Changes in Goodwill (Detail) $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($)
Goodwill [Line Items]
Balance at beginning of period	$ 2,159,337
Adjustments to purchase price allocations	723
Balance at end of period	2,160,060
Mid-America Group
Goodwill [Line Items]
Balance at beginning of period	281,403
Balance at end of period	281,403
Southeast Group
Goodwill [Line Items]
Balance at beginning of period	50,346
Balance at end of period	50,346
West Group
Goodwill [Line Items]
Balance at beginning of period	1,827,588
Adjustments to purchase price allocations	723
Balance at end of period	$ 1,828,311