Investment Securities - Narrative (Details)		3 Months Ended		9 Months Ended
	Jan. 01, 2019 USD ($)	Sep. 30, 2019 USD ($) Investments	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) Investments	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) Investments
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity		$ 94,900,000		$ 94,900,000		$ 406,800,000
Amortized cost of held to maturity securities transferred	$ 281,100,000
Fair value of held to maturity securities	$ 275,000,000.0
Unrealized loss				$ 6,000,000.0
Investment securities in an unrealized loss position (securities) | Investments		334		334		760
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		160,500,000		160,500,000
Fair value of investment securities callable after one year but within five years		160,800,000		160,800,000
Callable structured notes amortized costs		$ 1,000,000.0		$ 1,000,000.0