Employee Benefits - Asset Target Allocation Ranges Prescribed by Trusts' Investment Policies (Detail)	12 Months Ended
	Dec. 31, 2012
Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation, minimum	70.00%
Target Allocation, maximum	90.00%
Fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation, minimum	0.00%
Target Allocation, maximum	10.00%
Convertible securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation, minimum	0.00%
Target Allocation, maximum	10.00%
Cash equivalents and other assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Target Allocation, minimum	10.00%
Target Allocation, maximum	30.00%