Schedule of Target Asset Allocation (Detail)	12 Months Ended
	Dec. 31, 2012
Passive And Active Fixed Income | Barclays Capital Long Government Credit Index | Fixed Income Funds
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Target assets allocation percentage	85.00%
Equity (Large Cap) | S&P 500 Stock Index | Equity Securities
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Target assets allocation percentage	10.00%
International Equity | MSCI EAFE Index (Net Dividends) | Equity Securities
Schedule of Defined Benefit Plan Asset Allocation Targets [Line Items]
Target assets allocation percentage	5.00%