Cash and Cash Equivalents and Securities - Summary of Amortized Cost and Fair Value to Contractual Maturity (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Available for Sale, Amortized Cost, Within 1 year	$ 200.8
Available for Sale, Amortized Cost, After 1 but within 5 years	613.7
Available for Sale, Amortized Cost, After 5 but within 10 years	880.8
Available for Sale, Amortized Cost, After 10 years	2,461.1
Amortized Cost	4,156.4	$ 3,545.8
Available for Sale, Fair Value, Within 1 year	201.4
Available for Sale, Fair Value, After 1 but within 5 years	633.1
Available for Sale, Fair Value, After 5 but within 10 years	918.9
Available for Sale, Fair Value, After 10 years	2,523.2
Available for Sale, Fair Value, Total	4,276.6	3,564.3
Held to Maturity, Amortized Cost, Within 1 year	16.5
Held to Maturity, Amortized Cost, After 1 but within 5 years	1,089.2
Held to Maturity, Amortized Cost, After 5 but within 10 years	637.6
Held to Maturity, Amortized Cost, After 10 years	2,120.1
Amortized Cost	3,863.4	3,869.2
Held to Maturity, Fair Value, Within 1 year	16.5
Held to Maturity, Fair Value, After 1 but within 5 years	1,136.1
Held to Maturity, Fair Value, After 5 but within 10 years	672.6
Held to Maturity, Fair Value, After 10 years	2,240.0
Held to Maturity, Fair Value, Total	4,065.2	4,020.0
U.S. Treasury and agency
Schedule of Investments [Line Items]
Available for Sale, Amortized Cost, Within 1 year	200.8
Available for Sale, Amortized Cost, After 1 but within 5 years	530.5
Amortized Cost	731.3	689.5
Available for Sale, Fair Value, Within 1 year	201.4
Available for Sale, Fair Value, After 1 but within 5 years	545.8
Available for Sale, Fair Value, Total	747.2	687.1
Held to Maturity, Amortized Cost, Within 1 year	0.0
Held to Maturity, Amortized Cost, After 1 but within 5 years	0.0
Amortized Cost	0.0
Held to Maturity, Fair Value, Within 1 year	0.0
Held to Maturity, Fair Value, After 1 but within 5 years	0.0
Held to Maturity, Fair Value, Total	0.0
GSE mortgage-backed securities
Schedule of Investments [Line Items]
Available for Sale, Amortized Cost, After 1 but within 5 years	83.2
Available for Sale, Amortized Cost, After 5 but within 10 years	880.8
Available for Sale, Amortized Cost, After 10 years	2,461.1
Amortized Cost	3,425.1	2,856.3
Available for Sale, Fair Value, After 1 but within 5 years	87.3
Available for Sale, Fair Value, After 5 but within 10 years	918.9
Available for Sale, Fair Value, After 10 years	2,523.2
Available for Sale, Fair Value, Total	3,529.4	2,877.2
Held to Maturity, Amortized Cost, After 1 but within 5 years	841.2
Held to Maturity, Amortized Cost, After 5 but within 10 years	77.5
Held to Maturity, Amortized Cost, After 10 years	272.1
Amortized Cost	1,190.8	1,271.4
Held to Maturity, Fair Value, After 1 but within 5 years	879.3
Held to Maturity, Fair Value, After 5 but within 10 years	82.4
Held to Maturity, Fair Value, After 10 years	278.8
Held to Maturity, Fair Value, Total	1,240.5	1,279.1
State and municipal
Schedule of Investments [Line Items]
Available for Sale, Amortized Cost, Within 1 year	0.0
Available for Sale, Amortized Cost, After 1 but within 5 years	0.0
Available for Sale, Amortized Cost, After 5 but within 10 years	0.0
Available for Sale, Amortized Cost, After 10 years	0.0
Amortized Cost	0.0
Available for Sale, Fair Value, Within 1 year	0.0
Available for Sale, Fair Value, After 1 but within 5 years	0.0
Available for Sale, Fair Value, After 5 but within 10 years	0.0
Available for Sale, Fair Value, After 10 years	0.0
Available for Sale, Fair Value, Total	0.0
Held to Maturity, Amortized Cost, Within 1 year	10.0
Held to Maturity, Amortized Cost, After 1 but within 5 years	245.5
Held to Maturity, Amortized Cost, After 5 but within 10 years	471.4
Held to Maturity, Amortized Cost, After 10 years	1,848.0
Amortized Cost	2,574.9	2,503.9
Held to Maturity, Fair Value, Within 1 year	10.0
Held to Maturity, Fair Value, After 1 but within 5 years	254.3
Held to Maturity, Fair Value, After 5 but within 10 years	499.4
Held to Maturity, Fair Value, After 10 years	1,961.2
Held to Maturity, Fair Value, Total	2,724.9	2,645.5
Corporate
Schedule of Investments [Line Items]
Available for Sale, Amortized Cost, Within 1 year	0.0
Available for Sale, Amortized Cost, After 1 but within 5 years	0.0
Available for Sale, Amortized Cost, After 5 but within 10 years	0.0
Amortized Cost	0.0
Available for Sale, Fair Value, Within 1 year	0.0
Available for Sale, Fair Value, After 1 but within 5 years	0.0
Available for Sale, Fair Value, After 5 but within 10 years	0.0
Available for Sale, Fair Value, Total	0.0
Held to Maturity, Amortized Cost, Within 1 year	5.0
Held to Maturity, Amortized Cost, After 1 but within 5 years	2.5
Held to Maturity, Amortized Cost, After 5 but within 10 years	88.7
Amortized Cost	96.2	92.4
Held to Maturity, Fair Value, Within 1 year	5.0
Held to Maturity, Fair Value, After 1 but within 5 years	2.5
Held to Maturity, Fair Value, After 5 but within 10 years	90.8
Held to Maturity, Fair Value, Total	98.3	93.9
Other
Schedule of Investments [Line Items]
Available for Sale, Amortized Cost, Within 1 year	0.0
Amortized Cost	0.0
Available for Sale, Fair Value, Within 1 year	0.0
Available for Sale, Fair Value, Total	0.0
Held to Maturity, Amortized Cost, Within 1 year	1.5
Amortized Cost	1.5	1.5
Held to Maturity, Fair Value, Within 1 year	1.5
Held to Maturity, Fair Value, Total	$ 1.5	$ 1.5