Other investments - Narrative (Detail) (USD $)		1 Months Ended	3 Months Ended	12 Months Ended		12 Months Ended	1 Months Ended	3 Months Ended	12 Months Ended			3 Months Ended		3 Months Ended	12 Months Ended				3 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	1 Months Ended
	Mar. 31, 2012 Corporate Joint Venture [Member]	Apr. 18, 2011 Goldman Sachs Group Inc [Member] Other Fixed Maturity and Equity Securities [Member] Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2012 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 [Member]	Mar. 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]	Oct. 17, 2011 General Electric Company [Member] Other Fixed Maturity and Equity Securities [Member] Cumulative Perpetual Preferred Stock [Member]	Mar. 31, 2012 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 [Member]	Dec. 31, 2008 General Electric Company [Member] Other Fixed Maturity and Equity Securities [Member] Cumulative Perpetual Preferred Stock And Warrants [Member]	Mar. 31, 2012 General Electric Company [Member] Other Fixed Maturity and Equity Securities [Member] Warrants Expiring October 16, 2013 [Member]	Mar. 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]	Mar. 31, 2012 Wm Wrigley Jr Company [Member] Other Fixed Maturity and Equity Securities [Member] Preferred Stock [Member] Available-for-sale 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, 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]	Mar. 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]	Mar. 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]	Sep. 30, 2011 Bank of America Corporation [Member] Other Fixed Maturity and Equity Securities [Member] Cumulative Perpetual Preferred Stock [Member]	Mar. 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]	Mar. 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				50,000					30,000											3,000,000	50,000
Amount paid for an investment						$ 5,000,000,000				$ 3,000,000,000					$ 2,100,000,000					$ 3,000,000,000			$ 5,000,000,000
Conversion ratio of preferred stock into common stock																			24.201
Preferred stock conversion price, per share																			$ 41.32
Common stock threshold price, per share, to trigger the issuer's option to convert preferred stock into common stock																			$ 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
Investment in preferred stock, stated dividend rate			10.00%					10.00%						5.00%					8.50%			6.00%
Number of common shares that can be purchased					43,478,260						134,831,460													700,000,000
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
Proceeds from redemption of preferred stock		5,500,000,000					3,300,000,000
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 owned, face value																$ 800,000,000		$ 200,000,000
Investment in notes, stated interest rate												11.45%
Economic interest in joint venture	50.00%