Short-Term Investments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014
Short-Term Investments
Amortized Cost	$ 253,395
Gross Unrealized Gains	109
Gross Unrealized Losses	(204)
Fair Value	253,300
Contractual maturities of short-term investments
Under 1 Year	127,178
1 to 5 Years	126,122
Total	253,300
Corporate debt
Short-Term Investments
Amortized Cost	184,141
Gross Unrealized Gains	91
Gross Unrealized Losses	(193)
Fair Value	184,039
Contractual maturities of short-term investments
Under 1 Year	80,081
1 to 5 Years	103,958
Total	184,039
U.S. government and agency debt
Short-Term Investments
Amortized Cost	23,797
Gross Unrealized Gains	11
Gross Unrealized Losses	(2)
Fair Value	23,806
Contractual maturities of short-term investments
Under 1 Year	16,325
1 to 5 Years	7,481
Total	23,806
Municipal bonds
Short-Term Investments
Amortized Cost	22,924
Gross Unrealized Gains	7
Gross Unrealized Losses	(4)
Fair Value	22,927
Contractual maturities of short-term investments
Under 1 Year	8,244
1 to 5 Years	14,683
Total	22,927
Other fixed income securities
Short-Term Investments
Amortized Cost	22,533
Gross Unrealized Gains	0
Gross Unrealized Losses	(5)
Fair Value	22,528
Contractual maturities of short-term investments
Under 1 Year	22,528
1 to 5 Years	0
Total	$ 22,528