Consolidated Statements of Equity (Unaudited) (USD $) In Thousands, unless otherwise specified	Total	Preferred Stock	Common Stock	Capital in Excess of Par Value	Treasury Stock	Cumulative Net Income	Cumulative Dividends	Accumulated Other Comprehensive Income (Loss)	Other Equity	Noncontrolling Interests
Balances at beginning of period at Dec. 31, 2010	$ 4,733,100	$ 291,667	$ 147,155	$ 4,932,468	$ (11,352)	$ 1,676,196	$ (2,427,881)	$ (11,099)	$ 5,697	$ 130,249
Comprehensive income:
Net income (loss)	31,802					32,052				(250)
Other comprehensive income:
Unrealized gain (loss) on equity investments	322							322
Cash flow hedge activity	482							482
Total comprehensive income	32,606
Contributions by noncontrolling interests	33,503			6,017						27,486
Distributions to noncontrolling interests	(7,023)									(7,023)
Amounts related to issuance of common stock from dividend reinvestment and stock incentive plans, net of forfeitures	32,663		658	34,486	(2,128)				(353)
Proceeds from issuance of common stock	1,358,694		28,750	1,329,944
Proceeds from issuance of preferred stock	696,741	718,750		(22,009)
Preferred stock redeemed, amount	0	0
Conversion of preferred stock	0	0	0	0
Option compensation expense	1,039								1,039
Cash dividends paid:
Common stock cash dividends	(102,040)						(102,040)
Preferred stock cash dividends	(8,680)						(8,680)
Balances at end of period at Mar. 31, 2011	6,770,603	1,010,417	176,563	6,280,906	(13,480)	1,708,248	(2,538,601)	(10,295)	6,383	150,462
Balances at beginning of period at Dec. 31, 2011	7,278,647	1,010,417	192,299	7,019,714	(13,535)	1,893,806	(2,972,129)	(11,928)	6,120	153,883
Comprehensive income:
Net income (loss)	57,836					58,514				(678)
Other comprehensive income:
Unrealized gain (loss) on equity investments	8							8
Cash flow hedge activity	278							278
Total comprehensive income	58,122
Contributions by noncontrolling interests	8,101			874						7,227
Distributions to noncontrolling interests	(2,799)									(2,799)
Amounts related to issuance of common stock from dividend reinvestment and stock incentive plans, net of forfeitures	32,052		530	35,546	(3,730)				(294)
Proceeds from issuance of common stock	1,062,738		20,700	1,042,038
Proceeds from issuance of preferred stock	277,901	287,500		(9,599)
Preferred stock redeemed, amount	0
Conversion of preferred stock	0
Equity component of convertible debt	0
Option compensation expense	1,382								1,382
Cash dividends paid:
Common stock cash dividends	(142,919)						(142,919)
Preferred stock cash dividends	(19,207)						(19,207)
Balances at end of period at Mar. 31, 2012	$ 8,554,018	$ 1,297,917	$ 213,529	$ 8,088,573	$ (17,265)	$ 1,952,320	$ (3,134,255)	$ (11,642)	$ 7,208	$ 157,633