Debt Obligations (Details) (USD $)
In Millions, unless otherwise specified

3 Months Ended

Mar. 31, 2012

Dec. 31, 2011

Mar. 31, 2012

Convertible Senior Subordinated Notes [Member]

Dec. 13, 2006

Convertible Senior Subordinated Notes [Member]

Mar. 31, 2012

Credit Facility [Member]

Debt Instrument [Line Items]

Face amount

$ 143.8

Interest rate (in hundredths)

2.4375%

Maturity date

2027

Terms of conversion feature

Under certain circumstances, the convertible notes are convertible into cash, or a combination of cash and common stock at the company's election, based on an initial conversion rate of 40.8513 shares of the company's common stock per $1,000 in principal amount of the convertible notes (equivalent to an initial conversion price of approximately $24.48 per share). At any time on or after January 21, 2014, the convertible notes are subject to redemption at the option of the company, in whole or in part, for cash equal to 100% of the principal amount of the convertible notes, plus unpaid interest, if any, accrued to the redemption date. Holders of the convertible notes may require the company to repurchase the convertible notes, in whole or in part, on January 15, 2014, January 15, 2017 or January 15, 2022, for cash equal to 100% of the principal amount of the convertible notes, plus any unpaid interest, if any, accrued to the redemption date. In addition, holders of the convertible notes may require the company to repurchase the convertible notes, in whole or in part, for cash equal to 100% of the principal amount of the convertible notes, plus any unpaid interest, if any, accrued to the redemption date, if a 'fundamental change' occurs prior to maturity of the convertible notes.

Fair value of debt

143.7

145.2

Term of credit facility (in years)

Maximum borrowing capacity

300

Expiration date of credit facility

June 7, 2016

Capacity for the issuance of standby letters of credit

125

Maximum increase in borrowing capacity

150

Interest rate description

Loans under the Credit Facility bear interest at LIBOR plus an applicable margin ranging from 1.75% to 2.50%, with the applicable margin varying according to the company’s total leverage ratio, or, at the election of the company, at a prime base rate plus 0.75% to 1.50%. Letters of credit issued under the Credit Facility accrue fees at a rate equal to the applicable margin for LIBOR loans.

Unused capacity, commitment fee description

In addition the company is required to pay a quarterly commitment fee for the unused portion of the Credit Facility if any at a rate ranging from 0.30% to 0.50%

Letters of credit issued

15.4

Amount available for borrowings

$ 284.6

Debt Obligations (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		3 Months Ended		3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Convertible Senior Subordinated Notes [Member]	Dec. 13, 2006 Convertible Senior Subordinated Notes [Member]	Mar. 31, 2012 Credit Facility [Member]
Debt Instrument [Line Items]
Face amount				$ 143.8
Interest rate (in hundredths)				2.4375%
Maturity date			2027
Terms of conversion feature	Under certain circumstances, the convertible notes are convertible into cash, or a combination of cash and common stock at the company's election, based on an initial conversion rate of 40.8513 shares of the company's common stock per $1,000 in principal amount of the convertible notes (equivalent to an initial conversion price of approximately $24.48 per share). At any time on or after January 21, 2014, the convertible notes are subject to redemption at the option of the company, in whole or in part, for cash equal to 100% of the principal amount of the convertible notes, plus unpaid interest, if any, accrued to the redemption date. Holders of the convertible notes may require the company to repurchase the convertible notes, in whole or in part, on January 15, 2014, January 15, 2017 or January 15, 2022, for cash equal to 100% of the principal amount of the convertible notes, plus any unpaid interest, if any, accrued to the redemption date. In addition, holders of the convertible notes may require the company to repurchase the convertible notes, in whole or in part, for cash equal to 100% of the principal amount of the convertible notes, plus any unpaid interest, if any, accrued to the redemption date, if a 'fundamental change' occurs prior to maturity of the convertible notes.
Fair value of debt	143.7	145.2
Term of credit facility (in years)					5Y
Maximum borrowing capacity					300
Expiration date of credit facility					June 7, 2016
Capacity for the issuance of standby letters of credit					125
Maximum increase in borrowing capacity					150
Interest rate description	Loans under the Credit Facility bear interest at LIBOR plus an applicable margin ranging from 1.75% to 2.50%, with the applicable margin varying according to the company’s total leverage ratio, or, at the election of the company, at a prime base rate plus 0.75% to 1.50%. Letters of credit issued under the Credit Facility accrue fees at a rate equal to the applicable margin for LIBOR loans.
Unused capacity, commitment fee description	In addition the company is required to pay a quarterly commitment fee for the unused portion of the Credit Facility if any at a rate ranging from 0.30% to 0.50%
Letters of credit issued					15.4
Amount available for borrowings					$ 284.6