Benefit Obligations - Plan Asset Allocations for Defined Benefit Pension Plans and Post-retirement Benefit Plan (Detail)	Dec. 31, 2012	Dec. 31, 2011
Pension Benefits [Member] | Equity Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Securities	50.00%	54.00%
Pension Benefits [Member] | Debt Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Securities	43.00%	42.00%
Pension Benefits [Member] | Real Estate Equity Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Securities	7.00%	4.00%
Non-qualified Pension Benefits [Member] | Equity Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Securities	41.00%	34.00%
Non-qualified Pension Benefits [Member] | Debt Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Securities	59.00%	66.00%
Non-qualified Pension Benefits [Member] | Real Estate Equity Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Securities
Post-retirement Benefits [Member] | Equity Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Securities	40.00%	40.00%
Post-retirement Benefits [Member] | Debt Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Securities	60.00%	60.00%
Post-retirement Benefits [Member] | Real Estate Equity Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Securities