Segment Reporting - Total Assets of Company's Reportable Segments (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Total Assets	$ 3,673,827	$ 3,328,005
Corporate (Member)
Segment Reporting Information [Line Items]
Total Assets	170,394	75,694
Western [Member]
Segment Reporting Information [Line Items]
Total Assets	1,505,949	1,370,098
Central [Member]
Segment Reporting Information [Line Items]
Total Assets	1,167,727	1,040,962
Eastern [Member]
Segment Reporting Information [Line Items]
Total Assets	$ 829,757	$ 841,251