Subsequent Events (Details) $ in Thousands, shares in Millions	Oct. 15, 2018 USD ($) office shares	Sep. 30, 2018 USD ($) office	Dec. 31, 2017 USD ($)
Subsequent Event [Line Items]
Number of branch locations acquired | office		58
Assets		$ 3,363,769	$ 3,144,360
First Security
Subsequent Event [Line Items]
Assets		572,000
Deposits		432,000
Loans		$ 406,000
Subsequent Event | First Security
Subsequent Event [Line Items]
Number of branch locations acquired | office	11
Shares of common stock issued for acquisition (in shares) | shares	2.0
Cash consideration to shareholders	$ 31,200