Investment Securities - Investments In Private Equity Funds Subject to Lock-Up Periods (Details) - Private equity funds $ in Millions	Sep. 30, 2020 USD ($)
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	$ 118.2
1 – 3 years
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	4.7
3 – 5 years
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	36.4
5 – 10 years
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	71.7
Greater than 10 years
Debt and Equity Securities, FV-NI [Line Items]
Fair Value	$ 5.4