Investments Available for Sale (Details 2)	Sep. 30, 2017	Dec. 31, 2016
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	44.30%	40.60%
Consumer, non-cyclical | Corporate debt securities
Investments available for sale
Percentage of debt securities	15.20%	18.60%
Energy | Corporate debt securities
Investments available for sale
Percentage of debt securities	8.50%	9.30%
Communications | Corporate debt securities
Investments available for sale
Percentage of debt securities	7.90%	6.00%
Industrial | Corporate debt securities
Investments available for sale
Percentage of debt securities	6.60%	5.60%
Utilities | Corporate debt securities
Investments available for sale
Percentage of debt securities	5.90%	6.00%
Consumer, cyclical | Corporate debt securities
Investments available for sale
Percentage of debt securities	5.60%	6.30%
Technology | Corporate debt securities
Investments available for sale
Percentage of debt securities	3.70%	4.30%
Basic materials | Corporate debt securities
Investments available for sale
Percentage of debt securities	2.30%	3.30%
Special revenue bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	63.70%	63.60%
General obligation bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	30.80%	29.70%
Certificate of participation bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	4.30%	4.90%
Tax allocation bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	0.60%	1.10%
Special tax bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	0.60%	0.70%