Investments - Gross Unrealized Losses and Fair Value for Mortgage-Backed in Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months, Gross Unrealized Losses	$ (849)	$ (3,113)
12 months or more, Gross Unrealized Losses	(3,320)	(619)
Total, Gross Unrealized Losses	(4,169)	(3,732)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months, Estimated Fair Value	82,279	143,859
12 months or more, Estimated Fair Value	124,618	10,885
Total, Estimated Fair Value	206,897	154,744
Mortgage-backed securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months, Gross Unrealized Losses	(772)	(2,423)
12 months or more, Gross Unrealized Losses	(2,438)	(619)
Total, Gross Unrealized Losses	(3,210)	(3,042)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months, Estimated Fair Value	77,356	129,549
12 months or more, Estimated Fair Value	110,500	10,885
Total, Estimated Fair Value	187,856	140,434
Utah Housing Corporation bonds
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months, Gross Unrealized Losses	(77)	(690)
12 months or more, Gross Unrealized Losses	(882)	0
Total, Gross Unrealized Losses	(959)	(690)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months, Estimated Fair Value	4,923	14,310
12 months or more, Estimated Fair Value	14,118	0
Total, Estimated Fair Value	$ 19,041	$ 14,310