Available-for-Sale Investments (Details) (USD $)	12 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011 item
Available-for-Sale Investments
Amortized Cost	$ 31,523,000	$ 76,728,000
Gross Unrealized Gains	6,000	16,000
Gross Unrealized Losses	(1,000)	(10,000)
Fair Value	31,528,000	76,734,000
Contractual maturity period, maximum	1 year
Number of investments classified as available-for-sale securities in an unrealized loss position	3	12
Aggregate fair value of securities none of which had been in an unrealized loss position for more than twelve months	3,000,000	35,500,000
Proceeds from maturities of available-for-sale securities	89,800,000	212,300,000
Proceeds from sales of available-for-sale securities	51,000,000	10,000,000
U.S. government-sponsored securities
Available-for-Sale Investments
Amortized Cost	16,472,000	54,911,000
Gross Unrealized Gains	5,000	12,000
Gross Unrealized Losses	(1,000)	(10,000)
Fair Value	16,476,000	54,913,000
U.S. Treasury securities
Available-for-Sale Investments
Amortized Cost	15,051,000	21,817,000
Gross Unrealized Gains	1,000	4,000
Fair Value	$ 15,052,000	$ 21,821,000