Investments - Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Amortized cost
Due in 1 year or less, Amortized cost	$ 12.8
Due after 1 year through 5 years, Amortized cost	549.5
Due after 5 years through 10 years, Amortized cost	418.3
Due after 10 years, Amortized cost	526.6
U.S. government agencies mortgage-backed securities, Amortized cost	642.3
Total, Amortized cost	2,149.5	$ 2,095.9
Fair value
Due in 1 year or less, Fair value	12.8
Due after 1 year through 5 years, Fair value	554.5
Due after 5 years through 10 years, Fair value	422.8
Due after 10 years, Fair value	545.9
U.S. government agencies mortgage-backed securities, Fair value	642.7
Total, Fair value	$ 2,178.7	$ 2,109.3