Investment Information (Maturity Profile of Available For Sale Securities) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Estimated Fair Value:
Estimated fair value	$ 9,890,425	$ 9,839,988
Amortized Cost:
Amortized cost	9,682,653	9,567,290
Fixed maturities and fixed maturities pledged under securities lending agreements
Estimated Fair Value:
Due in one year or less	414,808	446,402
Due after one year through five years	3,692,725	3,876,062
Due after five years through 10 years	1,852,459	1,949,297
Due after 10 years	301,094	179,927
Single maturity date	6,261,086	6,451,688
Estimated fair value	9,980,366	9,882,588
Amortized Cost:
Due in one year or less	405,063	436,376
Due after one year through five years	3,617,078	3,769,426
Due after five years through 10 years	1,789,372	1,869,698
Due after 10 years	293,245	172,275
Single maturity date	6,104,758	6,247,775
Amortized cost	9,770,198	9,608,614
Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Estimated Fair Value:
Securities without single maturity date	1,704,813	1,532,736
Amortized Cost:
Securities without single maturity date	1,690,959	1,515,411
Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Estimated Fair Value:
Securities without single maturity date	867,856	824,165
Amortized Cost:
Securities without single maturity date	842,816	791,119
Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Estimated Fair Value:
Securities without single maturity date	1,146,611	1,073,999
Amortized Cost:
Securities without single maturity date	$ 1,131,665	$ 1,054,309