Benefit Plans - Weighted Average Assumptions Used to Determine Benefit Obligations (Detail)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.10%	3.50%	3.90%
Expected long term rate of return on plan assets	7.50%	7.50%	7.50%
Interest crediting rate	3.80%	3.70%	3.70%
Pension Benefits [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Rate of compensation increase	3.90%	3.90%	3.90%
Pension Benefits [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Rate of compensation increase	5.50%	5.50%	5.50%
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	4.10%	3.40%	3.70%
Expected long term rate of return on plan assets	5.30%	5.30%	5.30%