Weighted-Average Assumptions Used to Determine Benefit Obligations (Detail)	Dec. 31, 2011	Dec. 31, 2010
Qualified Pension Plan
Schedule of Benefit Obligations Weighted Average Assumptions [Line Items]
Discount rate	5.10%	5.85%
Qualified Pension Plan | Minimum
Schedule of Benefit Obligations Weighted Average Assumptions [Line Items]
Rate of compensation increase	3.50%	3.50%
Qualified Pension Plan | Maximum
Schedule of Benefit Obligations Weighted Average Assumptions [Line Items]
Rate of compensation increase	5.50%	5.50%
Supplemental Executive Retirement Plan (SERP)
Schedule of Benefit Obligations Weighted Average Assumptions [Line Items]
Discount rate	4.75%	5.55%
Supplemental Executive Retirement Plan (SERP) | Minimum
Schedule of Benefit Obligations Weighted Average Assumptions [Line Items]
Rate of compensation increase	3.50%	3.50%
Supplemental Executive Retirement Plan (SERP) | Maximum
Schedule of Benefit Obligations Weighted Average Assumptions [Line Items]
Rate of compensation increase	5.50%	5.50%