Nature of Operations and Summary of Significant Accounting Policies (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Cash and Cash Equivalents
Restricted cash	$ 11,582,000	$ 12,219,000
Marketable Securities
Total cash and cash equivalents	132,275,000	170,636,000	133,772,000	129,526,000
Amortized Cost	415,726,000	486,753,000
Gross unrealized holding gains	12,000	495,000
Gross unrealized holding losses	(465,000)	(9,000)
Total marketable securities	415,273,000	487,239,000
Total cash and cash equivalents and available for sale securities, amortized cost	548,001,000	657,389,000
Total cash and cash equivalents and available for sale securities, fair market value	547,548,000	657,875,000
Maturities of marketable securities
Year 2015	233,858,000
Years 2016 through 2019	181,406,000
Thereafter	2,317,000
Marketable securities	415,300,000	487,200,000
Maximum period for redemption	1 year
Investment as non-current	2,300,000	2,200,000
Bonds and bond funds
Marketable Securities
Amortized Cost	410,618,000	486,571,000
Gross unrealized holding gains	9,000	487,000
Gross unrealized holding losses	(464,000)	(9,000)
Total marketable securities	410,163,000	487,049,000
Asset-backed Securities
Marketable Securities
Amortized Cost	5,108,000	182,000
Gross unrealized holding gains	3,000	8,000
Gross unrealized holding losses	(1,000)
Total marketable securities	$ 5,110,000	$ 190,000