Goodwill (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Changes in carrying amounts of goodwill
Balance at the beginning of the period	$ 439,486	$ 132,417
Goodwill acquired during the period		306,599
Contingent payments for businesses acquired	342	332
Foreign currency translation	(325)	138
Balance at the end of the period	439,503	439,486
Region One
Changes in carrying amounts of goodwill
Balance at the beginning of the period	193,758	65,697
Goodwill acquired during the period		127,782
Contingent payments for businesses acquired	342	279
Balance at the end of the period	194,100	193,758
Region Two
Changes in carrying amounts of goodwill
Balance at the beginning of the period		3,760
Goodwill acquired during the period		28,485
Balance at the end of the period	32,245	32,245
Region Three
Changes in carrying amounts of goodwill
Balance at the beginning of the period	66,181	37,097
Goodwill acquired during the period		28,893
Contingent payments for businesses acquired		53
Foreign currency translation	(325)	138
Balance at the end of the period	65,856	66,181
Region Four
Changes in carrying amounts of goodwill
Balance at the beginning of the period		22,577
Goodwill acquired during the period		40,044
Balance at the end of the period	62,621	62,621
Region Five
Changes in carrying amounts of goodwill
Balance at the beginning of the period		3,286
Goodwill acquired during the period		81,395
Balance at the end of the period	$ 84,681	$ 84,681