Summary of Investments (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Amortized cost
Maturing in one year or less	$ 711,203		$ 711,203
Maturing after one year through five years	3,115,236		3,115,236
Maturing after five years through ten years	3,774,464		3,774,464
Maturing after ten years	4,658,928		4,658,928
Mortgage-backed and asset-backed securities	3,249,542		3,249,542
Amortized cost	15,509,373		15,509,373
Estimated fair value
Maturing in one year or less	745,013		745,013
Maturing after one year through five years	3,376,902		3,376,902
Maturing after five years through ten years	4,012,364		4,012,364
Maturing after ten years	4,706,795		4,706,795
Mortgage-backed and asset-backed securities	3,407,961		3,407,961
Estimated fair value and carrying value	16,249,035		16,249,035		18,188,344
Information regarding the sales of securities as available-for-sale
Proceeds from sales	2,661,364	2,010,884	8,022,277	4,400,898
Gross realized gains from sales	11,061	26,486	62,465	69,991
Gross realized losses from sales	$ 2		$ 26,914	$ 2,901