Consolidated Balance Sheets (Parenthetical) (USD $) In Thousands, except Share data	Jun. 30, 2011	Dec. 31, 2010
Assets
Fixed maturities, at amortized cost	$ 4,539,998	$ 4,772,037
Short-term investments, at amortized cost	725,230	273,444
Other investments, at amortized cost	$ 15,018	$ 18,392
Shareholders' equity
Common shares, par value	$ 0.175	$ 0.175
Common shares, shares authorized	571,428,571	571,428,571
Common shares, shares issued	133,795,913	132,838,111
Common shares, shares outstanding	98,763,928	98,001,226
Treasury stock, common shares	35,031,985	34,836,885