Retirement Benefit Plans (Narrative) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2020
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Accumulated Benefit Obligation	$ 245,700	$ 214,000
Net actuarial loss	$ (4,100)
Expected long-term return on assets (as a percent)	5.75%
Healthcare-cost trend rate in 2024 (as a percent)	4.60%			6.50%
Increase in net periodic postretirement benefit cost if trend increased by 1%	$ 3
Increase in accumulated benefit obligation if trend increased by 1%	97
Decrease in net periodic postretirement benefit cost if trend rate decreased by 1%	3
Decrease in accumulated benefit obligation if trend rate decreased 1%	89
Funded status of plan	$ 2,400
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%
Compensation expense	$ 13,200	12,400	$ 12,000
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 10,000	$ 0
Expected long-term return on assets (as a percent)	6.00%	6.00%	6.50%
Funded status of plan	$ (1,783)	$ (17,541)
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	128	11,364
Funded status of plan	(1,935)	(1,835)
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 195	$ 208
Assumed healthcare cost trend (as a percent)	6.50%	7.00%	7.50%
Funded status of plan	$ (2,399)	$ (2,612)