Marketable Investments (Details Textual) (USD $) In Millions, unless otherwise specified	3 Months Ended									3 Months Ended
	Mar. 31, 2013	Mar. 31, 2013 Minimum [Member]	Mar. 31, 2012 Minimum [Member]	Mar. 31, 2013 Maximum [Member]	Mar. 31, 2012 Maximum [Member]	Mar. 31, 2013 Accumulated Net Unrealized Investment Gain (Loss) [Member]	Dec. 31, 2012 Accumulated Net Unrealized Investment Gain (Loss) [Member]	Mar. 31, 2013 Federal agency and corporate obligations [Member]	Dec. 31, 2012 Federal agency and corporate obligations [Member]	Mar. 31, 2013 ARS [Member] Level 3 [Member]
Marketable Investments (Textual) [Abstract]
Cash and cash equivalents								$ 50.0	$ 32.1
Intervals for each security		7 days		35 days
Interest rates on securities ranged		0.10%	0.10%	0.40%	0.40%
Weighted average interest rate										1.10%
Weighted average discount rate										4.30%
Weighted average holding period										8 years 8 months 12 days
Unrealized loss recorded in accumulated other comprehensive income (loss)						$ 2.1	$ 2.0
Marketable Investments (Additional Textual) [Abstract]
Original maturities	90 days