Summary of Investments (Details 2) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Amortized cost
Maturing in one year or less	$ 672,982
Maturing after one year through five years	3,074,443
Maturing after five years through ten years	3,685,547
Maturing after ten years	3,895,658
Mortgage-backed and asset-backed securities	5,427,586
Amortized cost	16,756,216
Estimated fair value
Maturing in one year or less	711,671
Maturing after one year through five years	3,374,411
Maturing after five years through ten years	4,192,495
Maturing after ten years	4,271,934
Mortgage-backed and asset-backed securities	5,637,833
Estimated fair value and carrying value	18,188,344	16,589,783
Information regarding the sales of securities as available-for-sale
Proceeds from sales	6,821,092	3,958,589	3,222,700
Gross realized gains from sales	113,984	104,893	62,702
Gross realized losses from sales	4,371	23,138	26
Fair value of corporate fixed maturity security that has been non-income producing		$ 9,949
Non-income producing period of corporate fixed maturity security	12 months