Statutory Capital and Surplus (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Statutory Accounting Practices [Line Items]
Total amount of restricted net assets for the Company's consolidated subsidiaries	$ 2,499,863
Bermuda [Member]
Statutory Accounting Practices [Line Items]
Statutory shareholders' surplus	3,272,786	3,184,294
Minimum required statutory shareholders' surplus	718,344	591,050
Statutory net income (loss)	528,712	197,624	666,372
Maximum amount of ordinary dividends or distributions that can be paid without prior authorization	818,196
United States [Member]
Statutory Accounting Practices [Line Items]
Statutory shareholders' surplus	1,110,468	1,054,441
Minimum required statutory shareholders' surplus	173,078	146,284
Statutory net income (loss)	25,390	44,974	63,304
Maximum amount of ordinary dividends or distributions that can be paid without prior authorization	86,849
Ireland [Member]
Statutory Accounting Practices [Line Items]
Statutory shareholders' surplus	412,480	403,156
Minimum required statutory shareholders' surplus	19,513	22,057
Statutory net income (loss)	9,325	14,590	9,378
Maximum amount of ordinary dividends or distributions that can be paid without prior authorization	31,260
Switzerland [Member]
Statutory Accounting Practices [Line Items]
Statutory shareholders' surplus	174,259	57,195
Minimum required statutory shareholders' surplus	19,945	19,949
Statutory net income (loss)	10,657	(637)
Maximum amount of ordinary dividends or distributions that can be paid without prior authorization	8,276
United Kingdom [Member]
Statutory Accounting Practices [Line Items]
Statutory shareholders' surplus	224,854	91,000
Minimum required statutory shareholders' surplus	224,531	90,500
Statutory net income (loss)	1,800	290	(10,320)
Maximum amount of ordinary dividends or distributions that can be paid without prior authorization	$ 0