Investments in fixed maturity securities - Narrative (Detail) (Fixed Maturities [Member], USD $)	Jun. 30, 2012	Dec. 31, 2011
Summary of Investment Holdings [Line Items]
Unrealized losses related to securities that have been in an unrealized loss position for more than 12 months	$ 24,000,000	$ 20,000,000
Foreign governments [Member] | Germany, United Kingdom, Canada, Australia, and The Netherlands [Member]
Summary of Investment Holdings [Line Items]
Percentage of fixed maturity investments by geographic location	80.00%
Foreign governments [Member] | AA or Higher Credit Rating [Member]
Summary of Investment Holdings [Line Items]
Percentage of fixed maturity investments by credit rating	94.00%