Investments - Narrative (Details) (USD $)	3 Months Ended
	Mar. 31, 2013 positions		Mar. 31, 2012	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Percentage of Gross Unrealized losses in Fixed income securities portfolio related to securities below investment grade or not rated	30.80%			37.70%
Fixed maturities, at fair value	$ 27,349,185,000			$ 27,846,920,000
Amortized cost of fixed maturities	26,188,523,000			26,544,806,000
Equity securities, at fair value	850,970,000			649,359,000
Cost of equity securities	790,132,000			617,486,000
Percentage of Fixed Income Investment portfolio invested in below investment grade or not rated	2.90%
Pledged Assets Separately Reported, Other Debt Securities Available-for-sale or Held-for-investment	16,500,000,000			16,900,000,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	249,700,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	1,415
Available-for-sale Securities, Qualitative Disclosure, Number of Positions	7,569
Held-to-maturity Securities, Continuous Unrealized Loss Position, Aggregate Losses	1,900,000
Held-to-maturity, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions	4
Held-to-maturity Securities, Qualitative Disclosure, Number of Positions	204
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Securities Sold	6,475,000		17,195,000
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities	4,637,000		20,965,000
Government Related Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities Cost Or Amortized Cost	2,079,000,000			1,912,700,000
Marketable Securities	2,164,400,000			1,988,500,000
Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities Cost Or Amortized Cost	301,800,000			404,300,000
Marketable Securities	341,500,000			446,700,000
Covered bonds
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities Cost Or Amortized Cost	7,900,000			8,400,000
Fixed maturities, at fair value	521,000,000			647,100,000
Amortized cost of fixed maturities	485,500,000			605,400,000
Marketable Securities	8,100,000			8,600,000
Corporate
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, at fair value	10,305,324,000	[1],[2]		10,482,000,000	[1],[2]
Amortized cost of fixed maturities	9,712,063,000	[1],[2]		9,837,962,000	[1],[2]
Medium term notes
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, at fair value	193,700,000			194,300,000
Amortized cost of fixed maturities	192,400,000			194,800,000
Tier One And Upper Tier Two Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities Cost Or Amortized Cost	295,700,000			327,600,000
Marketable Securities	283,500,000			308,500,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Losses	12,100,000			19,100,000
Fixed income funds
Schedule of Available-for-sale Securities [Line Items]
Equity securities, at fair value	101,000,000			101,900,000
Cost of equity securities	100,000,000			100,000,000
Structured Credit Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities	8,200,000
Impaired Investment Cumulative Fair Value Decline Percentage Amortized Cost	50.00%
Marketable Securities Realized Gain Loss Other Than Temporary Impairments Amount Total	20,000,000
RMBS - Non-Agency
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, at fair value	593,391,000			647,733,000
Amortized cost of fixed maturities	600,120,000			678,469,000
Marketable Securities Realized Gain Loss Other Than Temporary Impairments Amount Total	6,200,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Reductions, Securities Sold	5,800,000		9,000,000
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities	2,800,000
Foreign Currency Adjustments [Member]
Schedule of Available-for-sale Securities [Line Items]
Other than Temporary Impairment Losses, Investments, Portion Recognized in Earnings, Net, Available-for-sale Securities	1,800,000
CMBS
Schedule of Available-for-sale Securities [Line Items]
Fixed maturities, at fair value	1,153,341,000			1,077,251,000
Amortized cost of fixed maturities	1,094,389,000			1,010,794,000
Marketable Securities Realized Gain Loss Other Than Temporary Impairments Amount Total	600,000
Core Cdo Holdings [Member]
Schedule of Available-for-sale Securities [Line Items]
Marketable Securities Realized Gain Loss Other Than Temporary Impairments Amount Total	$ 13,200,000

[1] 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 $193.7 million and an amortized cost of $192.4 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.

[2] 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 $283.5 million and an amortized cost of $295.7 million at March 31, 2013.