Retirement Benefit Obligations - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2018 USD ($)
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Contributions to benefit plan	$ 1,200,000
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Contributions to benefit plan	1,000,000
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Benefit payments under SERP Plan	356,300
Expected additional benefit payments under SERP Plan in 2016	$ 43,900