Investments (Summary of Unrealized Loss by 12 month Period) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Investments Unrealized Loss Position [Line Items]
Fair Value, Less than 12 Months	$ 0	$ 117,000
Unrealized Loss, Less than 12 Months	0	(284)
Fair Value, 12 Months or Greater	0	1,649
Unrealized Loss, 12 Months or Greater	0	(2)
Fair Value, Total	0	118,649
Unrealized Loss, Total	0	286
Corporate Debt Securities [Member]
Investments Unrealized Loss Position [Line Items]
Fair Value, Less than 12 Months	0	74,807
Unrealized Loss, Less than 12 Months	0	(212)
Fair Value, 12 Months or Greater	0	1,000
Unrealized Loss, 12 Months or Greater	0	(1)
Fair Value, Total	0	75,807
Unrealized Loss, Total	0	213
US Treasury and Government [Member]
Investments Unrealized Loss Position [Line Items]
Fair Value, Less than 12 Months	0	38,004
Unrealized Loss, Less than 12 Months	0	(62)
Fair Value, 12 Months or Greater	0	649
Unrealized Loss, 12 Months or Greater	0	(1)
Fair Value, Total	0	38,653
Unrealized Loss, Total	0	63
US States and Political Subdivisions Debt Securities [Member]
Investments Unrealized Loss Position [Line Items]
Fair Value, Less than 12 Months	0	4,189
Unrealized Loss, Less than 12 Months	0	(10)
Fair Value, 12 Months or Greater	0	0
Unrealized Loss, 12 Months or Greater	0	0
Fair Value, Total	0	4,189
Unrealized Loss, Total	$ 0	$ 10