Investments - Concentration of Credit Risk (Details)	Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract]
Insurance	16.00%
Electric utilities	12.00%
Oil and natural gas pipelines	7.00%
Banks	6.00%
Transportation	4.00%
Oil and natural gas exploration and production	4.00%
Chemicals	4.00%
Real estate investment trusts	4.00%
Food	3.00%
Metals and mining	3.00%