Investments - Summary of Municipal Debt Securities and Corporate Debt Securities (Details)	Dec. 31, 2020	Dec. 31, 2019
Municipal debt securities
Debt Securities, Available-for-sale [Line Items]
Percentage of debt securities	100.00%	100.00%
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Percentage of debt securities	100.00%	100.00%
Special revenue bonds | Municipal debt securities
Debt Securities, Available-for-sale [Line Items]
Percentage of debt securities	76.80%	74.50%
General obligation bonds | Municipal debt securities
Debt Securities, Available-for-sale [Line Items]
Percentage of debt securities	20.30%	21.30%
Certificate of participation bonds | Municipal debt securities
Debt Securities, Available-for-sale [Line Items]
Percentage of debt securities	2.30%	3.40%
Tax allocation bonds | Municipal debt securities
Debt Securities, Available-for-sale [Line Items]
Percentage of debt securities	0.60%	0.80%
Financial | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Percentage of debt securities	34.90%	34.40%
Consumer, non-cyclical | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Percentage of debt securities	19.10%	20.10%
Communications | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Percentage of debt securities	9.30%	10.30%
Energy | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Percentage of debt securities	8.20%	8.30%
Consumer, cyclical | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Percentage of debt securities	8.00%	7.60%
Technology | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Percentage of debt securities	6.10%	4.80%
Utilities | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Percentage of debt securities	5.90%	6.20%
Industrial | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Percentage of debt securities	5.30%	4.20%
Basic materials | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Percentage of debt securities	3.10%	4.10%
Government | Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Percentage of debt securities	0.10%	0.00%