Investments - Principal geographic distribution of municipal bonds (Details)	Dec. 31, 2020	Dec. 31, 2019
Texas
Municipal Bonds
Percentage of municipal bonds carrying value	11.00%	12.70%
California
Municipal Bonds
Percentage of municipal bonds carrying value	12.80%	8.60%
Colorado
Municipal Bonds
Percentage of municipal bonds carrying value	4.80%	5.80%
Washington
Municipal Bonds
Percentage of municipal bonds carrying value	4.00%	5.50%
New York
Municipal Bonds
Percentage of municipal bonds carrying value	5.30%	3.70%