Postretirement Benefit Plans - Summary of Actual Pension Plan Asset Allocation (Detail) |
12 Months Ended | |
|---|---|---|
Dec. 31, 2018 |
Dec. 31, 2017 |
|
| Defined Benefit Plan Disclosure [Line Items] | ||
| Actual benefit plan asset allocation | 100.00% | 100.00% |
| Equity Securities [Member] | ||
| Defined Benefit Plan Disclosure [Line Items] | ||
| Actual benefit plan asset allocation | 33.00% | 32.00% |
| Debt Securities [Member] | ||
| Defined Benefit Plan Disclosure [Line Items] | ||
| Actual benefit plan asset allocation | 44.00% | 47.00% |
| Insurance Contracts [Member] | ||
| Defined Benefit Plan Disclosure [Line Items] | ||
| Actual benefit plan asset allocation | 7.00% | 7.00% |
| Other [Member] | ||
| Defined Benefit Plan Disclosure [Line Items] | ||
| Actual benefit plan asset allocation | 16.00% | 14.00% |