Health-insurance stocks plunged Thursday, led by a 19% drop in Humana Inc. shares, after the Obama administration outlined plans to finance a big chunk of its health-reform agenda with $175 billion in cuts to private Medicare plans. Shares in Humana, which has more than 1.4 million Medicare Advantage members, took the biggest plunge, falling $5.72 to $23.64 in trading on the New York Stock Exchange.