Investment Information (Maturity Profile of Available For Sale Securities) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011		Dec. 31, 2010
Estimated Fair Value:
Due in one year or less	$ 486,986		$ 414,390
Due after one year through five years	2,850,578		2,924,879
Due after five years through 10 years	2,532,834		1,719,446
Due after 10 years	345,755		442,575
Subtotal	6,216,153		5,501,290
Estimated fair value	10,873,912		10,535,007
Amortized Cost:
Due in one year or less	476,734		398,795
Due after one year through five years	2,793,982		2,833,955
Due after five years through 10 years	2,441,800		1,671,306
Due after 10 years	326,451		442,626
Subtotal	6,038,967		5,346,682
Amortized cost	10,663,518		10,303,557
Fixed maturities fair value [Member]
Estimated Fair Value:
Estimated fair value	9,431,998	[1]	9,033,434	[1]
Amortized Cost:
Amortized cost	9,219,958	[1]	8,844,521	[1]
Fixed maturities fair value [Member] | Mortgage backed securities [Member]
Estimated Fair Value:
Securities without single maturity date	1,592,762		1,806,813
Estimated fair value	1,592,762	[1]	1,806,813	[1]
Amortized Cost:
Securities without single maturity date	1,588,355		1,814,905
Amortized cost	1,588,355	[1]	1,814,905	[1]
Fixed maturities fair value [Member] | Commercial mortgage backed securities [Member]
Estimated Fair Value:
Securities without single maturity date	1,046,326		1,167,299
Estimated fair value	1,046,326	[1]	1,167,299	[1]
Amortized Cost:
Securities without single maturity date	1,020,450		1,141,584
Amortized cost	1,020,450	[1]	1,141,584	[1]
Fixed maturities fair value [Member] | Asset backed securities [Member]
Estimated Fair Value:
Securities without single maturity date	576,757		558,032
Estimated fair value	576,757	[1]	558,032	[1]
Amortized Cost:
Securities without single maturity date	572,186		541,350
Amortized cost	$ 572,186	[1]	$ 541,350	[1]

[1] In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities and short-term investments pledged. For purposes of this table, the Company has excluded the collateral received and reinvested and included the fixed maturities and short-term investments pledged. See "—Securities Lending Agreements".