Investments - Narrative (Details) $ in Thousands	3 Months Ended
	Jun. 30, 2018 USD ($) security	Sep. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Amortized cost		$ 181,785	$ 247,607
Ginnie Mae
Debt Securities, Available-for-sale [Line Items]
Amortized cost		68,000
Fannie Mae
Debt Securities, Available-for-sale [Line Items]
Amortized cost		47,000
Freddie Mac
Debt Securities, Available-for-sale [Line Items]
Amortized cost		44,000
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost		$ 159,397	$ 227,607
Number of mortgage-backed securities sold | security	9
Securities sold to align the portfolio with the Community Reinvestment Act requirements	$ 41,000
Recognized loss on securities	$ 2,000
Number of mortgage-backed securities with unrealized losses | security		75	62
Number of mortgage-backed securities | security		84	92
Par value of mortgage-backed securities pledged		$ 153,000	$ 218,000
Mortgage-backed securities | Ginnie Mae
Debt Securities, Available-for-sale [Line Items]
Number of mortgage-backed securities with unrealized losses | security		35	31
Utah Housing Corporation bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost		$ 22,388	$ 20,000
Moody's, Aa3 Rating | Utah Housing Corporation bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis		$ 22,000	$ 20,000