Condensed Consolidated Statements of Changes in Equity (Unaudited) - USD ($) $ in Thousands		Retained Earnings [Member]	AOCI Attributable to Parent [Member]	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Parent [Member]	Noncontrolling Interest [Member]	Total
Beginning balance at Dec. 31, 2016		$ (676,867)	$ 5,766	$ 32	$ 4,250	$ 5,922,958	$ 5,256,139	$ 146,735	$ 5,402,874
Balance (in shares) at Dec. 31, 2016				32,000	425,034,000
Contributions/deemed contributions from noncontrolling interests								48,604	48,604
Comprehensive income:
Net income		220,149					220,149	12,740	232,889
Change in unrealized value related to available-for-sale securities			(1,619)				(1,619)		(1,619)
Change in unrealized value on interest rate swap			205				205		205
Change in foreign currency translation adjustment			1,721				1,721		1,721
Redeemable noncontrolling interests income								(1,109)	(1,109)
Dividends		(252,953)					(252,953)		(252,953)
Distributions to noncontrolling interests								(12,691)	(12,691)
Issuance of common stock					$ 8	(8)
Issuance of common stock (in shares)					776,000
Surrender of restricted common stock					$ (2)	(5,322)	(5,324)		(5,324)
Surrender of restricted common stock (in shares)					(224,000)
Exercise of common stock options						973	973		973
Exercise of common stock options (in shares)					51,000
Amortization of equity awards						12,032	12,032		12,032
Balance (in shares) at Jun. 30, 2017				32,000	425,637,000
Beginning balance at Dec. 31, 2016		(676,867)	5,766	$ 32	$ 4,250	5,922,958	5,256,139	146,735	5,402,874
Balance (in shares) at Dec. 31, 2016				32,000	425,034,000
Ending balance at Jun. 30, 2017		(709,671)	6,073	$ 32	$ 4,256	5,930,633	5,231,323	194,279	5,425,602
Beginning balance at Dec. 31, 2017		(761,337)	(1,480)	$ 41	$ 4,256	6,152,764	5,394,244	127,903	5,522,147
Balance (in shares) at Dec. 31, 2017				41,000	425,646,000
Comprehensive income:
Net income		309,476					309,476	315	309,791
Change in unrealized value related to available-for-sale securities
Change in unrealized value on interest rate swap			416				416		416
Redeemable noncontrolling interests income								(185)	(185)
Dividends		(266,082)					(266,082)		(266,082)
Distributions to noncontrolling interests								(1,870)	(1,870)
Issuance of common stock					$ 11	(11)
Issuance of common stock (in shares)					1,075,000
Surrender of restricted common stock					$ (2)	(3,490)	(3,492)		(3,492)
Surrender of restricted common stock (in shares)					(236,000)
Exercise of common stock options						30	30		30
Exercise of common stock options (in shares)					3,000
Amortization of equity awards						9,329	9,329		9,329
Balance (in shares) at Jun. 30, 2018				43,000	421,388,000
Beginning balance at Dec. 31, 2017		(761,337)	(1,480)	$ 41	$ 4,256	6,152,764	5,394,244	127,903	5,522,147
Balance (in shares) at Dec. 31, 2017				41,000	425,646,000
Adoption of accounting policy (Accounting Standards Update 2017-05 [Member]) at Dec. 31, 2017	[1]	8,098					8,098		8,098
Adoption of accounting policy (Accounting Standards Update 2016-01 [Member]) at Dec. 31, 2017	[1]	(1,136)	1,136
Balance at Dec. 31, 2017		(754,375)	(344)	$ 41	$ 4,256	6,152,764	5,402,342	127,903	5,530,245
Comprehensive income:
Repurchase of common stock					$ (51)	(75,075)	(75,126)		$ (75,126)
Repurchase of common stock (in shares)					(5,100,000)				(5,100,000)
Issuance of preferred stock				$ 2		33,112	33,114		$ 33,114
Issuance of preferred stock (in shares)				2,000
Acquisition/deconsolidation of noncontrolling interests						1,203	1,203	(48,395)	(47,192)
Ending balance at Jun. 30, 2018		$ (710,981)	$ 72	$ 43	$ 4,214	$ 6,117,862	$ 5,411,210	$ 77,768	$ 5,488,978

[1] Represents the impact of change in accounting principles for its respective Accounting Standard Updates ("ASU"). See Footnote 2 of the Notes to Condensed Consolidated Financial Statements for additional disclosure.