Investments in equity securities - Narrative (Detail) (Equity Securities [Member], USD $) In Millions, unless otherwise specified			9 Months Ended
Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Investment Concentration [Member] Item
Summary of Investment Holdings [Line Items]
Concentration percentage			55.00%
Number of companies in concentration percentage			4
Unrealized losses related to securities that have been in an unrealized loss position for more than 12 months	$ 75	$ 45

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 21

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13537-108611

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

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-URI http://asc.fasb.org/extlink&oid=28364263&loc=d3e13531-108611

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 275

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-Section 50

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-URI http://asc.fasb.org/extlink&oid=6927468&loc=d3e6351-108592

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