Investments - Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Amortized cost
Due in 1 year or less, Amortized cost	$ 52.1
Due after 1 year through 5 years, Amortized cost	540.6
Due after 5 years through 10 years, Amortized cost	306.6
Due after 10 years, Amortized cost	580.9
U.S. government agencies mortgage-backed securities, Amortized cost	624.5
Total, Amortized cost	2,104.7	$ 2,095.9
Fair value
Due in 1 year or less, Fair value	52.6
Due after 1 year through 5 years, Fair value	546.7
Due after 5 years through 10 years, Fair value	310.0
Due after 10 years, Fair value	591.5
U.S. government agencies mortgage-backed securities, Fair value	622.7
Total, Fair value	$ 2,123.5	$ 2,109.3