Investments - Gross Unrealized Losses and Fair Value for Mortgage-Backed in Unrealized Loss Position (Details) - Mortgage-backed securities - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss [Abstract]
Less than 12 months, Gross unrealized losses	$ (827)	$ (27)
12 months or more, Gross unrealized losses	(1,554)	(1,465)
Total, Gross unrealized losses	(2,381)	(1,492)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value [Abstract]
Less than 12 months, Estimated fair value	73,802	12,147
12 months or more, Estimated fair value	39,271	41,462
Total, Estimated fair value	$ 113,073	$ 53,609