Borrowing Arrangements - Bank Line of Credit (Details) - Revolving line of credit	Jun. 30, 2021 USD ($)
Bank line of credit
Maximum borrowing capacity under the bank revolving line of credit	$ 100,000,000
Maximum letter of credit sublimit under the bank revolving line of credit	25,000,000
Available borrowing capacity under the bank revolving line of credit, due to outstanding letters of credit	99,100,000
Outstanding amount of letters of credit under the bank revolving line of credit	900,000
Available amount of letters of credit under the bank revolving line of credit	24,100,000
Borrowings outstanding under the bank revolving line of credit	0
Fair value of investments pledged as collateral on the bank revolving line of credit	$ 123,400,000