Stockholders' Equity - Common Stock Activity (Details) - shares shares in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares outstanding at the beginning of the period	96.9	110.4	105.2
Shares issued for preferred share conversions	0.0	0.0	25.5
Shares issued for vested restricted stock units	1.3	1.5	1.1
Shares issued for disputed claims	0.0	0.0	0.1
Shares repurchased	(0.4)	(15.0)	(21.5)
Shares outstanding at the end of the period	97.8	96.9	110.4