Consolidated Statements of Changes in Stockholders' Equity - USD ($)	Common Stock [Member] Common Class A [Member]	Common Stock [Member] Common Class B [Member]	Additional Paid-in Capital [Member]	AOCI Attributable to Parent [Member]	Retained Earnings [Member]	Common Class A [Member]	Total
Balance, shares (in shares) at Dec. 31, 2012	22,916,000	2,762,000
Balance at Dec. 31, 2012	$ 268,801,000	$ 343,000	$ 12,399,000	$ (2,615,000)	$ 149,173,000		$ 428,101,000
Net income					106,000,000		106,000,000
Gain on cash flow hedges, net of tax expense of				1,077,000			1,077,000
Issuance of stock in connection with employee stock plans (in shares)	283,000
Issuance of stock in connection with employee stock plans	$ 5,149,000						5,149,000
Stock Issued During Period, Shares, Restricted Stock Award, Gross	117,000
Repurchase of Class A common stock (in shares)	(187,000)					(127,900)
Repurchase of Class A common stock	$ (7,903,000)						(7,903,000)
Class B common stock converted to Class A common stock (in shares)	200,000	(200,000)
Class B common stock converted to Class A common stock	$ 24,000	$ (24,000)
Compensation for stock and stock option issuances and excess tax benefits from option exercises	$ 2,184,000		10,199,000				12,383,000
Dividends paid					(10,085,000)		(10,085,000)
Balance, shares (in shares) at Dec. 31, 2013	23,329,000	2,562,000
Balance at Dec. 31, 2013	$ 268,255,000	$ 319,000	22,598,000	(1,538,000)	245,088,000		$ 534,722,000
Issuance of stock in connection with acquisitions
Net income					138,720,000		$ 138,720,000
Gain on cash flow hedges, net of tax expense of				612,000			612,000
Issuance of stock in connection with employee stock plans (in shares)	118,000
Issuance of stock in connection with employee stock plans	$ 4,590,000						4,590,000
Stock Issued During Period, Shares, Restricted Stock Award, Gross	288,000
Repurchase of Class A common stock (in shares)	(333,000)					(226,729)
Repurchase of Class A common stock	$ (22,968,000)						(22,968,000)
Compensation for stock and stock option issuances and excess tax benefits from option exercises	$ 6,445,000		7,177,000				13,622,000
Dividends paid					(15,929,000)		(15,929,000)
Balance, shares (in shares) at Dec. 31, 2014	23,671,000	2,562,000
Balance at Dec. 31, 2014	$ 276,058,000	$ 319,000	29,775,000	(926,000)	367,879,000		673,105,000
Issuance of stock in connection with acquisitions (in shares)	269,000
Issuance of stock in connection with acquisitions	$ 19,736,000						19,736,000
Net income					182,999,000		182,999,000
Gain on cash flow hedges, net of tax expense of				649,000			649,000
Issuance of stock in connection with employee stock plans (in shares)	74,000
Issuance of stock in connection with employee stock plans	$ 6,065,000						6,065,000
Stock Issued During Period, Shares, Restricted Stock Award, Gross	217,000
Repurchase of Class A common stock (in shares)	(306,000)					(228,737)
Repurchase of Class A common stock	$ (31,548,000)						(31,548,000)
Class B common stock converted to Class A common stock (in shares)	20,000	(20,000)
Class B common stock converted to Class A common stock	$ 3,000	$ (3,000)
Compensation for stock and stock option issuances and excess tax benefits from option exercises	$ 7,832,000		9,047,000				16,879,000
Dividends paid					(19,985,000)		(19,985,000)
Balance, shares (in shares) at Dec. 31, 2015	23,676,000	2,542,000
Balance at Dec. 31, 2015	$ 258,410,000	$ 316,000	$ 38,822,000	$ (277,000)	$ 530,893,000		$ 828,164,000
Issuance of stock in connection with acquisitions