Schedule of Asset Allocation Policy and Ranges for Overall Risk and Return Objectives of Portfolio (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Assets allocation	47.00%
Permitted Range, minimum	43.00%
Permitted Range, maximum	49.00%
Equity Funds | Policy
Defined Benefit Plan Disclosure [Line Items]
Assets allocation	46.00%
Fixed Income Funds
Defined Benefit Plan Disclosure [Line Items]
Assets allocation	49.00%
Permitted Range, minimum	45.00%
Permitted Range, maximum	55.00%
Fixed Income Funds | Policy
Defined Benefit Plan Disclosure [Line Items]
Assets allocation	50.00%
Other
Defined Benefit Plan Disclosure [Line Items]
Assets allocation	4.00%	5.00%
Permitted Range, minimum	0.00%
Permitted Range, maximum	10.00%
Other | Policy
Defined Benefit Plan Disclosure [Line Items]
Assets allocation	4.00%