Nature of Operations and Summary of Significant Accounting Policies (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Cash and Cash Equivalents
Restricted cash	$ 12,219,000	$ 19,553,000
Marketable Securities
Cost	489,253,000	559,409,000
Unrealized appreciation	231,000	552,000
Total marketable securities	489,484,000	559,961,000
Maturities of marketable securities
Year 2014	304,353,000
Years 2015 through 2018	182,873,000
Thereafter	2,258,000
Marketable securities	487,200,000
Maximum period for redemption	1 year
Investment as non-current	2,200,000
Commercial paper
Marketable Securities
Cost		3,510,000
Total marketable securities		3,514,000
Bond and bond funds
Marketable Securities
Cost	489,071,000	555,603,000
Total marketable securities	489,294,000	556,133,000
Asset backed securities
Marketable Securities
Cost	182,000	296,000
Total marketable securities	$ 190,000	$ 314,000