Fair Value Measurement (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Financial liabilities measured at fair value on a recurring basis
Total liabilities at fair value	$ 21,613	$ 20,907
Income approach valuation technique [Member]
Financial liabilities measured at fair value on a recurring basis
Contingent purchase price related to acquisitions	21,613	20,907
Level 1 [Member]
Financial liabilities measured at fair value on a recurring basis
Contingent purchase price related to acquisitions	0	0
Total liabilities at fair value
Level 2 [Member]
Financial liabilities measured at fair value on a recurring basis
Contingent purchase price related to acquisitions	0	0
Total liabilities at fair value	0	0
Level 3 [Member]
Financial liabilities measured at fair value on a recurring basis
Total liabilities at fair value	21,613	20,907
Balance, beginning of period	21,613	20,907	3,015
Earnouts and adjustments related to acquisitions	430	17,733
Payment of contingent purchase obligations	0	(284)
Adjustment to contingent purchase obligation	276	443
Balance, end of period	21,613	20,907	3,015
Level 3 [Member] | Income approach valuation technique [Member]
Financial liabilities measured at fair value on a recurring basis
Contingent purchase price related to acquisitions	$ 21,613	$ 20,907