Investments in fixed maturity securities - Amortized cost and estimated fair value of securities with fixed maturities (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Summary of Investment Holdings [Line Items]
Amortized cost of fixed maturity securities	$ 27,214	$ 27,764
Fair value of fixed maturity securities	28,951	29,370
Fixed Maturities [Member]
Summary of Investment Holdings [Line Items]
Due in one year or less - amortized cost	7,993
Due after one year through five years - amortized cost	12,004
Due after five years through ten years - amortized cost	2,889
Due after ten years - amortized cost	2,705
Due in one year or less - fair value	8,077
Due after one year through five years - fair value	12,696
Due after five years through ten years - fair value	3,121
Due after ten years - fair value	3,228
Fixed Maturities [Member] | Mortgage-backed securities [Member]
Summary of Investment Holdings [Line Items]
Mortgage-backed securities - amortized cost	1,623
Mortgage-backed securities - fair value	$ 1,829