Investment Securities - Narrative (Details)		3 Months Ended		6 Months Ended
	Jan. 01, 2019 USD ($)	Jun. 30, 2019 USD ($) Investments	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) Investments	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($) Investments
Schedule of Held-to-maturity Securities [Line Items]
Investment securities classified as held to maturity		$ 98,800,000		$ 98,800,000		$ 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		397		397		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		153,600,000		153,600,000
Fair value of investment securities callable after one year but within five years		153,900,000		153,900,000
Callable structured notes amortized costs		2,000,000.0		2,000,000.0
Callable structured notes fair value		$ 2,000,000		$ 2,000,000