Investments (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Short-term investments:
Investments, Cost Basis	$ 240,273	$ 261,767
Available-for-sale Securities, Gross Unrealized Gain (Loss)	2,715	119
Short-term investments, Fair Value	242,988	261,886
Certificates of deposit [Member]
Short-term investments:
Investments, Cost Basis	215,010	258,518
Available-for-sale Securities, Gross Unrealized Gain (Loss)	0	0
Short-term investments, Fair Value	215,010	258,518
U.S. dollar indexed bond funds [Member]
Short-term investments:
Investments, Cost Basis	25,263	3,249
Available-for-sale Securities, Gross Unrealized Gain (Loss)	2,715	119
Short-term investments, Fair Value	27,978	3,368
Assets held in rabbi trusts [Member]
Long-term investments:
Long-term investments, Cost Basis	10,085	6,266
Long-term investments, Unrealized Gain (Loss)	292	517
Long-term investments, Fair Value	$ 10,377	$ 6,783