Investment Portfolio Investment Portfolio - Investment Ratings (Details) $ in Millions	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Investment [Line Items]
Number of securities	44	45
Available-for-sale investments:	$ 3.5	$ 4.5
Percent of Investment	100.00%	100.00%
External Credit Rating, Investment Grade [Member]
Investment [Line Items]
Number of securities	9	10
Available-for-sale investments:	$ 3.0	$ 3.6
Percent of Investment	86.00%	80.00%
External Credit Rating, Non Investment Grade [Member]
Investment [Line Items]
Number of securities	35	35
Available-for-sale investments:	$ 0.5	$ 0.9
Percent of Investment	14.00%	20.00%