Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) $ in Millions	Dec. 31, 2018 USD ($)
Summary of investments - other than investments in related parties
Cost or amortized cost	$ 3,416.0
Fair value	3,409.7
Amount shown on the balance sheet	3,409.7
Fixed maturity investments
Summary of investments - other than investments in related parties
Cost or amortized cost	1,952.9
Fair value	1,949.2
Amount shown on the balance sheet	1,949.2
Corporate debt securities
Summary of investments - other than investments in related parties
Cost or amortized cost	694.1
Fair value	695.8
Amount shown on the balance sheet	695.8
Asset-backed securities
Summary of investments - other than investments in related parties
Cost or amortized cost	496.3
Fair value	494.5
Amount shown on the balance sheet	494.5
Residential mortgage-backed securities
Summary of investments - other than investments in related parties
Cost or amortized cost	413.0
Fair value	413.5
Amount shown on the balance sheet	413.5
Commercial mortgage-backed securities
Summary of investments - other than investments in related parties
Cost or amortized cost	163.9
Fair value	167.9
Amount shown on the balance sheet	167.9
Non-U.S. government and government agency
Summary of investments - other than investments in related parties
Cost or amortized cost	50.6
Fair value	50.3
Amount shown on the balance sheet	50.3
U.S. government and government agency
Summary of investments - other than investments in related parties
Cost or amortized cost	117.7
Fair value	115.9
Amount shown on the balance sheet	115.9
Preferred stocks
Summary of investments - other than investments in related parties
Cost or amortized cost	14.5
Fair value	8.5
Amount shown on the balance sheet	8.5
U.S. States, municipalities, and political subdivision
Summary of investments - other than investments in related parties
Cost or amortized cost	2.8
Fair value	2.8
Amount shown on the balance sheet	2.8
Short-term investments
Summary of investments - other than investments in related parties
Cost or amortized cost	716.1
Fair value	715.5
Amount shown on the balance sheet	715.5
Total equity securities
Summary of investments - other than investments in related parties
Cost or amortized cost	409.4
Fair value	380.0
Amount shown on the balance sheet	380.0
Other long-term investments
Summary of investments - other than investments in related parties
Cost or amortized cost	337.6
Fair value	365.0
Amount shown on the balance sheet	$ 365.0