Investments - Cost and Fair Value of Available-for-Sale Fixed-Maturity Securities by Contractual Maturity (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Amortized Cost And Fair Value Debt Securities [Abstract]
Available-for-sale Securities, Debt Maturities, Due in One Year or less, Amortized Cost	$ 705.4
Available-for-sale Securities, Debt Maturities, Due after One year through two Years, Amortized Cost	118.4
Available-for-sale Securities, Debt Maturities, Amortized Cost	823.8	807.3
Available-for-sale Securities, Debt Maturities, Due in One Year or less, Fair Value	705.4
Available-for-sale Securities, Debt Maturities, Due after One year through two Years, Fair Value	118.5
Available-for-sale Securities, Fair Value	$ 823.9	$ 807.7