Investment Securities - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) secuirty	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Gain (Loss) on Securities [Line Items]
Trading securities	$ 0
Securities pledged as collateral	914,500,000	$ 688,200,000
Sale of investment securities available for sale	7,012,000	457,046,000	$ 201,580,000
Sale of investment securities available for sale in 2016	100,000	2,100,000	2,400,000
Losses from sale of available-for-sale securities (less than during the current quarter end)	0	100,000	100,000
Proceeds from Sale of Debt Securities, Available-for-sale1			201,800,000
Proceeds from sale of equity securities	6,200,000	0	0
Realized gain on sale of equity investment	3,757,000	0	$ 0
Realized losses on equity securities	0
Unamortized premiums	12,700,000	14,100,000
Unaccreted discounts	2,500,000	1,300,000
Owned investment securities	940,800,000
Total unrealized losses on securities	$ 23,800,000
Number of securities, below investment grade | secuirty	1
Available-for-sale securities, below investment grade, fair value	$ 600,000
Weighted average duration of MBS portfolio	4 years 8 months 12 days
Collateralized Mortgage Backed Securities
Gain (Loss) on Securities [Line Items]
OTTI on evaluation of securities	$ 0	$ 0