Retirement Benefit Obligations - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2018 USD ($)
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Contributions to benefit plan	$ 1,200,000
Expected additional benefit payments for the remainder of 2018	15,000,000
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Contributions to benefit plan	2,000,000
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Expected additional benefit payments for the remainder of 2018	333,500
Benefit payments under SERP Plan	$ 65,700