Pension and other postretirement benefits - fair values of pension plan assets footnote (Detail)	12 Months Ended
	Dec. 31, 2017
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Maximum percentage of net assets invested in emerging market	35.00%
Foreign Companies
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Percentage of net assets invested in foreign equity securities	80.00%
High Yield Fixed Income
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Percentage of net assets invested	80.00%
Fixed Income Securities
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Percentage of net assets invested	80.00%
U.S. Russell 2500 Index | Small and Mid-Sized Companies | Equity Securities
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Percentage of net assets invested	80.00%
Minimum
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Effective average duration to maintain	3 years
Maximum
Schedule Of Pension Plan Assets By Fair Value [Line Items]
Effective average duration to maintain	10 years