Goodwill and Other Intangibles - Changes in Goodwill (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($)
Goodwill [Line Items]
Balance at January 1, 2018	$ 2,160,290
Acquisitions	240,071
Divestitures	(927)
Balance at September 30, 2018	2,399,434
Mid-America Group
Goodwill [Line Items]
Balance at January 1, 2018	281,403
Acquisitions	147,333
Balance at September 30, 2018	428,736
Southeast Group
Goodwill [Line Items]
Balance at January 1, 2018	50,346
Acquisitions	92,738
Divestitures	(927)
Balance at September 30, 2018	142,157
West Group
Goodwill [Line Items]
Balance at January 1, 2018	1,828,541
Balance at September 30, 2018	$ 1,828,541