Retirement Plans - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) RetirementPlans shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2008
Retirement Plans [Line Items]
Number of supplemental non-qualified retirement plans sponsored by the Corporation | RetirementPlans	2
Consecutive calendar years used to calculate BRP benefits			5 years
Term of employment for consideration of BRP			10 years
Percentage of non callable corporate bonds	A yield curve was produced for a universe containing the majority of U.S.-issued Aa-graded corporate bonds, all of which were non-callable (or callable with make-whole provisions), and after excluding the 10% of the bonds with the highest and lowest yields.
Corporation contribution	$ 1,345,000	$ 1,345,000
Benefits paid	$ 9,728,000	7,936,000
Percent of employer match	100.00%
Employee contribution percentage	6.00%
Additional discretionary contribution, annual financial goals, percentage	3.00%
Dividends received on common stock	$ 276,000	$ 276,000
Equity securities
Retirement Plans [Line Items]
Common stock, shares | shares	575,128	575,128
Percentage of common stock on total plan assets	4.90%	6.70%
Non Qualified Pension Plans
Retirement Plans [Line Items]
Benefits paid	$ 1,300,000	$ 1,300,000
Qualified Pension Plans
Retirement Plans [Line Items]
Corporation contribution	0	0
Benefits paid	$ 8,400,000	$ 6,600,000