Income Taxes - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Tax Expense Benefit [Line Items]
Reduction in taxes payable	$ 10	$ 14	$ 11
Purchased tax credits	2,951
2018 [Member]
Income Tax Expense Benefit [Line Items]
Unused tax credits expiration amount	438
2019 [Member]
Income Tax Expense Benefit [Line Items]
Unused tax credits expiration amount	1,061
2020 [Member]
Income Tax Expense Benefit [Line Items]
Unused tax credits expiration amount	664
2021 [Member]
Income Tax Expense Benefit [Line Items]
Unused tax credits expiration amount	521
2022 [Member]
Income Tax Expense Benefit [Line Items]
Unused tax credits expiration amount	261
2023 [Member]
Income Tax Expense Benefit [Line Items]
Unused tax credits expiration amount	3
2024 [Member]
Income Tax Expense Benefit [Line Items]
Unused tax credits expiration amount	$ 3
Minimum [Member]
Income Tax Expense Benefit [Line Items]
Purchase tax credits recognized period	5 years
Maximum [Member]
Income Tax Expense Benefit [Line Items]
Purchase tax credits recognized period	8 years