CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) shares in Thousands	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Dec. 31, 2018
Statement of Financial Position [Abstract]
Fixed maturities, available for sale, amortized cost	$ 12,201,832,000		$ 12,263,240,000
Fixed maturities, available for sale, allowance for credit loss	2,498,000	$ 6,257,000	0	$ 0	$ 0	$ 0
Equity securities, cost	356,176,000		398,956,000
Credit losses associated with commercial mortgage loans	0		0
Allowance for credit loss	10,727,000		7,000,000
Allowance for credit losses on reinsurance recoverable for unpaid losses and loss expenses	$ 22,825,000		$ 18,000,000
Common shares, shares issued (in shares)	176,580	176,580	176,580	176,580	176,580	176,580
Common shares, shares outstanding (in shares)	84,309		83,959	83,947
Treasury shares, shares (in shares)	92,271	92,274	92,621	92,633	92,633	92,994