Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Per Share data, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2011	Dec. 31, 2010
Fixed maturities - available for sale, at amortized cost	$ 11,618,929	$ 12,011,336
Equity securities - available for sale, at cost	457,075	363,283
Other invested assets, at cost	586,142	603,681
Preferred shares, par value	$ 0.01	$ 0.01
Preferred shares, authorized	50,000	50,000
Preferred shares, issued	0	0
Preferred shares, outstanding	0	0
Common shares, par value	$ 0.01	$ 0.01
Common shares, authorized	200,000	200,000
Common shares, outstanding	66,403	66,017
Accumulated other comprehensive income (loss), deferred income tax expense (benefit)	$ 127,236	$ 102,868
Treasury shares	12,614	11,589
Senior Long Term Notes [Member]
Debt instrument, interest rate, stated percentage	5.40%	5.40%
Debt instrument, maturity date	Oct. 15, 2014	Oct. 15, 2014
Long Term Notes Payable [Member]
Debt instrument, interest rate, stated percentage	6.60%	6.60%
Debt instrument, maturity date	May 01, 2067	May 01, 2067