Income Taxes - Net Operating Losses and Tax Rates (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net operating losses
Statutory Federal income tax rate (as a percent)	21.00%	21.00%	21.00%
Alternative minimum tax credit which does not expire		$ 4.4
Federal
Net operating losses
Operating loss carryforward	$ 987.0	1,581.1
Estimated effective tax rate on net operating losses	21.00%
State
Net operating losses
Operating loss carryforward	$ 653.1	$ 766.4
Estimated effective tax rate on net operating losses	3.45%