Investments - Amortized Cost and Fair Value of Available-for-Sale Fixed Maturities by Contractual Maturity (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Amortized cost
Due in 1 year or less, Amortized cost	$ 45.5
Due after 1 year through 5 years, Amortized cost	558.6
Due after 5 years through 10 years, Amortized cost	350.6
Due after 10 years, Amortized cost	577.2
U.S. government agencies mortgage-backed securities, Amortized cost	606.4
Total, Amortized cost	2,138.3	$ 2,095.9
Fair value
Due in 1 year or less, Fair value	45.6
Due after 1 year through 5 years, Fair value	563.8
Due after 5 years through 10 years, Fair value	354.4
Due after 10 years, Fair value	594.6
U.S. government agencies mortgage-backed securities, Fair value	608.7
Total, Fair value	$ 2,167.1	$ 2,109.3