Investments - Cost and Fair Value of Available-for-Sale Fixed-Maturity Securities by Contractual Maturity (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 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	$ 779.7
Available-for-sale Securities, Debt Maturities, Due after One year through two Years, Amortized Cost	253.6
Available-for-sale Securities, Debt Maturities, Amortized Cost	1,033.3	807.3
Available-for-sale Securities, Debt Maturities, Due in One Year or less, Fair Value	779.8
Available-for-sale Securities, Debt Maturities, Due after One year through two Years, Fair Value	253.7
Available-for-sale Securities, Fair Value	$ 1,033.5	$ 807.7