Investments (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Short-term investments:
Investments, Cost Basis	$ 197,784	$ 261,767
Available-for-sale Securities, Gross Unrealized Gain (Loss)	1,919	119
Short-term investments, Fair Value	199,703	261,886
Certificates Of Deposit [Member]
Short-term investments:
Investments, Cost Basis	160,286	258,518
Available-for-sale Securities, Gross Unrealized Gain (Loss)	0	0
Short-term investments, Fair Value	160,286	258,518
U.S. dollar indexed bond funds [Member]
Short-term investments:
Investments, Cost Basis	37,498	3,249
Available-for-sale Securities, Gross Unrealized Gain (Loss)	1,919	119
Short-term investments, Fair Value	39,417	3,368
Assets held in rabbi trust [Member]
Long-term investments:
Long-term investments, Cost Basis	5,867	6,266
Long-term investments, Unrealized Gain (Loss)	1,269	517
Fair Value Of Assets Held Under Trust	$ 7,136	$ 6,783