Investments (Details 2)	Mar. 31, 2019	Dec. 31, 2018
Municipal debt securities
Investments available for sale
Percentage of debt securities	100.00%	100.00%
Corporate debt securities
Investments available for sale
Percentage of debt securities	100.00%	100.00%
Financial | Corporate debt securities
Investments available for sale
Percentage of debt securities	36.10%	37.10%
Consumer, non-cyclical | Corporate debt securities
Investments available for sale
Percentage of debt securities	21.90%	20.70%
Communications | Corporate debt securities
Investments available for sale
Percentage of debt securities	11.80%	12.60%
Consumer, cyclical | Corporate debt securities
Investments available for sale
Percentage of debt securities	7.60%	7.60%
Energy | Corporate debt securities
Investments available for sale
Percentage of debt securities	5.50%	5.70%
Industrial | Corporate debt securities
Investments available for sale
Percentage of debt securities	5.00%	4.70%
Utilities | Corporate debt securities
Investments available for sale
Percentage of debt securities	4.90%	5.00%
Technology | Corporate debt securities
Investments available for sale
Percentage of debt securities	3.90%	3.10%
Basic materials | Corporate debt securities
Investments available for sale
Percentage of debt securities	3.30%	3.50%
Special revenue bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	68.90%	69.00%
General obligation bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	25.80%	26.00%
Certificate of participation bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	3.80%	3.60%
Tax allocation bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	1.00%	0.90%
Special tax bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	0.50%	0.50%