Summary of Significant Accounting Policies Cash and Cash Equivalents (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Debt Maturities, Next Twelve Months, Fair Value	$ 207,400,000	$ 212,300,000
Cash and Cash Equivalents, at Carrying Value	349,843,000	342,211,000	215,451,000	210,863,000
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	1,303,000	(550,000)	(166,000)
Accumulated Other Comprehensive Income (Loss) [Member]
Schedule of Available-for-sale Securities [Line Items]
Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax	1,303,000	(550,000)	(166,000)
Money Market Funds and Demand Deposits [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and Cash Equivalents, at Carrying Value	261,899,000	338,211,000
Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 87,944,000	$ 4,000,000