Nature of Operations and Summary of Significant Accounting Policies (Details 2) (USD $)	Dec. 31, 2012 Y	Dec. 31, 2011
Cash and Cash Equivalents
Restricted cash	$ 19,553,000	$ 19,434,000
Marketable Securities
Cost	559,409,000	501,181,000
Unrealized appreciation	552,000	164,000
Total marketable securities	559,961,000	501,345,000
Market Value	559,961,000	501,345,000
Maturities of marketable securities
Year 2013	327,942,000
Years 2014 through 2017	160,116,000
Years 2018 through 2022	4,060,000
Thereafter	67,843,000
Marketable securities	556,100,000
Maximum period for redemption (in year)	1
Investment as non-current	3,800,000
Commercial paper
Marketable Securities
Cost	3,510,000	4,555,000
Market Value	3,514,000	4,557,000
Bond and bond funds
Marketable Securities
Cost	555,603,000	496,170,000
Market Value	556,133,000	496,310,000
Asset backed securities
Marketable Securities
Cost	296,000	456,000
Market Value	$ 314,000	$ 478,000