Deposits - Schedule of Deposits (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Amount
Non-interest-bearing	$ 9,803.7	$ 8,543.0
Savings	4,987.7	4,116.5
Interest-bearing checking and money market	19,592.6	16,583.3
Time deposits maturing:
Within 3 months	3,984.7	1,480.0
After 3 but within 6 months	2,659.3	1,375.6
After 6 months but within 1 year	1,354.8	2,511.5
After 1 but within 2 years	816.3	1,275.5
After 2 but within 3 years	275.4	153.5
After 3 but within 4 years	97.8	102.1
After 4 but within 5 years	17.2	18.0
After 5 years (less than $0.1 million)	0.0	0.0
Total	9,205.5	6,916.2
Total deposits	$ 43,589.5	$ 36,159.0
Weighted- Average Rate
Non-interest-bearing	0.00%	0.00%
Savings	0.23%	0.09%
Interest-bearing checking and money market	0.92%	1.04%
Time deposits maturing:
Within 3 months	1.90%	1.66%
After 3 but within 6 months	2.12%	1.67%
After 6 months but within 1 year	1.77%	1.83%
After 1 but within 2 years	1.82%	1.98%
After 2 but within 3 years	2.23%	1.97%
After 3 but within 4 years	2.37%	1.63%
After 4 but within 5 years	1.09%	1.29%
After 5 years	0.99%	1.01%
Total	1.95%	1.78%
Total deposits	0.85%	0.83%
Maximum [Member]
Time deposits maturing:
After 5 years (less than $0.1 million)	$ 0.1	$ 0.1