Retirement Benefit Plans (Narrative) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2020
Defined Benefit Plan Disclosure [Line Items]
Healthcare-cost trend rate in 2024 (as a percent)	4.40%			6.50%
Funded status of plan	$ 2,600
Shortfall amortization period	7 years
Employer matching contribution, percent of match	100.00%
Employer matching contribution, percent of employees' gross pay	2.00%
Employer matching contribution, additional percent of match	50.00%
Employer additional matching contribution, percent of employees' gross pay	6.00%
Employer additional matching contribution, percent of employees' gross pay, default	3.00%
Compensation expense	$ 13,800	$ 13,200	$ 12,400
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Funded status of plan	(146)	(1,783)
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Funded status of plan	(2,046)	(1,935)
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Funded status of plan	$ (1,998)	$ (2,399)