Investments (Schedule of Available-for-sale Securities) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Investment [Line Items]
Amortized Cost	$ 1,515,117	$ 1,307,910
Gross Unrealized Gains	2,244	2,401
Gross Unrealized Losses	(689)	(635)
Fair Value	1,516,672	1,309,676
Agency Securities [Member]
Investment [Line Items]
Amortized Cost	622,936	453,922
Gross Unrealized Gains	1,269	1,177
Gross Unrealized Losses	(278)	(349)
Fair Value	623,927	454,750
Corporate Securities [Member]
Investment [Line Items]
Amortized Cost	744,199	643,360
Gross Unrealized Gains	813	947
Gross Unrealized Losses	(382)	(216)
Fair Value	744,630	644,091
Municipal Securities [Member]
Investment [Line Items]
Amortized Cost	31,214	53,698
Gross Unrealized Gains	40	81
Gross Unrealized Losses	(3)	(23)
Fair Value	31,251	53,756
Government Securities [Member]
Investment [Line Items]
Amortized Cost	116,768	156,930
Gross Unrealized Gains	122	196
Gross Unrealized Losses	(26)	(47)
Fair Value	$ 116,864	$ 157,079