Fair Value Measurements - Narrative (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013		Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	$ 27,282,442		$ 27,846,920
Amortized cost of fixed maturities	26,772,143		26,544,806
Equity securities, at fair value	974,733		649,359
Other investments - structured transactions	293,900		312,100
Spread on U.S. treasury for discounting deposit liabilities	0.652%		1.003%
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	10,759,332	[1],[2]	10,482,000	[3],[4]
Amortized cost of fixed maturities	10,395,005	[1],[2]	9,837,962	[3],[4]
Medium term notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	150,900		194,300
Amortized cost of fixed maturities	150,600		194,800
Other asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	1,151,135	[5]	1,470,711	[6]
Amortized cost of fixed maturities	1,122,958	[5]	1,426,483	[6]
Covered bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities, at fair value	560,700		647,100
Amortized cost of fixed maturities	531,400		605,400
Fixed income funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	$ 89,700		$ 101,900

[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.

[5] Covered Bonds within Fixed maturities - AFS with an amortized cost of $531.4 million and a fair value of $560.7 million and Covered Bonds within Fixed maturities - HTM with an amortized cost of $8.3 million and a fair value of $8.3 million are included within Other asset-backed securities to align the Company's classification to market indices.

[6] Covered Bonds within Fixed maturities - AFS with an amortized cost of $605.4 million and a fair value of $647.1 million and Covered Bonds within Fixed maturities - HTM with an amortized cost of $8.4 million