Significant Non-Cash Investing Activities (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Other Significant Noncash Transactions [Line Items]
Accounts receivable	$ 266,217,000		$ 265,642,000
Purchases of property and equipment accrued in accounts payable but not yet paid	12,200,000	14,000,000
Accounts receivable - grants [Member]
Other Significant Noncash Transactions [Line Items]
Accounts receivable	$ 20,898,000	$ 17,800,000	$ 25,036,000