Investments - Cost and Fair Value of Available-for-Sale Fixed-Maturity Securities by Contractual Maturity (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Investments Debt And Equity Securities [Abstract]
Available-for-sale Securities, Debt Maturities, Due in One Year or less, Amortized Cost	$ 726.7
Available-for-sale Securities, Debt Maturities, Due after One year through two Years, Amortized Cost	80.6
Available-for-sale Securities, Debt Maturities, Amortized Cost	807.3
Available-for-sale Securities, Debt Maturities, Due in One Year or less, Fair Value	727.0
Available-for-sale Securities, Debt Maturities, Due after One year through two Years, Fair Value	80.7
Available-for-sale Securities, Fair Value	$ 807.7	$ 915.7