Goodwill (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended						12 Months Ended				12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Region One	Dec. 31, 2011 Region One	Dec. 31, 2012 Region Two	Dec. 31, 2010 Region Two	Dec. 31, 2012 Region Three	Dec. 31, 2011 Region Three	Dec. 31, 2012 Region Four	Dec. 31, 2010 Region Four	Dec. 31, 2012 Region Five	Dec. 31, 2010 Region Five
Changes in carrying amounts of goodwill
Balance at the beginning of the period	$ 132,417	$ 132,196	$ 65,697	$ 65,396	$ 3,760	$ 3,760	$ 37,097	$ 37,177	$ 22,577	$ 22,577	$ 3,286	$ 3,286
Acquired during the period	306,599	73	127,782	73	28,485		28,893		40,044		81,395
Contingent payments related to acquisitions	332	262	279	228			53	34
Foreign currency translation	138	(114)					137	(114)
Balance at the end of the period	$ 439,486	$ 132,417	$ 193,788	$ 65,697	$ 32,245	$ 3,760	$ 66,181	$ 37,097	$ 62,621	$ 22,577	$ 84,681	$ 3,286