CONSOLIDATED STATEMENT OF EQUITY - USD ($) $ in Thousands	Total	Additional Paid-In Capital	Accumulated Deficit	Accumulated Other Comprehensive Loss	Noncontrolling Interests	Class A Common Stock	Class A Common Stock Common Stock	Class B Common Stock	Class B Common Stock Common Stock
BALANCE (shares) at Dec. 31, 2016							64,558,207		25,670,684
BALANCE at Dec. 31, 2016	$ 557,936	$ 843,691	$ (255,804)	$ (807)	$ (30,047)		$ 646		$ 257
Increase (Decrease) in Stockholders' Equity
Issuance of common stock, net of issuance costs (shares)							12,000,000
Issuance of common stock, net of issuance costs	487,883	487,763					$ 120
Dividends declared and paid on Class A and Class B Common Stock	(53,049)		(53,049)
Repurchases of Class A Common Stock (in shares)							(997,300)
Repurchases of Class A Common Stock	(30,287)	30,277					$ (10)
Class A Common Stock issued pursuant to employee benefit plans (shares)							471,617
Class A Common Stock issued pursuant to employee benefit plans	16,982	16,978					$ 4
Distributions to noncontrolling interests, net	(20,469)				(20,469)
Net income	149,303		132,483		16,820
BALANCE (shares) at Sep. 30, 2017							76,032,524		25,670,684
BALANCE at Sep. 30, 2017	1,108,299	1,318,155	(176,370)	(807)	(33,696)		$ 760		$ 257
BALANCE (shares) at Jun. 30, 2017							76,993,826		25,670,684
BALANCE at Jun. 30, 2017	1,124,972	1,346,657	(188,701)	(807)	(33,204)		$ 770		$ 257
Increase (Decrease) in Stockholders' Equity
Dividends declared and paid on Class A and Class B Common Stock	(18,306)		(18,306)
Repurchases of Class A Common Stock (in shares)							(997,300)
Repurchases of Class A Common Stock	(30,287)	(30,277)					$ (10)
Class A Common Stock issued pursuant to employee benefit plans (shares)							35,998
Class A Common Stock issued pursuant to employee benefit plans	1,775	1,775
Distributions to noncontrolling interests, net	(2,421)				(2,421)
Net income	32,566		30,637		1,929
BALANCE (shares) at Sep. 30, 2017							76,032,524		25,670,684
BALANCE at Sep. 30, 2017	1,108,299	1,318,155	(176,370)	(807)	(33,696)		$ 760		$ 257
BALANCE (shares) at Dec. 31, 2017						76,071,145	76,071,145	25,670,684	25,670,684
BALANCE at Dec. 31, 2017	1,534,366	1,320,298	248,845	(1,423)	(34,372)		$ 761		$ 257
Cumulative effect of adoption of new accounting standard at Dec. 31, 2017	2,100		2,100
Increase (Decrease) in Stockholders' Equity
Dividends declared and paid on Class A and Class B Common Stock	(55,205)		(55,205)
Repurchases of Class A Common Stock (in shares)							(1,636,019)
Repurchases of Class A Common Stock	(45,904)	(45,888)					$ (16)
Class A Common Stock issued pursuant to employee benefit plans (shares)							543,298
Class A Common Stock issued pursuant to employee benefit plans	18,764	18,759					$ 5
Distributions to noncontrolling interests, net	(6,871)				(6,871)
Net income	138,304		135,040		3,264
BALANCE (shares) at Sep. 30, 2018						74,978,424	74,978,424	25,670,684	25,670,684
BALANCE at Sep. 30, 2018	1,585,554	1,293,169	330,780	(1,423)	(37,979)		$ 750		$ 257
BALANCE (shares) at Jun. 30, 2018							76,576,980		25,670,684
BALANCE at Jun. 30, 2018	1,585,418	1,336,251	285,316	(1,423)	(35,749)		$ 766		$ 257
Increase (Decrease) in Stockholders' Equity
Dividends declared and paid on Class A and Class B Common Stock	(18,411)		(18,411)
Repurchases of Class A Common Stock (in shares)							(1,636,019)
Repurchases of Class A Common Stock	(45,904)	(45,888)					$ (16)
Class A Common Stock issued pursuant to employee benefit plans (shares)							37,463
Class A Common Stock issued pursuant to employee benefit plans	2,806	2,806
Distributions to noncontrolling interests, net	(3,355)				(3,355)
Net income	65,000		63,875		1,125
BALANCE (shares) at Sep. 30, 2018						74,978,424	74,978,424	25,670,684	25,670,684
BALANCE at Sep. 30, 2018	$ 1,585,554	$ 1,293,169	$ 330,780	$ (1,423)	$ (37,979)		$ 750		$ 257