Consolidated Balance Sheets (Parenthetical) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019
Fixed maturities - available for sale, at market value, amortized cost	$ 17,131,414,000	$ 16,473,491,000
Fixed maturities - available for sale, at market value, allowance for credit loss	19,641,000	0
Short-term investments, at cost	1,221,198,000	414,639,000
Other invested assets, at cost	$ 1,911,757,000	$ 1,763,531,000
Preferred shares, par value	$ 0.01	$ 0.01
Preferred shares, authorized	50,000,000	50,000,000
Preferred shares, issued	0	0
Preferred shares, outstanding	0	0
Common shares, par value	$ 0.01	$ 0.01
Common shares, authorized	200,000,000	200,000,000
Common shares, outstanding	69,603,000	69,464,000
Accumulated other comprehensive income (loss), net of deferred income tax expense (benefit)	$ 74,481,000	$ 30,996,000
Treasury shares	29,636,000	28,665,000
Senior notes due 6/1/2044 [Member] | Senior Notes [Member]
Debt instrument, maturity date	Jun. 01, 2044
Long term notes due 05/01/2067 [Member] | Subordinated Debt [Member]
Debt instrument, maturity date	May 01, 2067