Trust's Assets at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	$ 36,355	$ 32,643	$ 32,797
Us Large Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	10,835	9,958
Us Mid Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	4,256	3,332
Non Us Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	6,187	5,549
Emerging Markets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	2,201	1,636
Us Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	9,487	9,645
Non Us Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	1,645	1,493
Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	184	133
Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	1,560	897
Fair Value, Inputs, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	36,355	28,290
Fair Value, Inputs, Level 1 [Member] | Us Large Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	10,835	9,958
Fair Value, Inputs, Level 1 [Member] | Us Mid Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	4,256	3,332
Fair Value, Inputs, Level 1 [Member] | Non Us Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	6,187	5,549
Fair Value, Inputs, Level 1 [Member] | Emerging Markets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	2,201	1,636
Fair Value, Inputs, Level 1 [Member] | Us Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	9,487	5,292
Fair Value, Inputs, Level 1 [Member] | Non Us Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	1,645	1,493
Fair Value, Inputs, Level 1 [Member] | Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	184	133
Fair Value, Inputs, Level 1 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	1,560	897
Fair Value, Inputs, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	4,353
Fair Value, Inputs, Level 2 [Member] | Us Large Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Fair Value, Inputs, Level 2 [Member] | Us Mid Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Fair Value, Inputs, Level 2 [Member] | Non Us Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Fair Value, Inputs, Level 2 [Member] | Emerging Markets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Fair Value, Inputs, Level 2 [Member] | Us Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	4,353
Fair Value, Inputs, Level 2 [Member] | Non Us Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Fair Value, Inputs, Level 2 [Member] | Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Fair Value, Inputs, Level 2 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Fair Value, Inputs, Level 3 [Member] | Us Large Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Fair Value, Inputs, Level 3 [Member] | Us Mid Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Fair Value, Inputs, Level 3 [Member] | Non Us Cap [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Fair Value, Inputs, Level 3 [Member] | Emerging Markets [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Fair Value, Inputs, Level 3 [Member] | Us Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Fair Value, Inputs, Level 3 [Member] | Non Us Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Fair Value, Inputs, Level 3 [Member] | Real Estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	0	0
Fair Value, Inputs, Level 3 [Member] | Cash and Cash Equivalents [Member]
Defined Benefit Plan Disclosure [Line Items]
Total Assets at Fair Value	$ 0	$ 0