Dividends (Parenthetical) (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2012 Restricted Stock Units (RSUs)	Dec. 31, 2011 Restricted Stock Units (RSUs)	Dec. 31, 2010 Restricted Stock Units (RSUs)
Dividend Declared [Line Items]
Dividends related to outstanding restricted stock units		$ 894	$ 684	$ 635
Effective Date of change in amount of quarterly dividend	Nov. 02, 2010
Revised quarterly dividend	$ 0.21