Reporting Segments and Related Information - Distribution of Total Assets and Operating Performance (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
TOTAL ASSETS	$ 170,003.9	$ 136,828.4	$ 132,212.5
TOTAL REVENUE	6,100.8	6,073.1	5,960.2
INCOME BEFORE INCOME TAXES	1,627.6	1,944.1	1,957.8
Net Income	1,209.3	1,492.2	1,556.4
Non-U.S.
Segment Reporting Information [Line Items]
TOTAL ASSETS	38,393.8	27,888.6	32,712.9
TOTAL REVENUE	1,737.6	1,889.5	2,018.1
INCOME BEFORE INCOME TAXES	404.0	600.0	786.4
Net Income	302.6	451.0	625.7
U.S.
Segment Reporting Information [Line Items]
TOTAL ASSETS	131,610.1	108,939.8	99,499.6
TOTAL REVENUE	4,363.2	4,183.6	3,942.1
INCOME BEFORE INCOME TAXES	1,223.6	1,344.1	1,171.4
Net Income	$ 906.7	$ 1,041.2	$ 930.7