Employer Contributions, Participant Contributions and Benefit Payments (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plans
Defined Contribution Plan Disclosure [Line Items]
Employer Contributions	$ 4,665	$ 6,916	$ 16,628
Benefit Payments	6,038	6,877	4,986
Other Postretirement Benefit Plans, Defined Benefit
Defined Contribution Plan Disclosure [Line Items]
Employer Contributions	4,156	4,000	4,000
Participant Contributions	240	121	153
Benefit Payments	2,568	1,758	2,193
Supplemental Employee Retirement Plans, Defined Benefit
Defined Contribution Plan Disclosure [Line Items]
Employer Contributions	654	610	401
Benefit Payments	$ 654	$ 610	$ 401