Unaudited Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Unaudited Quarterly Financial Data
Revenues	$ 1,201,877	$ 1,250,689	$ 1,084,510	$ 955,233	$ 1,099,291	$ 1,030,388	$ 1,053,065	$ 992,928	$ 4,492,309	$ 4,175,672	$ 4,111,471
Gross profit	129,723	140,841	129,531	105,347	139,735	120,857	105,955	100,357	505,442	466,904	415,132
Income from construction operations	64,396	70,354	65,443	41,497	70,175	58,094	39,474	36,079	241,690	203,822	(221,811)
Income before income taxes	53,917	58,616	47,612	27,293	53,507	37,035	25,157	23,916	187,438	139,615	(267,842)
Net income (loss)	$ 27,722	$ 35,730	$ 28,545	$ 15,939	$ 33,259	$ 23,759	$ 15,478	$ 14,800	$ 107,936	$ 87,296	$ (265,400)
Earnings per share:
Basic (in dollars per share)	$ 0.57	$ 0.74	$ 0.59	$ 0.33	$ 0.69	$ 0.50	$ 0.32	$ 0.31	$ 2.22	$ 1.82	$ (5.59)
Diluted (in dollars per share)	$ 0.56	$ 0.73	$ 0.58	$ 0.33	$ 0.68	$ 0.49	$ 0.32	$ 0.31	$ 2.20	$ 1.80	$ (5.59)