CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Thousands	Total	Common Stock	Treasury	Additional Paid-In Capital	Retained Earnings	Noncontrolling Interest Non Redeemable
Beginning balance, Amount at Dec. 31, 2018	$ 1,424,474	$ 358	$ (49,373)	$ 937,965	$ 427,975	$ 107,549
Beginning balance (in shares) at Dec. 31, 2018		35,835,348
Beginning balance, treasury (in shares) at Dec. 31, 2018			5,029,429
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	20,542				18,856	1,686
Acquired noncontrolling interest	820					820
Noncontrolling interest distributions	(6,799)					(6,799)
Purchase of additional controlling interest	(18,000)					(18,000)
Nonvested stock compensation	1,804			1,804
Issuance of vested stock	2	$ 2
Issuance of vested stock (in shares)		174,562
Treasury shares redeemed to pay income tax	(6,841)		$ (6,726)	(115)
Treasury shares redeemed to pay income tax (in shares)			61,465
Exercise of options (in shares)		44,387	24,044
Exercise of options	(850)	$ 1	$ (851)
Issuance of common stock under Employee Stock Purchase Plan (in shares)		5,357
Issuance of common stock under Employee Stock Purchase Plan	478			478
Ending balance, Amount at Mar. 31, 2019	1,415,630	$ 361	$ (56,950)	940,132	446,831	85,256
Ending balance (in shares) at Mar. 31, 2019		36,059,654
Ending balance, treasury (in shares) at Mar. 31, 2019			5,114,938
Beginning balance, Amount at Dec. 31, 2018	1,424,474	$ 358	$ (49,373)	937,965	427,975	107,549
Beginning balance (in shares) at Dec. 31, 2018		35,835,348
Beginning balance, treasury (in shares) at Dec. 31, 2018			5,029,429
Ending balance, Amount at Jun. 30, 2019	1,448,048	$ 361	$ (57,892)	941,920	471,831	91,828
Ending balance (in shares) at Jun. 30, 2019		36,084,393
Ending balance, treasury (in shares) at Jun. 30, 2019			5,124,003
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to noncontrolling interest-redeemable	5,900
Beginning balance, Amount at Mar. 31, 2019	1,415,630	$ 361	$ (56,950)	940,132	446,831	85,256
Beginning balance (in shares) at Mar. 31, 2019		36,059,654
Beginning balance, treasury (in shares) at Mar. 31, 2019			5,114,938
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	26,897				25,000	1,897
Acquired noncontrolling interest	6,170					6,170
Noncontrolling interest distributions	(2,026)					(2,026)
Purchase of additional controlling interest	(752)			(1,283)		531
Nonvested stock compensation	2,588			2,588
Issuance of vested stock	0
Issuance of vested stock (in shares)		17,145
Treasury shares redeemed to pay income tax	(700)		$ (730)	30
Treasury shares redeemed to pay income tax (in shares)			7,199
Exercise of options (in shares)		3,293	1,866
Exercise of options	(212)		$ (212)
Issuance of common stock under Employee Stock Purchase Plan (in shares)		4,301
Issuance of common stock under Employee Stock Purchase Plan	453			453
Ending balance, Amount at Jun. 30, 2019	1,448,048	$ 361	$ (57,892)	941,920	471,831	91,828
Ending balance (in shares) at Jun. 30, 2019		36,084,393
Ending balance, treasury (in shares) at Jun. 30, 2019			5,124,003
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to noncontrolling interest-redeemable	3,000
Beginning balance, Amount at Dec. 31, 2019	$ 1,507,251	$ 361	$ (60,060)	949,321	523,701	93,928
Beginning balance (in shares) at Dec. 31, 2019		36,129,280
Beginning balance, treasury (in shares) at Dec. 31, 2019	5,136,890		5,136,890
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	$ 24,123				22,024	2,099
Acquired noncontrolling interest	2,880					2,880
Noncontrolling interest distributions	(2,093)					(2,093)
Purchase of additional controlling interest	(23,575)			(2,470)		(21,105)
Nonvested stock compensation	3,680			3,680
Issuance of vested stock	2	$ 2
Issuance of vested stock (in shares)		163,163
Treasury shares redeemed to pay income tax	$ (6,933)		$ (7,122)	189
Treasury shares redeemed to pay income tax (in shares)			59,390
Issuance of common stock under Employee Stock Purchase Plan (in shares)	4,663	4,663
Issuance of common stock under Employee Stock Purchase Plan	$ 610			610
Ending balance, Amount at Mar. 31, 2020	1,505,945	$ 363	$ (67,182)	951,330	545,725	75,709
Ending balance (in shares) at Mar. 31, 2020		36,297,106
Ending balance, treasury (in shares) at Mar. 31, 2020			5,196,280
Beginning balance, Amount at Dec. 31, 2019	$ 1,507,251	$ 361	$ (60,060)	949,321	523,701	93,928
Beginning balance (in shares) at Dec. 31, 2019		36,129,280
Beginning balance, treasury (in shares) at Dec. 31, 2019	5,136,890		5,136,890
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Treasury shares redeemed to pay income tax (in shares)	65,646
Ending balance, Amount at Jun. 30, 2020	$ 1,558,103	$ 363	$ (68,654)	955,119	590,417	80,858
Ending balance (in shares) at Jun. 30, 2020		36,327,819
Ending balance, treasury (in shares) at Jun. 30, 2020	5,205,851		5,205,851
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to noncontrolling interest-redeemable	$ 9,604
Beginning balance, Amount at Mar. 31, 2020	1,505,945	$ 363	$ (67,182)	951,330	545,725	75,709
Beginning balance (in shares) at Mar. 31, 2020		36,297,106
Beginning balance, treasury (in shares) at Mar. 31, 2020			5,196,280
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	51,750				44,692	7,058
Noncontrolling interest distributions	(1,909)					(1,909)
Nonvested stock compensation	3,263			3,263
Issuance of vested stock	0	$ 0
Issuance of vested stock (in shares)		19,846
Treasury shares redeemed to pay income tax	(1,139)		$ (950)	(189)
Treasury shares redeemed to pay income tax (in shares)			6,256
Exercise of options (in shares)		7,137	3,315
Exercise of options	$ (304)		$ (522)	218
Issuance of common stock under Employee Stock Purchase Plan (in shares)	3,730	3,730
Issuance of common stock under Employee Stock Purchase Plan	$ 497			497
Ending balance, Amount at Jun. 30, 2020	$ 1,558,103	$ 363	$ (68,654)	$ 955,119	$ 590,417	$ 80,858
Ending balance (in shares) at Jun. 30, 2020		36,327,819
Ending balance, treasury (in shares) at Jun. 30, 2020	5,205,851		5,205,851
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income attributable to noncontrolling interest-redeemable	$ 3,500