Fair Value Measurements (Details) (USD $)			6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014 Auction rate securities	Jun. 30, 2014 Recurring Fair value	Dec. 31, 2013 Recurring Fair value	Jun. 30, 2014 Recurring Fair value Bonds and bond funds	Dec. 31, 2013 Recurring Fair value Bonds and bond funds	Jun. 30, 2014 Recurring Fair value Asset backed securities	Dec. 31, 2013 Recurring Fair value Asset backed securities	Jun. 30, 2014 Recurring Level 1	Dec. 31, 2013 Recurring Level 1	Jun. 30, 2014 Recurring Level 2	Dec. 31, 2013 Recurring Level 2	Jun. 30, 2014 Recurring Level 2 Bonds and bond funds	Dec. 31, 2013 Recurring Level 2 Bonds and bond funds	Jun. 30, 2014 Recurring Level 2 Asset backed securities	Dec. 31, 2013 Recurring Level 2 Asset backed securities	Jun. 30, 2014 Recurring Level 3	Dec. 31, 2013 Recurring Level 3
Fair Value Measurements
Marketable securities	$ 384,944,000	$ 487,239,000		$ 384,944,000	$ 487,239,000	$ 384,800,000	$ 487,049,000	$ 144,000	$ 190,000			$ 384,944,000	$ 487,239,000	$ 384,800,000	$ 487,049,000	$ 144,000	$ 190,000
Cash, Cash Equivalents and Restricted Cash				82,011,000	182,855,000					82,011,000	182,855,000
Other Assets				2,265,000	2,245,000													2,265,000	2,245,000
Total Assets Measured at Fair Value				469,220,000	672,339,000					82,011,000	182,855,000	384,944,000	487,239,000					2,265,000	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			20,000
Balance at the end of the period			2,265,000
Fair Value of Financial Instruments
Fair value of long-term debt												1,605,700,000	1,509,200,000
Carrying amount of long-term debt												$ 1,569,700,000	$ 1,470,600,000