Investments - Additional Information (Details) $ in Thousands	Dec. 31, 2017 USD ($) security	Dec. 31, 2016 USD ($) security
Investment Holdings [Line Items]
Amortized Cost	$ 247,607	$ 211,406
Par value of mortgage-backed securities pledged to FRB	218,400	188,000
Mortgage-backed securities
Investment Holdings [Line Items]
Amortized Cost	$ 227,607	$ 196,406
Number of mortgage-backed securities with unrealized losses | security	62	48
Number of mortgage-backed securities | security	92	84
Utah Housing Corporation bonds
Investment Holdings [Line Items]
Amortized Cost	$ 20,000	$ 15,000
Ginnie Mae | Mortgage-backed securities
Investment Holdings [Line Items]
Amortized Cost	$ 110,200
Number of mortgage-backed securities with unrealized losses | security	31	21
Fannie Mae | Mortgage-backed securities
Investment Holdings [Line Items]
Amortized Cost	$ 70,800
Freddie Mac | Mortgage-backed securities
Investment Holdings [Line Items]
Amortized Cost	46,600
Aa3 Rating | Utah Housing Corporation bonds
Investment Holdings [Line Items]
Amortized Cost	$ 20,000