Employee Benefits - Asset Target Allocations Prescribed by Trusts' Investment Policies (Detail)	12 Months Ended
	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	100.00%
Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	100.00%
Equity Securities [Member] | Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	80.00%
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	40.00%
Fixed Income Securities [Member] | Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	10.00%
Convertible Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	5.00%
Convertible Securities [Member] | Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	5.00%
Cash Equivalents [Member] | Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation	5.00%