Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Fixed maturities - available for sale, at amortized cost	$ 11,731,173	$ 12,011,336
Equity securities - available for sale, at cost	463,620	363,283
Other invested assets, at cost	558,232	603,681
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	66,455,000	66,017,000
Accumulated other comprehensive income (loss), deferred income tax expense (benefit)	$ 112,969	$ 102,868
Treasury shares	12,719,000	11,589,000
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