Employee Benefit Plans - Schedule of assumptions used in the valuation and disclosures for the Pension Plan and the PRB Plan (Detail)	12 Months Ended
	Dec. 31, 2021
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate used for net periodic pension cost	2.75%
Expected return on plan assets	7.25%
Expected future interest crediting rate	3.00%
Pension Plan [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate used for disclosure (range)	3.08%
Pension Plan [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate used for disclosure (range)	1.02%
PRB Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate used for net periodic pension cost	2.50%
Discount rate used for disclosure (range)	2.75%