Investments (Details) (USD $)	Dec. 31, 2013	Dec. 31, 2012
Available-for-sale investments, balance sheet
Fair value	$ 30,000,000
Total short-term investments	33,000,000	416,000,000
Available-for-sale investments
Amortized cost, U.S. treasuries and government agency securities due in 1 year or less	21,000,000
Amortized cost, Auction rate securities due after ten years	8,000,000
Amortized cost	29,000,000
Fair value, U.S. treasuries and government agency securities due in 1 year or less	21,000,000
Fair value, Auction rate securities due after ten years	9,000,000
Fair value	30,000,000
U.S. treasuries and government agency securities | Short-term investments.
Available-for-sale investments, balance sheet
Available-for-sale securities, debt maturities, amortized cost	21,000,000	387,000,000
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Fair value	21,000,000	387,000,000
Available-for-sale investments
Fair value	21,000,000	387,000,000
Corporate bonds | Short-term investments.
Available-for-sale investments, balance sheet
Available-for-sale securities, debt maturities, amortized cost		11,000,000
Gross unrealized gains		0
Gross unrealized losses		0
Fair value		11,000,000
Available-for-sale investments
Fair value		11,000,000
Restricted cash | Short-term investments.
Available-for-sale investments, balance sheet
Fair value	12,000,000	18,000,000
Available-for-sale investments
Fair value	12,000,000	18,000,000
Auction rate securities held through Morgan Stanley Smith Barney LLC | Long-term investments.
Available-for-sale investments, balance sheet
Available-for-sale securities, debt maturities, amortized cost	8,000,000	8,000,000
Gross unrealized gains	1,000,000	0
Gross unrealized losses	0	0
Fair value	9,000,000	8,000,000
Available-for-sale investments
Fair value	$ 9,000,000	$ 8,000,000