Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Preferred Stock	Common Stock	Capital in Excess of Par Value	Treasury Stock	Cumulative Net Income	Cumulative Dividends	Accumulated Other Comprehensive Income	Other Equity	Noncontrolling Interests
Balances at beginning of period at Dec. 31, 2014	$ 13,473,049	$ 1,006,250	$ 328,835	$ 14,740,712	$ (35,241)	$ 2,842,022	$ (5,635,923)	$ (77,009)	$ 5,507	$ 297,896
Comprehensive income:
Net income (loss)	888,628					883,750				4,878
Other comprehensive income	(47,199)							(11,234)		(35,965)
Total comprehensive Income	841,429
Increase in noncontrolling interests	295,439									318,516
Decrease in noncontrolling interests				(23,077)
Amounts related to issuance of common stock from dividend reinvestment and stock incentive plans, net of forfeitures	13,941		126	25,053	(9,131)				(2,107)
Net proceeds from sale of common stock	1,754,701		24,520	1,730,181
Equity component of convertible debt	6,761		1,330	5,431
Option compensation expense	698								698
Cash dividends paid:
Common stock cash dividends	(1,144,727)						(1,144,727)
Preferred stock cash dividends	(65,406)						(65,406)
Balances at end of period at Dec. 31, 2015	15,175,885	1,006,250	354,811	16,478,300	(44,372)	3,725,772	(6,846,056)	(88,243)	4,098	585,325
Comprehensive income:
Net income (loss)	1,087,080					1,077,803				9,277
Other comprehensive income	(78,833)							(81,288)		2,455
Total comprehensive Income	1,008,247
Decrease in noncontrolling interests	(173,456)			(51,478)						(121,978)
Amounts related to issuance of common stock from dividend reinvestment and stock incentive plans, net of forfeitures	36,103		839	46,938	(10,369)				(1,305)
Net proceeds from sale of common stock	533,352		7,421	525,931
Option compensation expense	266								266
Cash dividends paid:
Common stock cash dividends	(1,233,519)						(1,233,519)
Preferred stock cash dividends	(65,406)						(65,406)
Balances at end of period at Dec. 31, 2016	15,281,472	1,006,250	363,071	16,999,691	(54,741)	4,803,575	(8,144,981)	(169,531)	3,059	475,079
Comprehensive income:
Net income (loss)	543,593					522,774				20,819
Other comprehensive income	80,414							58,066		22,348
Total comprehensive Income	624,007
Increase in noncontrolling interests				13,473
Decrease in noncontrolling interests	(2,468)									(15,941)
Amounts related to issuance of common stock from dividend reinvestment and stock incentive plans, net of forfeitures	9,690		402	21,494	(9,807)				(2,399)
Net proceeds from sale of common stock	621,436		8,881	612,555
Redemption of equity membership units	5,545		91	5,465	(11)
Redemption of preferred stock	(287,509)	(287,500)		9,760		(9,769)
Conversion of preferred stock	0	(247)	4	243
Option compensation expense	10								10
Cash dividends paid:
Common stock cash dividends	(1,277,321)						(1,277,321)
Preferred stock cash dividends	(49,410)						(49,410)
Balances at end of period at Dec. 31, 2017	$ 14,925,452	$ 718,503	$ 372,449	$ 17,662,681	$ (64,559)	$ 5,316,580	$ (9,471,712)	$ (111,465)	$ 670	$ 502,305