Condensed Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Mar. 31, 2012	Mar. 31, 2011
Statement of Financial Position [Abstract]
Fixed maturities, available-for-sale, amortized cost (in dollars)	$ 16,769.7	$ 16,577.9
Equity securities, available-for-sale, cost (in dollars)	131.1	131.8
Securities pledged, amortized cost (in dollars)	$ 187.9	$ 572.5
Common stock, shares authorized	100,000	100,000
Common stock, shares issued	55,000	55,000
Common stock, shares outstanding	55,000	55,000
Common stock, per share value (in dollars per share)	$ 50.00	$ 50.00