Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Fixed maturities - available for sale, at market value, amortized cost	$ 12,831,159	$ 12,391,164
Equity securities - available for sale, at market value, cost	148,326	148,342
Other invested assets, at cost	601,925	508,447
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	68,336,000	67,965,000
Accumulated other comprehensive income (loss), net of deferred income tax expense (benefit)	$ 20,715	$ 57,661
Treasury shares	23,650,000	20,422,000
Long Term Subordinated Notes [Member]
Debt instrument, stated interest rate	6.60%	6.60%
Debt instrument, maturity date	May 01, 2067	May 01, 2067
4.868% Senior notes due 6/1/2044 [Member] | Senior Notes [Member]
Debt instrument, stated interest rate	4.868%	0.00%
Debt instrument, maturity date	Jun. 01, 2044	Jun. 01, 2044
5.4% Senior notes due 10/15/2014 [Member] | Senior Notes [Member]
Debt instrument, stated interest rate	5.40%	5.40%
Debt instrument, maturity date	Oct. 15, 2014	Oct. 15, 2014