Investments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Short-term investments:
Investments, Cost Basis	$ 207,953	$ 240,273
Available-for-sale Securities, Gross Unrealized Gain	1,064	2,715
Short-term investments, Fair Value	209,017	242,988
Certificates Of Deposit [Member]
Short-term investments:
Investments, Cost Basis	169,652	215,010
Available-for-sale Securities, Gross Unrealized Gain	0	0
Short-term investments, Fair Value	169,652	215,010
U.S. dollar indexed bond funds [Member]
Short-term investments:
Investments, Cost Basis	38,301	25,263
Available-for-sale Securities, Gross Unrealized Gain	1,064	2,715
Short-term investments, Fair Value	39,365	27,978
Assets held in rabbi trusts [Member]
Long-term investments:
Long-term investments, Cost Basis	9,738	10,085
Long-term investments, Unrealized Gain	423	292
Fair Value Of Assets Held Under Trust	$ 10,161	$ 10,377