Supplemental health and life insurer Aflac ( AFL) may be best known for its unlucky cartoon duck mascot, but the firm is anything but unlucky in its own affairs. Aflac is a major insurer in two of the most lucrative insurance markets in the world: the U.S. and Japan. In particular, Japan has been fuelling some impressive sales for Aflac in 2013, helped along by an equity rally that's even outpaced the impressive performance here at home. >>4 Hot Stocks to Trade (or Not) Aflac's flagship supplemental policies pay out preset cash benefits if customers meet a predetermined condition -- normally contracting a disease or being involved in an accident. These sorts of loss-of-income policies are proving popular in the wake of the Great Recession as consumers look for way to protect income. And since they're deducted directly from paychecks in many cases, there's no sticker shock effect from seeing money go out each month. That provides a sticky customer base, especially in Japan, where renewal rates hover around 95%. While the U.S. has largely been underserved with supplemental coverage (it only makes up a quarter or so of Aflac's premium income), it's also a less attractive insurance market where retention rates are lower. Despite the demographic shortfalls here at home, the U.S. market should provide some real growth for AFL. So should the continued equity rally, as Aflac's investment portfolios get pushed higher by market forces, leaving shareholders with more assets for the same level of risk.