Investments - Narrative (Details) $ in Thousands	3 Months Ended
	Jun. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 185,749	$ 247,607
Available-for-sale investments	178,145	244,088
Ginnie Mae
Debt Securities, Available-for-sale [Line Items]
Amortized cost	72,000
Fannie Mae
Debt Securities, Available-for-sale [Line Items]
Amortized cost	49,000
Freddie Mac
Debt Securities, Available-for-sale [Line Items]
Amortized cost	46,000
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 167,203	227,607
Number of mortgage-backed securities sold | security	9
Securities sold to align the portfolio with the Community Reinvestment Act requirements	$ 41,000
Available-for-sale investments	160,416	$ 225,047
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	$ 161,000	$ 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	$ 18,546	$ 20,000
Available-for-sale investments	17,729	19,041
Moody's, Aa3 Rating | Utah Housing Corporation bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost basis	$ 19,000	$ 20,000