Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Retained Earnings [Member]	Treasury Stock Value [Member]	Accumulated Other Comprehensive (Loss) Income, Net of Tax [Member]	Total Shareholders' Equity [Member]	Nonredeemable Noncontrolling Interest in Subsidiary [Member]	Redeemable Noncontrolling Interest in Subsidiaries / Temporary Equity [Member]	Treasury Stock [Member]	Class A [Member]	Class B [Member]
Common stock, shares outstanding										9,000	104,918,647
Treasury Stock, Common, Shares									4,586,809
Stock award activity, shares											871,837
Stock award activity, treasury shares reissued									(871,837)
Repurchase of class B shares											(1,696,457)
Purchase of treasury stock, shares									1,696,457
Balance at Dec. 31, 2014	$ 609,652	$ 271,020	$ 505,394	$ (165,258)	$ (1,662)	$ 609,494	$ 158
Balance - Temporary Equity at Dec. 31, 2014								$ 3,697
Net income	173,249		169,807			169,807	3,442	(1,263)
Other comprehensive income (loss), net of tax	(2,947)				(2,947)	(2,947)		0
Subscriptions – redeemable noncontrolling interest holders								16,409
Consolidation/(deconsolidation)								(6,867)
Stock award activity	29,111	27,559	(24,810)	26,362		29,111
Dividends declared	(104,606)		(104,606)			(104,606)
Distributions to noncontrolling interest in subsidiaries	(2,444)						(2,444)	(3,242)
Purchase of treasury stock	(53,043)			(53,043)		(53,043)
Balance at Dec. 31, 2015	648,972	298,579	545,785	(191,939)	(4,609)	647,816	1,156
Balance - Temporary Equity at Dec. 31, 2015								8,734
Common stock, shares outstanding										9,000	104,094,027
Treasury Stock, Common, Shares									5,411,429
Adoption of new accounting pronouncements | Adjustments for New Accounting Pronouncement [Member]	43	123	(911)		831	43		14,850
Stock award activity, shares											948,860
Stock award activity, treasury shares reissued									(948,860)
Repurchase of class B shares											(3,053,204)
Purchase of treasury stock, shares									3,053,204
Net income	218,312		208,919			208,919	9,393	3,202
Other comprehensive income (loss), net of tax	3,255				3,255	3,255		(13)
Subscriptions – redeemable noncontrolling interest holders								17,868
Consolidation/(deconsolidation)								(4,579)
Stock award activity	23,715	22,280	(18,715)	20,150		23,715
Dividends declared	(205,329)		(205,329)			(205,329)
Distributions to noncontrolling interest in subsidiaries	(9,591)						(9,591)	(8,700)
Purchase of treasury stock	(83,593)			(83,593)		(83,593)
Balance at Dec. 31, 2016	$ 595,784	320,982	529,749	(255,382)	(523)	594,826	958
Balance - Temporary Equity at Dec. 31, 2016								31,362
Common stock, shares outstanding										9,000	101,989,683
Treasury Stock, Common, Shares	7,515,773								7,515,773
Stock award activity, shares											952,570
Stock award activity, treasury shares reissued									(952,570)
Repurchase of class B shares											(1,841,800)
Purchase of treasury stock, shares									1,841,800
Net income	$ 291,817		291,341			291,341	476	3,084
Other comprehensive income (loss), net of tax	(267)				(267)	(267)		0
Subscriptions – redeemable noncontrolling interest holders								4,687
Consolidation/(deconsolidation)								(67)
Stock award activity	23,695	22,396	(22,308)	23,607		23,695
Dividends declared	(101,423)		(101,423)			(101,423)
Distributions to noncontrolling interest in subsidiaries	(1,434)						(1,434)	(8,903)
Purchase of treasury stock	(46,957)			(46,957)		(46,957)
Balance at Dec. 31, 2017	$ 761,215	$ 343,378	$ 697,359	$ (278,732)	$ (790)	$ 761,215	$ 0
Balance - Temporary Equity at Dec. 31, 2017								$ 30,163
Common stock, shares outstanding										9,000	101,100,453
Treasury Stock, Common, Shares	8,405,003								8,405,003