Investments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Short-term investments:
Investments, Cost Basis	$ 257,256	$ 261,767
Available-for-sale Securities, Gross Unrealized Gain (Loss)	9	119
Short-term investments, Fair Value	257,265	261,886
Certificates Of Deposit [Member]
Short-term investments:
Investments, Cost Basis	223,339	258,518
Available-for-sale Securities, Gross Unrealized Gain (Loss)	0	0
Short-term investments, Fair Value	223,339	258,518
U.S. dollar indexed bond funds [Member]
Short-term investments:
Investments, Cost Basis	33,917	3,249
Available-for-sale Securities, Gross Unrealized Gain (Loss)	9	119
Short-term investments, Fair Value	33,926	3,368
Assets held in rabbi trust [Member]
Long-term investments:
Long-term investments, Cost Basis	6,348	6,266
Long-term investments, Unrealized Gain (Loss)	1,016	517
Fair Value Of Assets Held Under Trust	$ 7,364	$ 6,783