Investments Available for Sale (Details 2)	Dec. 31, 2016	Dec. 31, 2015
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	40.60%	44.90%
Consumer, non-cyclical | Corporate debt securities
Investments available for sale
Percentage of debt securities	18.60%	14.80%
Energy | Corporate debt securities
Investments available for sale
Percentage of debt securities	9.30%	9.00%
Consumer, cyclical | Corporate debt securities
Investments available for sale
Percentage of debt securities	6.30%	6.20%
Utilities | Corporate debt securities
Investments available for sale
Percentage of debt securities	6.00%	5.00%
Communications | Corporate debt securities
Investments available for sale
Percentage of debt securities	6.00%	7.10%
Industrial | Corporate debt securities
Investments available for sale
Percentage of debt securities	5.60%	5.20%
Technology | Corporate debt securities
Investments available for sale
Percentage of debt securities	4.30%	3.80%
Basic materials | Corporate debt securities
Investments available for sale
Percentage of debt securities	3.30%	4.00%
Special revenue bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	63.60%	70.40%
General obligation bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	29.70%	24.50%
Certificate of participation bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	4.90%	4.00%
Tax allocation bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	1.10%	1.10%
Special tax bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	0.70%	0.00%