Investments in equity securities - Narrative (Detail) (Equity Securities [Member], USD $)	Jun. 30, 2012	Dec. 31, 2011
Equity Securities [Member]
Summary of Investment Holdings [Line Items]
Unrealized losses related to securities that have been in an unrealized loss position for more than 12 months	$ 52,000,000	$ 0

Amount of the excess of amortized cost basis over fair value of securities that have been in a loss position for twelve months or longer for securities which are categorized neither as held-to-maturity nor trading securities.

No definition available.

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant