Accounts Receivable - Summary of components of accounts receivable (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Trade receivables:
Billed	$ 310,997	$ 367,718
Unbilled	82,649	99,913
Other receivables	2,994	1,734
Total	396,640	469,365
Less: allowance for doubtful accounts	(9,110)	(8,093)
Accounts receivable, net	$ 387,530	$ 461,272