Goodwill (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013
Changes in carrying amounts of goodwill
Balance at the beginning of the period	$ 435,122
Contingent payments related to acquisitions	(6)
Adjustment to purchase price allocation	(651)
Foreign currency translation	(122)
Balance at the end of the period	434,343
Region One
Changes in carrying amounts of goodwill
Balance at the beginning of the period	191,941
Adjustment to purchase price allocation	(272)
Balance at the end of the period	191,669
Region Two
Changes in carrying amounts of goodwill
Balance at the beginning of the period	31,840
Adjustment to purchase price allocation	(61)
Foreign currency translation	(122)
Balance at the end of the period	31,657
Region Three
Changes in carrying amounts of goodwill
Balance at the beginning of the period	65,770
Contingent payments related to acquisitions	(6)
Adjustment to purchase price allocation	(61)
Balance at the end of the period	65,703
Region Four
Changes in carrying amounts of goodwill
Balance at the beginning of the period	62,052
Adjustment to purchase price allocation	(85)
Balance at the end of the period	61,967
Region Five
Changes in carrying amounts of goodwill
Balance at the beginning of the period	83,519
Adjustment to purchase price allocation	(172)
Balance at the end of the period	$ 83,347