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Long-term debt (Details) (USD $)
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12 Months Ended | 12 Months Ended | 12 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended | 12 Months Ended | 1 Months Ended | 12 Months Ended | |||||||||||||
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Dec. 31, 2013
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Dec. 31, 2012
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Dec. 31, 2011
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Dec. 31, 2013
Construction Loan [Member]
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Dec. 31, 2012
Construction Loan [Member]
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Dec. 31, 2012
Construction Loan [Member]
PNC Bank [Member]
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Dec. 31, 2013
Equipment Loan [Member]
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Dec. 31, 2012
Equipment Loan [Member]
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Dec. 31, 2012
Equipment Loan [Member]
PNC Bank [Member]
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Dec. 31, 2013
Term Loan December 2009 [Member]
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Dec. 31, 2012
Term Loan December 2009 [Member]
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Dec. 31, 2012
Term Loan December 2009 [Member]
HSBC Bank [Member]
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Dec. 31, 2009
Prior Term Loan Replaced in December 2009 [Member]
HSBC Bank [Member]
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Dec. 31, 2013
Term Loan November 2009 [Member]
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Dec. 31, 2012
Term Loan November 2009 [Member]
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Oct. 31, 2009
Term Loan November 2009 [Member]
HSBC Bank [Member]
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Dec. 31, 2013
BAML Revolving Credit Facility [Member]
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Dec. 31, 2012
BAML Revolving Credit Facility [Member]
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Jan. 29, 2014
BAML Revolving Credit Facility [Member]
Subsequent Event [Member]
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Dec. 31, 2013
Credit Agreement [Member]
Institution
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Dec. 11, 2013
Credit Agreement [Member]
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Dec. 31, 2013
Term Loan Facility [Member]
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Jan. 29, 2014
2.875% Convertible Senior Notes Due 2021 [Member]
Subsequent Event [Member]
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Dec. 31, 2013
BAML Term Loan Member
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| Long-Term Debt [Line Items] | ||||||||||||||||||||||||
| Long-term Debt, Total | $ 62,000,000 | $ 62,774,000 | $ 0 | $ 29,375,000 | $ 0 | $ 11,068,000 | $ 0 | $ 18,200,000 | $ 0 | $ 4,131,000 | $ 0 | |||||||||||||
| Less current portion of long-term indebtedness | 0 | (4,470,000) | ||||||||||||||||||||||
| Noncurrent portion of long-term indebtedness | 62,000,000 | 58,304,000 | ||||||||||||||||||||||
| Interest rate, stated percentage (in hundredths) | 3.00% | 3.00% | 3.25% | 3.25% | 2.75% | 2.875% | ||||||||||||||||||
| Debt instrument, type of variable interest rate | one month LIBOR | one month LIBOR | three month LIBOR | three month LIBOR | one month LIBOR | |||||||||||||||||||
| Maturity date | Dec. 31, 2014 | Mar. 31, 2014 | Jan. 15, 2021 | Dec. 11, 2018 | ||||||||||||||||||||
| Number of lending institutions | 3 | |||||||||||||||||||||||
| Maximum borrowing capacity | 30,000,000 | 12,000,000 | 30,000,000 | 100,000,000 | 125,000,000 | |||||||||||||||||||
| Proceeds from borrowings on long-term indebtedness | 62,000,000 | |||||||||||||||||||||||
| Deferred financing costs | 3,400,000 | 3,500,000 | ||||||||||||||||||||||
| Debt covenant, consolidated debt service coverage ratio, minimum | 2.50 | |||||||||||||||||||||||
| Debt covenant, leverage ratio, maximum | 3.50 | |||||||||||||||||||||||
| Debt Instrument, covenant, senior leverage ratio, maximum | 2.00 | |||||||||||||||||||||||
| Debt covenant, minimum cash and liquid investments balance | 50,000,000 | |||||||||||||||||||||||
| Loan advance expiration date | Jul. 31, 2012 | Dec. 31, 2012 | ||||||||||||||||||||||
| Face amount of debt instrument | 22,800,000 | 250,000,000 | ||||||||||||||||||||||
| Conversion rate of notes per $1,000 principal amount (in shares) | 30.8821 | |||||||||||||||||||||||
| Conversion price per share (in dollars per share) | $ 32.38 | |||||||||||||||||||||||
| Payments on borrowings on long-term indebtedness | 62,774,000 | 10,227,000 | 15,494,000 | 62,000,000 | ||||||||||||||||||||
| Long-term Debt, Fiscal Year Maturity [Abstract] | ||||||||||||||||||||||||
| 2014 | 0 | |||||||||||||||||||||||
| 2015 | 0 | |||||||||||||||||||||||
| 2016 | 0 | |||||||||||||||||||||||
| 2017 | 0 | |||||||||||||||||||||||
| 2018 and thereafter | 62,000,000 | |||||||||||||||||||||||
| Long-term Debt, Total | $ 62,000,000 | $ 62,774,000 | $ 0 | $ 29,375,000 | $ 0 | $ 11,068,000 | $ 0 | $ 18,200,000 | $ 0 | $ 4,131,000 | $ 0 | |||||||||||||