Earnings Per Common Share - Basic and Diluted Earnings Per Common Share, Reflecting Application of Two-Class Method (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net income available to common shareholders	$ 134.0	$ 131.6	$ 129.7	$ 111.1	$ 129.4	$ 113.5	$ 106.7	$ 104.4	$ 506.3	$ 454.0	$ 323.1
Dividends paid on and undistributed earnings allocated to participating securities									(0.2)	(0.2)	(0.5)
Earnings attributable to common shareholders									$ 506.1	$ 453.8	$ 322.6
Weighted average common shares outstanding for basic EPS (in shares)	421,850	391,660	391,270	370,720	370,220	341,430	340,640	339,760	394,000	348,100	330,300
Effect of dilutive equity-based awards (in shares)									3,200	3,600	2,600
Weighted average common shares and common-equivalent shares for diluted EPS (in shares)	424,980	394,450	394,570	374,090	372,830	345,040	344,470	344,000	397,200	351,700	332,900
EPS:
Basic (in dollars per share)	$ 0.31	$ 0.34	$ 0.33	$ 0.30	$ 0.35	$ 0.33	$ 0.31	$ 0.31	$ 1.28	$ 1.30	$ 0.98
Diluted (in dollars per share)	$ 0.31	$ 0.33	$ 0.33	$ 0.30	$ 0.35	$ 0.33	$ 0.31	$ 0.30	$ 1.27	$ 1.29	$ 0.97