Fair Value Measurements (Details) (USD $)			9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Auction Rate Securities [Member]	Sep. 30, 2014 Fair Value, Measurements, Recurring [Member] Estimate of Fair Value Measurement [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Estimate of Fair Value Measurement [Member]	Sep. 30, 2014 Fair Value, Measurements, Recurring [Member] Estimate of Fair Value Measurement [Member] Corporate Bond Securities [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Estimate of Fair Value Measurement [Member] Corporate Bond Securities [Member]	Sep. 30, 2014 Fair Value, Measurements, Recurring [Member] Estimate of Fair Value Measurement [Member] Asset-backed Securities [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Estimate of Fair Value Measurement [Member] Asset-backed Securities [Member]	Sep. 30, 2014 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member]	Sep. 30, 2014 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member]	Sep. 30, 2014 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member] Corporate Bond Securities [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member] Corporate Bond Securities [Member]	Sep. 30, 2014 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member] Asset-backed Securities [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member] Asset-backed Securities [Member]	Sep. 30, 2014 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]	Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]
Fair Value Measurements
Marketable securities	$ 392,690,000	$ 487,239,000		$ 392,690,000	$ 487,239,000	$ 392,565,000	$ 487,049,000	$ 125,000	$ 190,000			$ 392,690,000	$ 487,239,000	$ 392,565,000	$ 487,049,000	$ 125,000	$ 190,000
Cash, Cash Equivalents and Restricted Cash				162,975,000	182,855,000					162,975,000	182,855,000
Other Assets				2,272,000	2,245,000													2,272,000	2,245,000
Total Assets Measured at Fair Value				557,937,000	672,339,000					162,975,000	182,855,000	392,690,000	487,239,000					2,272,000	2,245,000
Changes in assets measured at fair value on a recurring basis using significant unobservable inputs (level 3)
Balance at the beginning of the period			2,245,000
Total realized and unrealized gains or (losses) Included in other comprehensive income			27,000
Balance at the end of the period			2,272,000
Fair Value of Financial Instruments
Fair value of long-term debt												1,706,300,000	1,509,200,000
Carrying amount of long-term debt												$ 1,669,600,000	$ 1,470,600,000