Fair Value Measurements - Credit Ratings (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Debt securities, held-to-maturity, amortized cost, before allowance for credit loss	$ 71.9	$ 72.1
AA+
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Debt securities, held-to-maturity, amortized cost, before allowance for credit loss	13.4	13.5
AA
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Debt securities, held-to-maturity, amortized cost, before allowance for credit loss	0.0	10.6
Standard & Poor's, AA- Rating
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Debt securities, held-to-maturity, amortized cost, before allowance for credit loss	10.6	0.0
A
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Debt securities, held-to-maturity, amortized cost, before allowance for credit loss	13.3	24.2
A-
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Debt securities, held-to-maturity, amortized cost, before allowance for credit loss	14.1	13.4
BBB+
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Debt securities, held-to-maturity, amortized cost, before allowance for credit loss	$ 20.5	$ 10.4