Retirement Benefit Obligations - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assumed health care cost trend rate, description	The health care cost trend rate used to determine benefit plan costs for 2024 for pre-65 retirees is 8.00%, with an ultimate rate of 4.50% in 2033, and for post-65 retirees, the health care cost trend rate is 6.00%, with an ultimate rate of 4.50% in 2033. The health care cost trend rate used to determine benefit plan costs for 2023 for pre-65 retirees was 8.00%, with an ultimate rate of 4.50% in 2030, and for post-65 retirees, the health care cost trend rate was 6.25%, with an ultimate rate of 4.50% in 2030.
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer	$ 0.0
Other Postretirement Benefits Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Plan Assets, Contributions by Employer	0.0
Supplemental Employee Retirement Plans, Defined Benefit
Defined Benefit Plan Disclosure [Line Items]
Benefit payments under SERP Plan	0.2
Expected additional benefit payments for the remainder of 2020	$ 0.5
Pre-65 retirees
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate, assumed	8.00%	8.00%
Health care cost trend rate, ultimate rate	4.50%	4.50%
Health care cost trend rate, ultimate rate in year	2033	2030
Post-65 retirees
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate, assumed	6.00%	6.25%
Health care cost trend rate, ultimate rate	4.50%	4.50%
Health care cost trend rate, ultimate rate in year	2033	2030