Supplementary Financial Information (Marketable Securities) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 30,375	$ 30,374
Gross Unrealized Gains	266	243
Gross Unrealized Losses	(23)	(29)
Estimated Fair Value	30,618	30,588
U.S. treasury securities
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	30,049	30,049
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(23)	(29)
Estimated Fair Value	30,026	30,020
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49	48
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	49	48
Mutual funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	277	277
Gross Unrealized Gains	266	243
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 543	$ 520