Investment Information - Maturity Profile of Available For Sale Securities (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Estimated Fair Value:
Estimated fair value	$ 10,750,770	$ 9,571,776
Amortized Cost:
Amortized cost	10,701,557	9,564,634
Fixed maturities and fixed maturities pledged under securities lending agreements
Estimated Fair Value:
Due in one year or less	235,930	235,330
Due after one year through five years	4,074,562	3,738,500
Due after five years through 10 years	2,475,726	1,966,536
Due after 10 years	279,542	196,305
Single maturity date	7,065,760	6,136,671
Estimated fair value	10,801,572	9,676,857
Amortized Cost:
Due in one year or less	233,794	232,652
Due after one year through five years	4,077,408	3,718,920
Due after five years through 10 years	2,461,356	1,979,510
Due after 10 years	273,372	198,286
Single maturity date	7,045,930	6,129,368
Amortized cost	10,753,633	9,670,507
Fixed maturities and fixed maturities pledged under securities lending agreements | Mortgage backed securities
Estimated Fair Value:
Securities without single maturity date	943,343	1,133,095
Amortized Cost:
Securities without single maturity date	928,342	1,139,034
Fixed maturities and fixed maturities pledged under securities lending agreements | Commercial mortgage backed securities
Estimated Fair Value:
Securities without single maturity date	1,114,528	1,074,497
Amortized Cost:
Securities without single maturity date	1,103,756	1,075,749
Fixed maturities and fixed maturities pledged under securities lending agreements | Asset backed securities
Estimated Fair Value:
Securities without single maturity date	1,677,941	1,332,594
Amortized Cost:
Securities without single maturity date	$ 1,675,605	$ 1,326,356