Other investments - Narrative (Detail) (USD $)		12 Months Ended		48 Months Ended	12 Months Ended	40 Months Ended		12 Months Ended		46 Months Ended	12 Months Ended		60 Months Ended		12 Months Ended	60 Months Ended				1 Months Ended	16 Months Ended	1 Months Ended
	Dec. 31, 2012 Corporate Joint Venture [Member]	Dec. 31, 2012 The Dow Chemical Company [Member] Other Fixed Maturity and Equity Securities [Member] Series A Cumulative Convertible Perpetual Preferred Stock [Member]	Dec. 31, 2009 The Dow Chemical Company [Member] Other Fixed Maturity and Equity Securities [Member] Series A Cumulative Convertible Perpetual Preferred Stock [Member]	Dec. 31, 2012 The Dow Chemical Company [Member] Other Fixed Maturity and Equity Securities [Member] Series A Cumulative Convertible Perpetual Preferred Stock [Member]	Dec. 31, 2008 Goldman Sachs Group Inc [Member] Other Fixed Maturity and Equity Securities [Member] Cumulative Perpetual Preferred Stock [Member]	Apr. 18, 2011 Goldman Sachs Group Inc [Member] Other Fixed Maturity and Equity Securities [Member] Cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2012 Goldman Sachs Group Inc [Member] Other Fixed Maturity and Equity Securities [Member] Warrants Expiring October 1, 2013 [Member]	Dec. 31, 2008 Goldman Sachs Group Inc [Member] Other Fixed Maturity and Equity Securities [Member] Cumulative Perpetual Preferred Stock And Warrants [Member]	Dec. 31, 2008 General Electric Company [Member] Other Fixed Maturity and Equity Securities [Member] Cumulative Perpetual Preferred Stock [Member]	Oct. 17, 2011 General Electric Company [Member] Other Fixed Maturity and Equity Securities [Member] Cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2008 General Electric Company [Member] Other Fixed Maturity and Equity Securities [Member] Cumulative Perpetual Preferred Stock And Warrants [Member]	Dec. 31, 2012 General Electric Company [Member] Other Fixed Maturity and Equity Securities [Member] Warrants Expiring October 16, 2013 [Member]	Dec. 31, 2012 Wm Wrigley Jr Company [Member] Other Fixed Maturity and Equity Securities [Member] Subordinated Debt Due 2018 [Member] Held-to-maturity Securities [Member]	Dec. 31, 2008 Wm Wrigley Jr Company [Member] Other Fixed Maturity and Equity Securities [Member] Subordinated Debt Due 2018 [Member] Held-to-maturity Securities [Member]	Dec. 31, 2008 Wm Wrigley Jr Company [Member] Other Fixed Maturity and Equity Securities [Member] Preferred Stock [Member] Available-for-sale Securities [Member]	Dec. 31, 2012 Wm Wrigley Jr Company [Member] Other Fixed Maturity and Equity Securities [Member] Preferred Stock [Member] Available-for-sale Securities [Member]	Dec. 31, 2012 Wm Wrigley Jr Company [Member] Other Fixed Maturity and Equity Securities [Member] Senior Notes Due 2013 and 2014 [Member] Held-to-maturity Securities [Member]	Dec. 31, 2009 Wm Wrigley Jr Company [Member] Other Fixed Maturity and Equity Securities [Member] Senior Notes Due 2013 and 2014 [Member] Held-to-maturity Securities [Member]	Dec. 31, 2012 Wm Wrigley Jr Company [Member] Other Fixed Maturity and Equity Securities [Member] Senior Notes Due 2013 and 2014 [Member] Held-to-maturity Securities [Member] Corporate Joint Venture [Member]	Sep. 30, 2011 Bank of America Corporation [Member] Other Fixed Maturity and Equity Securities [Member] Cumulative Perpetual Preferred Stock [Member]	Dec. 31, 2012 Bank of America Corporation [Member] Other Fixed Maturity and Equity Securities [Member] Cumulative Perpetual Preferred Stock [Member]	Sep. 30, 2011 Bank of America Corporation [Member] Other Fixed Maturity and Equity Securities [Member] Cumulative Perpetual Preferred Stock And Warrants [Member]	Dec. 31, 2012 Bank of America Corporation [Member] Other Fixed Maturity and Equity Securities [Member] Warrants Expiring 2021 [Member]
Other Investments [Line Items]
Investment, number of shares purchased			3,000,000		50,000				30,000											50,000
Investment in preferred stock, stated dividend rate				8.50%		10.00%				10.00%						5.00%					6.00%
Number of common shares that can be purchased							43,478,260					134,831,460											700,000,000
Amount paid for an investment			$ 3,000,000,000					$ 5,000,000,000			$ 3,000,000,000				$ 2,100,000,000							$ 5,000,000,000
Redemption value of Cumulative Perpetual Preferred Stock, per share						$ 110,000				$ 110,000											$ 105,000
Redemption value of Cumulative Perpetual Preferred Stock, aggregate value						5,500,000,000				3,300,000,000											5,250,000,000
Aggregate cost to exercise warrants							5,000,000,000					3,000,000,000											5,000,000,000
Exercise price for warrants, per share							$ 115					$ 22.25											$ 7.142857
Acquisition of debt investments, par value when acquired														4,400,000,000				1,000,000,000
Investment in notes, stated interest rate													11.45%
Investment owned, face value																	$ 800,000,000		$ 200,000,000
Economic interest in joint venture	50.00%
Conversion ratio of preferred stock into common stock		24.201		24.201
Preferred stock conversion price, per share		$ 41.32		$ 41.32
Common stock threshold price, per share, to trigger the issuer's option to convert preferred stock into common stock		$ 53.72		$ 53.72
Minimum number of trading days in a consecutive-day period that the issuer's common stock price must exceed to trigger the issuer's option to convert preferred stock to common stock		20 days
Range of trading days in a consecutive day period, number of days		30 days