Income Taxes - Schedule of Income Tax Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Deferred Tax Liabilities, Unrealized Gains on Trading Securities	$ (19.2)
Current:
Federal	42.4	$ 40.5	$ 23.1
State	12.3	8.2	7.2
Total current	54.7	48.7	30.3
Deferred:
Federal	(5.7)	3.7	12.3
State	(0.9)	1.7	3.5
Total deferred	(6.6)	5.4	15.8
Total income tax expense	48.1	54.1	46.1
Operating Loss Carryforwards [Line Items]
Benefit of stock-based compensation plans			$ (1.1)
Common Stock Including Additional Paid in Capital [Member]
Operating Loss Carryforwards [Line Items]
Benefit of stock-based compensation plans	$ (0.4)	$ (1.2)