Investments (Schedule of Available-for-sale Securities) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Investment [Line Items]
Amortized Cost	$ 1,577,079	$ 1,307,910
Gross Unrealized Gains	1,991	2,401
Gross Unrealized Losses	(1,402)	(635)
Fair Value	1,577,668	1,309,676
Agency Securities [Member]
Investment [Line Items]
Amortized Cost	644,491	453,922
Gross Unrealized Gains	1,392	1,177
Gross Unrealized Losses	(539)	(349)
Fair Value	645,344	454,750
Corporate Securities [Member]
Investment [Line Items]
Amortized Cost	768,474	643,360
Gross Unrealized Gains	540	947
Gross Unrealized Losses	(822)	(216)
Fair Value	768,192	644,091
Municipal Securities [Member]
Investment [Line Items]
Amortized Cost	41,263	53,698
Gross Unrealized Gains	23	81
Gross Unrealized Losses	(13)	(23)
Fair Value	41,273	53,756
Government Securities [Member]
Investment [Line Items]
Amortized Cost	122,851	156,930
Gross Unrealized Gains	36	196
Gross Unrealized Losses	(28)	(47)
Fair Value	$ 122,859	$ 157,079