Summary of Investments (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Amortized cost
Maturing in one year or less	$ 773,417
Maturing after one year through five years	2,964,892
Maturing after five years through ten years	3,728,447
Maturing after ten years	4,392,104
Mortgage-backed and asset-backed securities	3,538,569
Amortized cost	15,397,429
Estimated fair value
Maturing in one year or less	814,578
Maturing after one year through five years	3,262,484
Maturing after five years through ten years	4,195,157
Maturing after ten years	4,735,803
Mortgage-backed and asset-backed securities	3,752,818
Estimated fair value and carrying value	16,760,840		18,188,344
Information regarding the sales of securities as available-for-sale
Proceeds from sales	3,301,164	1,425,912
Gross realized gains from sales	33,213	20,006
Gross realized losses from sales	$ 7,276	$ 1,481