Fair Value Measurements (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Assets
Restricted cash	$ 47,019	$ 38,717
Fair value measured on a recurring basis | Quoted prices in active markets (Level 1)
Assets
Cash and cash equivalents	142,678	168,056
Restricted cash	47,019	38,717
Investments in lieu of retainage	8,705	10,553
Total assets	198,402	217,326
Fair value measured on a recurring basis | Significant other observable inputs (Level 2)
Assets
Short-term investments	2,183	2,679
Investments in lieu of retainage	12,707	11,381
Total assets	14,890	14,060
Liabilities
Interest rate swap contract	1,166	1,923
Total liabilities	1,166	1,923
Fair value measured on a recurring basis | Significant unobservable inputs (Level 3)
Assets
Long-term investments - auction rate securities	46,283	46,283
Total assets	46,283	46,283
Liabilities
Contingent consideration	47,918	42,624
Total liabilities	47,918	42,624
Total Carrying Value
Assets
Cash and cash equivalents	142,678	168,056
Restricted cash	47,019	38,717
Short-term investments	2,183	2,679
Investments in lieu of retainage	21,412	21,934
Long-term investments - auction rate securities	46,283	46,283
Total assets	259,575	277,669
Liabilities
Interest rate swap contract	1,166	1,923
Contingent consideration	47,918	42,624
Total liabilities	$ 49,084	$ 44,547