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	(850)	$ 1	$ (851)
Exercise of options (in shares)		44,387	24,044
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, 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		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		5,196,280