CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) shares in Thousands	Jun. 30, 2021	Dec. 31, 2020
Statement of Financial Position [Abstract]
Fixed maturities, available for sale, amortized cost	$ 11,638,359,000	$ 11,566,930,000
Fixed maturities, available for sale, allowance for credit loss	84,000	323,000
Fixed maturities, held to maturity, fair value	403,034,000
Fixed maturities, held to maturity, allowance for credit loss	0
Equity securities, cost	472,214,000	421,744,000
Credit losses associated with commercial mortgage loans	0	0
Allowance for expected credit losses on insurance and reinsurance premium balances receivable	9,898,000	8,836,000
Allowance for expected credit losses on reinsurance recoverable for unpaid losses and loss expenses	$ 26,209,000	$ 23,711,000
Common shares, shares issued (in shares)	176,580	176,580
Common shares, shares outstanding (in shares)	84,767	84,353
Treasury shares, shares (in shares)	91,813	92,227