Postretirement Benefits - Activity for the SERP (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Premiums paid for insurance coverage	$ 2,050,000	$ 2,050,000	$ 10,068,000
Company owned life insurance	40,273,000	37,267,000
Exchange traded funds	55,442,000	48,999,000
Total investments of SERP	95,715,000	86,266,000
Cash deposited into investment fund		0	$ 0
Nonqualified | Active plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liability	81,457,000	74,687,000
Nonqualified | Closed
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Liability	$ 3,008,000	$ 3,220,000