Income Taxes - Schedule of Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Deferred Tax Liabilities, Unrealized Gains on Trading Securities	$ (3.9)
Current:
Federal	40.5	$ 23.1	$ 24.3
State	8.2	7.2	5.0
Total current	48.7	30.3	29.3
Deferred:
Federal	3.7	12.3	18.4
State	1.7	3.5	2.5
Total deferred	5.4	15.8	20.9
Total income tax expense	$ 54.1	$ 46.1	$ 50.2