Summary of Investments (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012
Amortized cost
Maturing in one year or less	$ 815,999		$ 815,999
Maturing after one year through five years	3,071,738		3,071,738
Maturing after five years through ten years	3,626,928		3,626,928
Maturing after ten years	4,607,710		4,607,710
Mortgage-backed and asset-backed securities	3,387,392		3,387,392
Amortized cost	15,509,767		15,509,767
Estimated fair value
Maturing in one year or less	849,614		849,614
Maturing after one year through five years	3,321,878		3,321,878
Maturing after five years through ten years	3,889,260		3,889,260
Maturing after ten years	4,669,382		4,669,382
Mortgage-backed and asset-backed securities	3,551,425		3,551,425
Estimated fair value and carrying value	16,281,559		16,281,559		18,188,344
Information regarding the sales of securities as available-for-sale
Proceeds from sales	2,059,748	964,142	5,360,913	2,390,054
Gross realized gains from sales	18,191	23,499	51,404	43,505
Gross realized losses from sales	19,636	1,420	26,912	2,901
Unrealized losses on held-for-trading fixed maturity investments			$ 6,575		$ 634