Investments - Narrative (Details) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2013 positions		Sep. 30, 2012	Sep. 30, 2013 positions		Sep. 30, 2012	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Reinsurance Recoverables on Unpaid Losses	$ 3,352,667,000			$ 3,352,667,000			$ 3,382,102,000
Percentage of Gross Unrealized losses in Fixed income securities portfolio related to securities below investment grade or not rated	16.70%			16.70%			37.70%
Fixed maturities, at fair value	27,282,442,000			27,282,442,000			27,846,920,000
Investments In Structured Transactions Not Carried at Fair Value	293,900,000			293,900,000			312,100,000
Fixed maturities, held to maturity	2,802,111,000			2,802,111,000			2,814,447,000
Fair value of held to maturity securities	3,116,940,000			3,116,940,000			3,262,804,000
Amortized cost of fixed maturities	26,772,143,000			26,772,143,000			26,544,806,000
Equity securities, at fair value	974,733,000			974,733,000			649,359,000
Cost of equity securities	878,679,000			878,679,000			617,486,000
Percentage of Fixed Income Investment portfolio invested in below investment grade or not rated	2.50%			2.50%			2.90%
Pledged Assets Separately Reported, Other Debt Securities Available-for-sale or Held-for-investment	16,000,000,000			16,000,000,000			16,900,000,000
Available for sale securities, Gross unrealized losses	402,339,000			402,339,000			282,812,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	2,037			2,037
Available-for-sale Securities, Qualitative Disclosure, Number of Positions	7,602			7,602
Fixed maturites held to maturity, gross unrealized losses	2,786,000			2,786,000			2,681,000
Held-to-maturity, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	5			5
Held-to-maturity Securities, Qualitative Disclosure, Number of Positions	204			204
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Securities Sold	17,341,000		33,114,000	78,960,000		87,092,000
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities	3,187,000		12,629,000	10,214,000		61,787,000
Reinsurance Receivables	97,100,000			97,100,000			58,428,000
Government Related Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities Cost Or Amortized Cost	2,373,400,000			2,373,400,000			1,912,700,000
Marketable Securities	2,414,700,000			2,414,700,000			1,988,500,000
Agency Securities
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities Cost Or Amortized Cost	268,700,000			268,700,000			404,300,000
Marketable Securities	290,600,000			290,600,000			446,700,000
Covered bonds
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, at fair value	560,700,000			560,700,000			647,100,000
Fixed maturities, held to maturity	8,300,000			8,300,000			8,400,000
Fair value of held to maturity securities	8,300,000			8,300,000			8,600,000
Amortized cost of fixed maturities	531,400,000			531,400,000			605,400,000
Corporate
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, at fair value	10,759,332,000	[1],[2]		10,759,332,000	[1],[2]		10,482,000,000	[3],[4]
Fixed maturities, held to maturity	1,370,391,000			1,370,391,000			1,425,320,000
Fair value of held to maturity securities	1,506,281,000			1,506,281,000			1,615,397,000
Amortized cost of fixed maturities	10,395,005,000	[1],[2]		10,395,005,000	[1],[2]		9,837,962,000	[3],[4]
Fixed maturites held to maturity, gross unrealized losses	1,436,000			1,436,000			794,000
Medium term notes
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, at fair value	150,900,000			150,900,000			194,300,000
Amortized cost of fixed maturities	150,600,000			150,600,000			194,800,000
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities	1,100,000
Tier One And Upper Tier Two Securities
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities Cost Or Amortized Cost	280,200,000			280,200,000			327,600,000
Marketable Securities	271,500,000			271,500,000			308,500,000
Available For Sale Securities Continuous Unrealized Loss Position Aggregate Losses Accumulated In Investments	8,800,000			8,800,000			19,100,000
Fixed income funds
Schedule of Available-for-sale Securities [Line Items]
Equity securities, at fair value	89,700,000			89,700,000			101,900,000
Cost of equity securities	100,000,000			100,000,000			100,000,000
Structured Credit Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities	4,200,000			4,200,000
Impaired Investment Cumulative Fair Value Decline Percentage Amortized Cost	50.00%			50.00%
Marketable Securities Unrealized Loss Other Than Temporary Impairments Amount	13,000,000			13,000,000
RMBS - Non-Agency
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, at fair value	399,399,000			399,399,000			647,733,000
Fixed maturities, held to maturity	65,542,000			65,542,000			83,205,000
Fair value of held to maturity securities	71,122,000			71,122,000			93,707,000
Amortized cost of fixed maturities	407,789,000			407,789,000			678,469,000
Fixed maturites held to maturity, gross unrealized losses	0			0			0
Marketable Securities Unrealized Loss Other Than Temporary Impairments Amount	700,000			700,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Securities Sold	3,100,000		11,600,000	63,300,000		38,300,000
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities	2,100,000
CMBS
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, at fair value	1,197,881,000			1,197,881,000			1,077,251,000
Fixed maturities, held to maturity	141,884,000			141,884,000			12,751,000
Fair value of held to maturity securities	156,182,000			156,182,000			14,799,000
Amortized cost of fixed maturities	1,166,285,000			1,166,285,000			1,010,794,000
Fixed maturites held to maturity, gross unrealized losses	0			0			0
Marketable Securities Unrealized Loss Other Than Temporary Impairments Amount	500,000			500,000
Core Cdo Holdings
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities Unrealized Loss Other Than Temporary Impairments Amount	$ 11,700,000			$ 11,700,000

[1] Included within Corporate are Tier One and Upper Tier Two securities, representing committed term debt and hybrid instruments, which are senior to the common and preferred equities of the financial institutions. These securities have a fair value of $271.5 million and an amortized cost of $280.2 million at September 30, 2013.

[2] Included within Corporate are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes have a fair value of $150.9 million and an amortized cost of $150.6 million. These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate values of these notes.

[3] Included within Corporate are certain medium term notes supported primarily by pools of European investment grade credit with varying degrees of leverage. The notes have a fair value of $194.3 million and an amortized cost of $194.8 million. These notes allow the investor to participate in cash flows of the underlying bonds including certain residual values, which could serve to either decrease or increase the ultimate values of these notes.

[4] Included within Corporate are Tier One and Upper Tier Two securities, representing committed term debt and hybrid instruments, which are senior to the common and preferred equities of the financial institutions. These securities have a fair value of $308.5 million and an amortized cost of $327.6 million at December 31, 2012.