Investments - Unrealized Losses and Fair Value for Mortgage-Backed Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Less than 12 months
Gross Unrealized Losses	$ (1,568)	$ (849)
Estimated Fair Value	49,747	82,279
12 months or more
Gross Unrealized Losses	(6,155)	(3,320)
Estimated Fair Value	124,911	124,618
Gross Unrealized Losses	(7,723)	(4,169)
Estimated Fair Value	174,658	206,897
Mortgage-backed securities
Less than 12 months
Gross Unrealized Losses	(1,568)	(772)
Estimated Fair Value	49,747	77,356
12 months or more
Gross Unrealized Losses	(5,338)	(2,438)
Estimated Fair Value	107,182	110,500
Gross Unrealized Losses	(6,906)	(3,210)
Estimated Fair Value	156,929	187,856
Utah Housing Corporation bonds
Less than 12 months
Gross Unrealized Losses	0	(77)
Estimated Fair Value	0	4,923
12 months or more
Gross Unrealized Losses	(817)	(882)
Estimated Fair Value	17,729	14,118
Gross Unrealized Losses	(817)	(959)
Estimated Fair Value	$ 17,729	$ 19,041