Dec. 31, 2011 Series I [Member]	Jul. 31, 2011 Series R [Member]	Dec. 31, 2011 Series R [Member]	Aug. 31, 2011 Series K [Member]	Dec. 31, 2011 Series K [Member]	Sep. 30, 2011 Series G [Member]	Dec. 31, 2011 Series G [Member]	Nov. 28, 2011 Series H [Member]	Dec. 31, 2011 Series H [Member]	Nov. 05, 2010 Series B [Member]	Dec. 31, 2011 Series C [Member]	Mar. 31, 2009 Series C [Member]	Dec. 31, 2011 Series F [Member]	Mar. 31, 2009 Series F [Member]	Oct. 07, 2010 Series P [Member]	Dec. 31, 2011 Series P [Member]	Aug. 03, 2010 Series Y [Member]	Dec. 31, 2011 Series Y [Member]	May 18, 2010 Series V [Member]	Mar. 31, 2009 Series V [Member]
Class of Stock [Line Items]
Redeemable preferred stock redemption price per share				$ 25.00
Income reclassified from common to preferred										$ 15,900,000				$ 13,100,000		$ 3,100,000		$ 3,500,000		$ 3,600,000							$ 800,000		$ 5,100,000
Dividend Rate %						6.50%	6.50%	6.875%	6.875%		7.25%	6.35%	6.35%		7.25%		7.00%		6.95%		6.60%		6.45%		6.50%	6.50%	6.85%	6.85%
Proceeds from issuance of preferred stock				835,627,000	261,103,000	375,000,000		145,000,000				487,500,000													125,000,000
Depositary shares repurchased, value																						2,700,000		1,600,000						13,200,000
Depositary shares repurchased, shares																						0.2		0.1						0.7
Number of stock issued in sale						15.0		5.8				19.5													5.0
Redemption of cumulative preferred shares										517,500,000				424,800,000		100,000,000		105,000,000		108,800,000									155,000,000
Original issuance costs on preferred shares redeemed during the period	15,600,000		11,300,000					4,800,000																	4,100,000
Repurchase of preferred stock																											0.4
Differential between carrying amount and amount paid for preferred shares redeemed during the period		6,200,000
Aggregate repurchase cost of preferred stock																											$ 9,200,000