Employee Postretirement Benefits - Schedule of Pension Plan Assets (Detail)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Target Allocation [Member]
Asset Category
Target Allocation	100.00%
Actual Asset Allocation [Member]
Asset Category
Actual Allocation, End of Year	100.00%	100.00%
Equity Securities [Member] | Target Allocation [Member] | Minimum [Member]
Asset Category
Target Allocation	47.00%
Equity Securities [Member] | Target Allocation [Member] | Maximum [Member]
Asset Category
Target Allocation	53.00%
Equity Securities [Member] | Actual Asset Allocation [Member]
Asset Category
Actual Allocation, End of Year	53.00%	50.00%
Fixed Income Securities [Member] | Target Allocation [Member] | Minimum [Member]
Asset Category
Target Allocation	47.00%
Fixed Income Securities [Member] | Target Allocation [Member] | Maximum [Member]
Asset Category
Target Allocation	53.00%
Fixed Income Securities [Member] | Actual Asset Allocation [Member]
Asset Category
Actual Allocation, End of Year	47.00%	50.00%