Investment Securities - Narrative (Details)	3 Months Ended		9 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($) Investments	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2019 USD ($) Investments
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity	$ 67,900,000			$ 92,300,000
Investment securities in an unrealized loss position (securities) | Investments	142			331
Impairment loss	$ 0	$ 0	$ 0	$ 0
Callable Within One Year
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost of investment securities callable after one year but within five years	245,000,000.0
Fair value of investment securities callable after one year but within five years	245,700,000
Callable structured notes amortized costs	$ 0