Schedule of Funded Status Plan and Change in Funded Status Plan (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
At beginning of year	$ 28,471
At end of year	32,776
Benefit obligations	3.87%	4.41%
Expected return on plan assets	7.00%
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
At beginning of year	44,246	36,690
Service cost	863	774	731
Interest cost	1,906	1,819	1,883
Plan participants' contributions	245	238
Actuarial loss (gain)	4,275	6,122
Benefits paid	(1,647)	(1,397)
At end of year	49,888	44,246	36,690
Funded status - net liability at year end	(17,112)	(15,775)
Amounts recognized in accumulated other comprehensive loss, Net actuarial loss	20,894	19,744
Net actuarial loss	2,256	1,885
Components of cost	4.41%	5.09%
Benefit obligations	3.87%	4.41%
Expected return on plan assets	7.00%	8.00%
Rate of compensation increase
Pension Benefits | Defined Benefit Plan Change In Plan Assets
Defined Benefit Plan Disclosure [Line Items]
At beginning of year	28,471	26,858
Actual return on plan assets	3,276	724
Employer contribution	2,431	2,048
Plan participants' contributions	245	238
Benefits paid	(1,647)	(1,397)
At end of year	32,776	28,471
Supplemental Retirement Benefits
Defined Benefit Plan Disclosure [Line Items]
At beginning of year	3,700	3,204
Service cost	29	20
Interest cost	160	159
Plan participants' contributions	20	11
Actuarial loss (gain)	385	464
Benefits paid	(158)	(158)
At end of year	4,136	3,700
Funded status - net liability at year end	4,136	3,700
Amounts recognized in accumulated other comprehensive loss, Net actuarial loss	2,056	1,863
Net actuarial loss	227	192
Components of cost	4.41%	5.09%
Benefit obligations	3.87%	4.41%
Rate of compensation increase
Supplemental Retirement Benefits | Defined Benefit Plan Change In Plan Assets
Defined Benefit Plan Disclosure [Line Items]
Employer contribution	138	147
Plan participants' contributions	20	11
Benefits paid	(158)	(158)
Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
At beginning of year	1,907	1,837
Service cost	37	38	38
Interest cost	72	81	89
Plan participants' contributions	52	55
Actuarial loss (gain)	(98)	(35)
Benefits paid	(128)	(69)
At end of year	1,842	1,907	1,837
Amounts recognized in accumulated other comprehensive loss, Net actuarial loss	230	348
Net actuarial loss	7	21
Components of cost	4.56%	4.56%
Benefit obligations	3.26%	3.91%
Rate of compensation increase
Postretirement Plan | Defined Benefit Plan Change In Plan Assets
Defined Benefit Plan Disclosure [Line Items]
Employer contribution	76	14
Plan participants' contributions	52	55
Benefits paid	(128)	(69)
Funded status - net liability at year end	$ 1,842	$ 1,907