Investments, Securities with Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Available-for-sale securities, continuous unrealized loss position, Fair Value [Abstract]
Less than 12 months	$ 98,721	$ 53,282
12 months or longer	71,746	45,404
Total	170,467	98,686
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	4,429	816
12 months or longer	6,610	3,105
Total	11,039	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	3,923	12,175
12 months or longer	20,812	12,737
Total	24,735	24,912
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	178	162
12 months or longer	1,271	349
Total	1,449	511
Obligations of States and Political Subdivisions [Member]
Available-for-sale securities, continuous unrealized loss position, Fair Value [Abstract]
Less than 12 months	4,402	999
12 months or longer	0	0
Total	4,402	999
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	135	1
12 months or longer	0	0
Total	135	1
Corporate Securities [Member]
Available-for-sale securities, continuous unrealized loss position, Fair Value [Abstract]
Less than 12 months	90,396	40,108
12 months or longer	50,934	32,667
Total	141,330	72,775
Available-for-sale Securities, Continuous Unrealized Loss Position, Unrealized Losses [Abstract]
Less than 12 months	4,116	653
12 months or longer	5,339	2,756
Total	$ 9,455	$ 3,409