Investments - Unrealized Losses and Fair Value for Mortgage-Backed Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Less than 12 months
Gross Unrealized Losses	$ (1,893)	$ (849)
Estimated Fair Value	48,128	82,279
12 months or more
Gross Unrealized Losses	(7,614)	(3,320)
Estimated Fair Value	120,893	124,618
Gross Unrealized Losses	(9,507)	(4,169)
Estimated Fair Value	169,021	206,897
Mortgage-backed securities
Less than 12 months
Gross Unrealized Losses	(1,768)	(772)
Estimated Fair Value	43,253	77,356
12 months or more
Gross Unrealized Losses	(6,278)	(2,438)
Estimated Fair Value	104,841	110,500
Gross Unrealized Losses	(8,046)	(3,210)
Estimated Fair Value	148,094	187,856
Utah Housing Corporation bonds
Less than 12 months
Gross Unrealized Losses	(125)	(77)
Estimated Fair Value	4,875	4,923
12 months or more
Gross Unrealized Losses	(1,336)	(882)
Estimated Fair Value	16,052	14,118
Gross Unrealized Losses	(1,461)	(959)
Estimated Fair Value	$ 20,927	$ 19,041