Investments Available for Sale (Details 2)	Mar. 31, 2016	Dec. 31, 2015
Corporate debt securities
Investments available for sale
Percentage of debt securities	100.00%	100.00%
Municipal debt securities
Investments available for sale
Percentage of debt securities	100.00%	100.00%
Special revenue bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	67.80%	70.40%
General obligation bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	27.50%	24.50%
Certificate of participation bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	3.70%	4.00%
Tax allocation bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	1.00%	1.10%
Financial | Corporate debt securities
Investments available for sale
Percentage of debt securities	42.20%	44.90%
Consumer, non-cyclical | Corporate debt securities
Investments available for sale
Percentage of debt securities	17.40%	14.80%
Energy | Corporate debt securities
Investments available for sale
Percentage of debt securities	7.90%	9.00%
Communications | Corporate debt securities
Investments available for sale
Percentage of debt securities	6.90%	7.10%
Industrial | Corporate debt securities
Investments available for sale
Percentage of debt securities	6.40%	5.20%
Consumer, cyclical | Corporate debt securities
Investments available for sale
Percentage of debt securities	6.00%	6.20%
Utilities | Corporate debt securities
Investments available for sale
Percentage of debt securities	6.00%	5.00%
Basic materials | Corporate debt securities
Investments available for sale
Percentage of debt securities	3.80%	4.00%
Technology | Corporate debt securities
Investments available for sale
Percentage of debt securities	3.40%	3.80%