Investments (Schedule of Available-for-sale Securities) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,256,782	$ 876,988
Gross Unrealized Gains	2,122	3,539
Gross Unrealized Losses	(2,317)	(192)
Fair Value	1,256,587	880,335
Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	461,338	400,365
Gross Unrealized Gains	1,465	2,347
Gross Unrealized Losses	(978)	(5)
Fair Value	461,825	402,707
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	629,911	404,546
Gross Unrealized Gains	578	947
Gross Unrealized Losses	(1,015)	(171)
Fair Value	629,474	405,322
Municipal Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	64,238	32,214
Gross Unrealized Gains	17	114
Gross Unrealized Losses	(171)	(15)
Fair Value	64,084	32,313
Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	101,295	39,863
Gross Unrealized Gains	62	131
Gross Unrealized Losses	(153)	(1)
Fair Value	$ 101,204	$ 39,993