Other investments - Narrative (Detail) (USD $)		3 Months Ended	12 Months Ended	51 Months Ended	1 Months Ended	19 Months Ended		1 Months Ended	12 Months Ended	40 Months Ended	12 Months Ended		12 Months Ended	46 Months Ended	12 Months Ended		63 Months Ended		12 Months Ended	63 Months Ended				3 Months Ended
	Mar. 31, 2013 Corporate Joint Venture [Member]	Mar. 31, 2013 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]	Mar. 31, 2013 The Dow Chemical Company [Member] Other Fixed Maturity and Equity Securities [Member] Series A Cumulative Convertible Perpetual Preferred Stock [Member]	Sep. 30, 2011 Bank of America Corporation [Member] Other Fixed Maturity and Equity Securities [Member] Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2013 Bank of America Corporation [Member] Other Fixed Maturity and Equity Securities [Member] Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2013 Bank of America Corporation [Member] Other Fixed Maturity and Equity Securities [Member] Warrants Expiring 2021 [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, 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, 2008 Goldman Sachs Group Inc [Member] Other Fixed Maturity and Equity Securities [Member] Cumulative Perpetual Preferred Stock And Warrants [Member]	Mar. 31, 2013 Goldman Sachs Group Inc [Member] Other Fixed Maturity and Equity Securities [Member] Warrants Expiring October 1, 2013 [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]	Mar. 31, 2013 General Electric Company [Member] Other Fixed Maturity and Equity Securities [Member] Warrants Expiring October 16, 2013 [Member]	Mar. 31, 2013 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]	Mar. 31, 2013 Wm Wrigley Jr Company [Member] Other Fixed Maturity and Equity Securities [Member] Preferred Stock [Member] Available-for-sale Securities [Member]	Mar. 31, 2013 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]	Mar. 31, 2013 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]	Mar. 31, 2013 Goldman Sachs and General Electric [Member] Other Fixed Maturity and Equity Securities [Member] Common Stock Warrants [Member]
Other Investments [Line Items]
Investment, number of shares purchased			3,000,000		50,000				50,000				30,000
Investment in preferred stock, stated dividend rate				8.50%		6.00%				10.00%				10.00%						5.00%
Number of common shares that can be purchased							700,000,000					43,478,260				134,831,460
Amount paid for an investment			$ 3,000,000,000					$ 5,000,000,000			$ 5,000,000,000				$ 3,000,000,000				$ 2,100,000,000
Aggregate cost to exercise warrants							5,000,000,000					5,000,000,000				3,000,000,000
Exercise price for warrants, per share							$ 7.142857					$ 115				$ 22.25
Unrealized gains and losses on securities																								520,000,000
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
Redemption value of Cumulative Perpetual Preferred Stock, per share						$ 105,000
Redemption value of Cumulative Perpetual Preferred Stock, aggregate value						$ 5,250,000,000