Investments (Investments Classified by Contractual Maturity Date) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Investments [Abstract]
Due in one year or less, Amortized Cost	$ 452.2
Due after one year through five years, Amortized Cost	2,363.3
Due after five years through ten years, Amortized Cost	2,206.1
Due after ten years, Amortized Cost	592.7
Gross fixed maturities, Amortized Cost	5,614.3
Mortgage-backed and asset-backed securities, Amortized Cost	1,442.2
Total fixed maturities, Amortized Cost	7,056.5	7,145.7
Due in one year or less, Fair Value	457.1
Due after one year through five years, Fair Value	2,462.0
Due after five years through ten years, Fair Value	2,302.7
Due after ten years, Fair Value	635.2
Gross fixed maturities, Fair Value	5,857.0
Mortgage-backed and asset-backed securities, Fair Value	1,484.6
Fixed maturities, Fair Value	$ 7,341.6	$ 7,378.1