Outstanding and Weighted Average Common Shares - Changes in Outstanding Common Shares (Detail) - shares shares in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Stockholders Equity Note [Abstract]
Balance at January 1	291.4	315.1	316.5
Conversion of preferred stock	11.6	0.0	0.0
Activity related to restricted stock awards, net	0.9	0.8	0.8
Stock options exercised	0.7	0.6	0.2
PSU vested	0.3	0.1	0.2
Shares repurchased	0.0	(25.2)	(2.6)
Balance at December 31	304.9	291.4	315.1