Investments, Securities with Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Available-for-sale securities, continuous unrealized loss position, Fair Value [Abstract]
Less than 12 months	$ 116,980	$ 53,282
12 months or longer	41,068	45,404
Total	158,048	98,686
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	4,613	816
12 months or longer	3,943	3,105
Total	8,556	3,921
U.S. Treasury Securities and Obligations of U.S. Government Agencies and Authorities [Member]
Available-for-sale securities, continuous unrealized loss position, Fair Value [Abstract]
Less than 12 months	15,379	12,175
12 months or longer	12,018	12,737
Total	27,397	24,912
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	445	162
12 months or longer	521	349
Total	966	511
Obligations of States and Political Subdivisions [Member]
Available-for-sale securities, continuous unrealized loss position, Fair Value [Abstract]
Less than 12 months	4,029	999
12 months or longer	0	0
Total	4,029	999
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	50	1
12 months or longer	0	0
Total	50	1
Corporate Securities [Member]
Available-for-sale securities, continuous unrealized loss position, Fair Value [Abstract]
Less than 12 months	97,572	40,108
12 months or longer	29,050	32,667
Total	126,622	72,775
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	4,118	653
12 months or longer	3,422	2,756
Total	$ 7,540	$ 3,409