Parent company financial statements - Condensed Statement of Income (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Condensed Financial Statements, Captions [Line Items]
Asset write-downs	$ 63,790	$ 170,632	$ 101,106
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Asset write-downs		$ 79,012