Investments, Securities with Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale securities, continuous unrealized loss position, Fair Value [Abstract]
Less than 12 months	$ 125,291	$ 53,282
12 months or longer	44,324	45,404
Total	169,615	98,686
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	6,509	816
12 months or longer	4,491	3,105
Total	11,000	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	11,636	12,175
12 months or longer	14,630	12,737
Total	26,266	24,912
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	359	162
12 months or longer	730	349
Total	1,089	511
Obligations of States and Political Subdivisions [Member]
Available-for-sale securities, continuous unrealized loss position, Fair Value [Abstract]
Less than 12 months	3,982	999
12 months or longer	0	0
Total	3,982	999
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	88	1
12 months or longer	0	0
Total	88	1
Corporate Securities [Member]
Available-for-sale securities, continuous unrealized loss position, Fair Value [Abstract]
Less than 12 months	109,673	40,108
12 months or longer	29,694	32,667
Total	139,367	72,775
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	6,062	653
12 months or longer	3,761	2,756
Total	$ 9,823	$ 3,409