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Income Taxes - Effective Tax Rate Reconciliation and Pretax Income (Loss) and Revenue by Tax Jurisdiction (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pre-tax Income Taxes and Revenue [Line Items]
Expected tax provision (benefit) at statutory rates in taxable jurisdictions									$ 490	$ 390	$ 76
Tax-exempt interest									(53)	(57)	(61)
Change in liability for uncertain tax positions									9	(2)	2
Other									(3)	3	5
Total provision (benefit) for income taxes	224us-gaap_IncomeTaxExpenseBenefit	133us-gaap_IncomeTaxExpenseBenefit	59us-gaap_IncomeTaxExpenseBenefit	27us-gaap_IncomeTaxExpenseBenefit	140us-gaap_IncomeTaxExpenseBenefit	152us-gaap_IncomeTaxExpenseBenefit	110us-gaap_IncomeTaxExpenseBenefit	(68)us-gaap_IncomeTaxExpenseBenefit	443us-gaap_IncomeTaxExpenseBenefit	334us-gaap_IncomeTaxExpenseBenefit	22us-gaap_IncomeTaxExpenseBenefit
Effective tax rate (as a percent)									28.90%	29.20%	16.50%
Income (loss) before provision for income taxes	756	488	218	69	489	536	329	(212)	1,531	1,142	132
Revenue									1,994us-gaap_Revenues	1,608us-gaap_Revenues	954us-gaap_Revenues
United States [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before provision for income taxes									1,420	1,118	218
Revenue									1,633	1,389	875
Bermuda [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before provision for income taxes									142	27	(86)
Revenue									365	219	79
United Kingdom [Member]
Pre-tax Income Taxes and Revenue [Line Items]
Income (loss) before provision for income taxes									(31)	(3)	0
Revenue									$ (4)	$ 0	$ 0