Fair Value Measurement (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2011
Financial liabilities measured at fair value on a recurring basis
Total liabilities at fair value	$ 22,429		$ 22,429		$ 20,907
Income approach valuation technique [Member]
Financial liabilities measured at fair value on a recurring basis
Contingent Liability, Fair Value Disclosure	22,429		22,429		20,907
Level 1 [Member]
Financial liabilities measured at fair value on a recurring basis
Contingent Liability, Fair Value Disclosure	0		0		0
Total liabilities at fair value
Level 2 [Member]
Financial liabilities measured at fair value on a recurring basis
Contingent Liability, Fair Value Disclosure	0		0		0
Total liabilities at fair value	0		0		0
Level 3 [Member]
Financial liabilities measured at fair value on a recurring basis
Total liabilities at fair value	22,429		22,429		20,907
Balance, beginning of period	22,429	14,734	22,429	14,734	20,907	23,698	7,210	3,015
Earnouts and adjustments related to acquisitions	1,900	7,371	7,678	11,733	17,733
Payments of contingent purchase obligations	0	0	(2,407)	(284)	(284)
Adjustments to contingent purchase obligation	(3,169)	153	(3,749)	270	443
Balance, end of period	22,429	14,734	22,429	14,734	20,907	23,698	7,210	3,015
Level 3 [Member] | Income approach valuation technique [Member]
Financial liabilities measured at fair value on a recurring basis
Contingent Liability, Fair Value Disclosure	$ 22,429		$ 22,429		$ 20,907