Investments - Gross Unrealized Losses and Fair Value for Mortgage-Backed in Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months, Gross Unrealized Losses	$ (3,113)
12 months or more, Gross Unrealized Losses	(619)
Total, Gross Unrealized Losses	(3,732)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months, Estimated Fair Value	143,859
12 months or more, Estimated Fair Value	10,885
Total, Estimated Fair Value	154,744
Mortgage-backed securities
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months, Gross Unrealized Losses	(2,423)	$ (827)
12 months or more, Gross Unrealized Losses	(619)	(1,554)
Total, Gross Unrealized Losses	(3,042)	(2,381)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months, Estimated Fair Value	129,549	73,802
12 months or more, Estimated Fair Value	10,885	39,271
Total, Estimated Fair Value	140,434	$ 113,073
Utah Housing Corporation bonds
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months, Gross Unrealized Losses	(690)
12 months or more, Gross Unrealized Losses	0
Total, Gross Unrealized Losses	(690)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months, Estimated Fair Value	14,310
12 months or more, Estimated Fair Value	0
Total, Estimated Fair Value	$ 14,310