Securities - Additional Information (Details) - USD ($)	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract]
Available-for-sale securities pledged	$ 1,700,000,000			$ 1,700,000,000		$ 1,200,000,000
Held-to-maturity securities pledged as collateral	0			0		0
Realized gains on securities available-for-sale	0		$ 0	0	$ 0
Proceeds from sales of securities available-for-sale		$ 93,300,000		93,332,000	0
Other-than-temporary securities impairment (OTTI)	$ 0		$ 0	$ 0	$ 0	$ 0