Retirement Benefit Obligations - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2018 USD ($)
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Contributions to benefit plan	$ 16,600,000
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Contributions to benefit plan	3,000,000
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Benefit payments under SERP Plan	87,500
Expected additional benefit payments for the remainder of 2018	$ 313,100