Taxation (Details) - Components of deferred tax assets and liabilities (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Deferred tax assets:
Net unpaid loss reserve discount	$ 108,285	$ 108,981
Unearned premium	70,071	62,162
Compensation liabilities	56,245	35,830
Net operating loss	83,061	263,600
Investment adjustments	12,927	12,861
Pension	11,976	9,753
Bad debt reserve	14,901	10,829
Amortizable goodwill	8,958	10,732
Net unrealized depreciation on investments	38,652	320
Stock options	13,035	13,842
Depreciation	1,910	4,072
Net realized capital losses	98,971	119,307
Deferred intercompany capital losses	97,566	117,892
Other	9,392	7,573
Deferred tax asset, gross of valuation allowance	625,950	777,754
Valuation allowance	297,637	497,236
Deferred tax asset, net of valuation allowance	328,313	280,518
Deferred tax liabilities:
Net unrealized appreciation on investments	106,520	96,797
Deferred acquisition costs	16,135	12,787
Currency translation adjustments	11,225	8,062
Regulatory reserves	146,101	125,249
Untaxed Lloyd's result	10,155	8,340
Other	13,838	4,786
Deferred tax liability	303,974	256,021
Net deferred tax assets	$ 24,339	$ 24,497