Financial Commitments (Narrative) (Details) (USD $)	0 Months Ended	12 Months Ended			1 Months Ended
	Jun. 05, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 20, 2010	Aug. 03, 2011	Aug. 03, 2012	Aug. 02, 2012
Principal payments required under long-term debt obligations
2015		$ 81,300,000
2016		95,100,000
2017		51,000,000
2018		359,700,000
2019		126,500,000
2020 and beyond		151,800,000
Amended Credit Agreement
Percentage of the original total outstanding principal repaid in year 1	6.00%
Percentage of the original total outstanding principal repaid in year 2	9.00%
Percentage of the original total outstanding principal repaid in year 3	12.00%
Percentage of the original total outstanding principal repaid in year 4	15.00%
Percentage of the original total outstanding principal repaid in year 5	13.50%
Percentage of the original total outstanding principal repaid at maturity	44.50%
Minimum fixed charge ratio		1.25
Maximum consolidated leverage ratio, term		5 years
Loan outstanding		865,359,000	733,884,000
Loans repaid during the period		1,026,349,000	676,795,000	626,122,000
Operating leases, rent expense
Operating leases, rent expense		24,400,000	18,500,000	17,700,000
Senior Unsecured Notes [Member]
Principal payments required under long-term debt obligations
Long-term debt, gross		300,000,000	300,000,000
Interest rate (as a percent)		7.625%	7.625%
Private placement offering price (as a percent)					99.258%
Imputed interest rate (as a percent)		7.75%	7.75%
Proceeds from the placement of Senior Notes					293,200,000
Unamortized debt discount		1,223,000	1,493,000		2,200,000
Debt issuance costs					4,600,000
Redemption price, change of control triggering event (as a percent)					101.00%
Default, minimum percentage of principal amount of Senior Notes held (as a percent)					25.00%
Amended Credit Agreement
Loan outstanding		298,777,000	298,507,000
Term Loan [Member]
Amended Credit Agreement
Face amount	250,000,000	250,000,000	200,000,000			200,000,000
Period of term loan		5 years	5 years
Loan outstanding		242,500,000	115,000,000
Lunda Seller Notes [Member]
Principal payments required under long-term debt obligations
Interest rate (as a percent)		5.00%	5.00%
Amended Credit Agreement
Loan outstanding		21,750,000	21,750,000
Equipment Financing: Fixed rates 2.28% to 3.09% [Member]
Amended Credit Agreement
Face amount		25,800,000
Period of term loan		60 months
Interest rate, minimum (as a percent)		2.28%
Interest rate, maximum (as a percent)		3.09%
Equipment Financing With Fixed Rate Range From 2.12 To 2.69% [Member]
Amended Credit Agreement
Loan outstanding		46,500,000
Interest rate, minimum (as a percent)		2.12%
Interest rate, maximum (as a percent)		2.69%
Land and Improvements Mortgages [Member]
Amended Credit Agreement
Basis points added to reference rate (as a percent)		3.00%	3.00%
Period of term loan		10 years
Loan outstanding		9,144,000	9,404,000
Minimum [Member]
Amended Credit Agreement
Maximum consolidated leverage ratio		2.75
Minimum [Member] | Equipment Financing With Fixed Rate Range From 2.12 To 2.69% [Member]
Amended Credit Agreement
Number of equal monthly installments of principal and interest		48 months
Maximum [Member]
Amended Credit Agreement
Maximum consolidated leverage ratio		3.75
Maximum [Member] | Term Loan [Member]
Amended Credit Agreement
Face amount	300,000,000
Maximum [Member] | Equipment Financing With Fixed Rate Range From 2.12 To 2.69% [Member]
Amended Credit Agreement
Number of equal monthly installments of principal and interest		60 months
Revolving Credit Facility [Member]
Amended Credit Agreement
Maximum borrowing capacity	300,000,000					300,000,000
Available borrowing capacity		169,000,000
Letters Of Credit [Member]
Amended Credit Agreement
Maximum borrowing capacity	150,000,000					50,000,000	150,000,000	50,000,000
Letters of credit outstanding		$ 1,000,000	$ 200,000
Swap [Member]
Amended Credit Agreement
Maximum adjustment to Applicable Rate (as a percent)		97.50%