Investments - Principal geographic distribution of municipal bonds (Details)	Dec. 31, 2021	Dec. 31, 2020
California
Municipal Bonds
Percentage of municipal bonds carrying value	11.80%	11.80%
Texas
Municipal Bonds
Percentage of municipal bonds carrying value	8.70%	9.50%
Pennsylvania
Municipal Bonds
Percentage of municipal bonds carrying value	5.40%	5.80%
New York
Municipal Bonds
Percentage of municipal bonds carrying value	5.10%	5.20%
Colorado
Municipal Bonds
Percentage of municipal bonds carrying value	4.40%	5.50%
Florida
Municipal Bonds
Percentage of municipal bonds carrying value	4.20%	5.50%