Securities - Narrative (Details)	3 Months Ended	6 Months Ended
	Mar. 31, 2017 USD ($)	Jun. 30, 2018 USD ($) security institution	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Available for sale debt securities	$ 2,946,000	$ 1,106,028,000	$ 674,104,000
Sale proceeds	$ 2,946,000
Fair Value		$ 1,088,779,000	671,488,000
Number of institutions issuing debt | institution		160
Trust preferred securities
Schedule of Available-for-sale Securities [Line Items]
Available for sale debt securities		$ 12,402,000	12,442,000
Fair Value		$ 10,401,000	9,388,000
Number of securities representing interests in tranches of trusts (tranches) | security		2
Impairments		$ 0
Secure government, public and trust deposits
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities pledged as collateral		443,011,000	217,867,000
Short-term borrowings
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities pledged as collateral		$ 18,278,000	$ 25,888,000