Fair Value Measurements - Fair Value Hierarchy (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets
U.S. Government obligations	$ 6.6	$ 5.9
Total	6.6	5.9
Level 1
Assets
U.S. Government obligations	0.0	0.0
Total	0.0	0.0
Level 2
Assets
U.S. Government obligations	6.6	5.9
Total	6.6	5.9
Level 3
Assets
U.S. Government obligations	0.0	0.0
Total	$ 0.0	$ 0.0