Investments - Special Deposits (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
U.S. Government obligations	$ 691.4	$ 6.6
Deposits with state insurance departments
Debt Securities, Available-for-sale [Line Items]
U.S. Government obligations	6.8	6.5
Deposits with state insurance departments | New York
Debt Securities, Available-for-sale [Line Items]
U.S. Government obligations	2.8	2.8
Deposits with state insurance departments | Washington
Debt Securities, Available-for-sale [Line Items]
U.S. Government obligations	1.2	1.2
Deposits with state insurance departments | Colorado
Debt Securities, Available-for-sale [Line Items]
U.S. Government obligations	1.1	1.1
Deposits with state insurance departments | Nevada
Debt Securities, Available-for-sale [Line Items]
U.S. Government obligations	0.3	0.3
Deposits with state insurance departments | North Carolina
Debt Securities, Available-for-sale [Line Items]
U.S. Government obligations	0.3	0.3
Deposits with state insurance departments | Virginia
Debt Securities, Available-for-sale [Line Items]
U.S. Government obligations	0.3	0.3
Deposits with state insurance departments | Virginia
Debt Securities, Available-for-sale [Line Items]
U.S. Government obligations	0.2	0.2
Deposits with state insurance departments | Florida
Debt Securities, Available-for-sale [Line Items]
U.S. Government obligations	0.4	0.2
Deposits with state insurance departments | Arkansas
Debt Securities, Available-for-sale [Line Items]
U.S. Government obligations	0.1	0.0
Deposits with state insurance departments | Massachusetts
Debt Securities, Available-for-sale [Line Items]
U.S. Government obligations	$ 0.1	$ 0.1