Investments Available for Sale (Details 2)	Mar. 31, 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	42.50%	40.60%
Consumer, non-cyclical | Corporate debt securities
Investments available for sale
Percentage of debt securities	16.40%	18.60%
Energy | Corporate debt securities
Investments available for sale
Percentage of debt securities	10.60%	9.30%
Communications | Corporate debt securities
Investments available for sale
Percentage of debt securities	6.70%	6.00%
Consumer, cyclical | Corporate debt securities
Investments available for sale
Percentage of debt securities	5.80%	6.30%
Utilities | Corporate debt securities
Investments available for sale
Percentage of debt securities	5.70%	6.00%
Technology | Corporate debt securities
Investments available for sale
Percentage of debt securities	5.20%	4.30%
Industrial | Corporate debt securities
Investments available for sale
Percentage of debt securities	4.70%	5.60%
Basic materials | Corporate debt securities
Investments available for sale
Percentage of debt securities	2.40%	3.30%
Special revenue bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	62.40%	63.60%
General obligation bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	31.80%	29.70%
Certificate of participation bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	4.50%	4.90%
Tax allocation bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	0.70%	1.10%
Special tax bonds | Municipal debt securities
Investments available for sale
Percentage of debt securities	0.60%	0.70%