Acquisitions - Additional Information (Detail) (USD $)	0 Months Ended	3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended
	May 08, 2012	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Feb. 04, 2013 Beall Corporation	Jun. 30, 2013 Walker Group Holdings LLC	Jun. 30, 2012 Walker Group Holdings LLC	Jun. 30, 2013 Walker Group Holdings LLC	Jun. 30, 2012 Walker Group Holdings LLC	Jun. 30, 2013 Convertible Senior Notes	Apr. 25, 2013 Convertible Senior Notes	Jun. 30, 2013 Unsecured Debt Convertible Senior Notes	Apr. 23, 2012 Unsecured Debt Convertible Senior Notes
Business Acquisition [Line Items]
Walker acquisition, purchase price	$ 377,000,000
Notes issued, interest rate											3.375%	3.375%	3.375%	3.375%
Notes issued, due date													2018	2018
Intangible assets preliminarily, Amount						8,900,000
Acquisition expenses		239,000	12,224,000	857,000	13,902,000
Estimated amortization expense for 2013		21,700,000		21,700,000
Estimated amortization expense for 2014		21,900,000		21,900,000
Estimated amortization expense for 2015		21,300,000		21,300,000
Estimated amortization expense for 2016		20,100,000		20,100,000
Estimated amortization expense for 2017		16,900,000		16,900,000
Goodwill						1,800,000
Acquisition of goodwill amount	148,500,000
Business acquisition cost of acquired entity purchase price						13,900,000
Business acquisition, purchase price allocation, capital lease obligation accrual		4,700,000		4,700,000
Amortization of intangibles		5,454,000	3,454,000	10,824,000	4,192,000
Business acquisition cost of revenue			401,142,000		776,155,000		103,100,000	44,300,000	198,300,000	44,300,000
Intangible Assets Purchase Price Allocation	162,800,000
Walker acquisition additional payment	2,100,000
Business acquisitions pro forma income before income taxes							$ 13,400,000	$ 3,700,000	$ 25,300,000	$ 3,700,000