Statement of Cash Flows - Noncash Investing and Financing Activities - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Cash Flow Elements [Line Items]
Noncash investing activities incurred for fixed asset acquisitions	$ 12,600,000	$ 10,410,000	$ 9,515,000
Common Stock [Member] | Stock Award Plan [Member]
Supplemental Cash Flow Elements [Line Items]
Noncash financing activities shares issued	35,372
Noncash financing activities issued value	$ 2,941,000
Tebras And PBC [Member]
Supplemental Cash Flow Elements [Line Items]
Noncash investing activities unpaid working capital adjustments related to 2016 acquisitions		$ 4,459,000