Investments (Details) - Narrative (USD $)	12 Months Ended
	Dec. 31, 2012 positions	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Fixed maturities, at fair value	$ 27,846,920,000	$ 26,190,025,000
Cost or Amortized cost of Fixed maturities	26,544,806,000	25,771,715,000
Fixed maturities, held to maturity	2,814,447,000	2,668,978,000
Held To Maturity Securities Fair Value	3,262,804,000	2,895,688,000
Percentage of fixed income portfolio invested in securities below investment grade	2.90%	2.40%
Percentage of gross unrealized losses relating to securities below investment grade	37.70%	31.40%
Pledged assets, amount	16,900,000	17,200,000
Unrealized losses on AFS portfolio	282,800,000
Total number of positions - AFS	7,331
AFS securities in unrealized loss position (number of positions)	1,223
Unrealized losses on HTM portfolio	2,700,000
Total number of positions - HTM	206
HTM securities in unrealized loss position (number of positions)	5
Marketable Securities Realized Gain Loss Other Than Temporary Impairments Amount Total	(54,306,000)	(174,102,000)	(170,643,000)
Credit loss impairments previously recognized on securities which matured, paid down, prepaid or were sold during the period	114,056,000	209,187,000
Net impairment charges	80,700,000
Maximum
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Short term investments maturity horizon from reporting date	1 year
Currency losses
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Net impairment charges	5,300,000
Government Related Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Fair value	1,988,500,000	1,915,600,000
Cost or amortized cost	1,912,700,000	1,878,300,000
U.S. Agency Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Fair value	446,700,000	541,200,000
Cost or amortized cost	404,300,000	494,000,000
Other asset-backed securities
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Fixed maturities, at fair value	1,470,711,000	1,340,249,000
Cost or Amortized cost of Fixed maturities	1,426,483,000	1,341,309,000
Fixed maturities, held to maturity	222,340,000	288,741,000
Held To Maturity Securities Fair Value	251,460,000	309,296,000
Covered Bonds [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Fixed maturities, at fair value	647,100,000	353,900,000
Cost or Amortized cost of Fixed maturities	605,400,000	345,400,000
Fixed maturities, held to maturity	8,400,000	8,100,000
Held To Maturity Securities Fair Value	8,600,000	7,700,000
Corporate
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Fixed maturities, at fair value	10,482,000,000	10,108,622,000
Cost or Amortized cost of Fixed maturities	9,837,962,000	9,866,677,000
Fixed maturities, held to maturity	1,425,320,000	1,290,209,000
Held To Maturity Securities Fair Value	1,615,397,000	1,367,089,000
Medium-term Notes [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Fair value	194,300,000	266,000,000
Cost or amortized cost	194,800,000	297,700,000
Fixed maturities, at fair value	194,300,000	266,000,000
Cost or Amortized cost of Fixed maturities	194,800,000	297,700,000
Net impairment charges	19,100,000
Tier One and Upper Tier Two Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Fair value	308,500,000	386,100,000
Cost or amortized cost	327,600,000	494,900,000
Unrealized losses on AFS portfolio	19,100,000	108,800,000
Equity Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
OTTI on investments, net of amounts transferred to other comprehensive income	3,746,000	0	0
Length of time in loss position before impairment	11 months
Fixed Income Funds Member
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Fair value	101,900,000	91,600,000
Cost or amortized cost	100,000,000	100,000,000
Structured Credit Securities [Member]
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Fair value	10,300,000
Net impairment charges	48,700,000
Cumulative fair value decline, percentage of amortized cost	50.00%
Structured Credit Securities [Member] | P&C
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Marketable Securities Realized Gain Loss Other Than Temporary Impairments Amount Total	26,000,000
RMBS - Non-Agency
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Fixed maturities, at fair value	647,733,000	641,815,000
Cost or Amortized cost of Fixed maturities	678,469,000	851,557,000
Fixed maturities, held to maturity	83,205,000	80,955,000
Held To Maturity Securities Fair Value	93,707,000	87,443,000
Marketable Securities Realized Gain Loss Other Than Temporary Impairments Amount Total	10,600,000
Credit loss impairments previously recognized on securities which matured, paid down, prepaid or were sold during the period	56,900,000	128,900,000
CMBS
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Fixed maturities, at fair value	1,077,251,000	974,835,000
Cost or Amortized cost of Fixed maturities	1,010,794,000	927,684,000
Fixed maturities, held to maturity	12,751,000
Held To Maturity Securities Fair Value	14,799,000
Marketable Securities Realized Gain Loss Other Than Temporary Impairments Amount Total	600,000
Core CDO
Schedule Of Available For Sale And Held To Maturity Securities [Line Items]
Marketable Securities Realized Gain Loss Other Than Temporary Impairments Amount Total	$ 14,800,000