Long-Term Debt (Narrative) (Details) (USD $)	1 Months Ended	3 Months Ended			6 Months Ended			6 Months Ended					1 Months Ended					1 Months Ended				6 Months Ended	12 Months Ended		1 Months Ended
	Mar. 31, 2011	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 Minimum [Member] Unrestricted Cash Reduction [Member]	Jun. 30, 2011 Maximum [Member] Unrestricted Cash Reduction [Member]	Jun. 30, 2011 Unrestricted Cash Reduction [Member]	Jun. 30, 2011 Including Acquisition Bridge Debt [Member]	Jun. 30, 2011 Excluding Acquisition Bridge Debt [Member]	Apr. 30, 2011 AR Credit Facility [Member]	Jun. 30, 2011 AR Credit Facility [Member]	Mar. 31, 2011 AR Credit Facility [Member]	Dec. 31, 2010 AR Credit Facility [Member]	Jun. 30, 2010 AR Credit Facility [Member]	Apr. 30, 2011 6.875% Notes, Due 2011 [Member]	Jun. 30, 2011 6.875% Notes, Due 2011 [Member]	Dec. 31, 2010 6.875% Notes, Due 2011 [Member]	Jun. 30, 2010 6.875% Notes, Due 2011 [Member]	Jun. 30, 2011 6.6% Senior Notes, Due 2018 [Member]	Dec. 31, 2010 6.6% Senior Notes, Due 2018 [Member]	Jun. 30, 2010 6.6% Senior Notes, Due 2018 [Member]	Mar. 31, 2011 Term Loan Due 2012 [Member]	Mar. 31, 2011 Term Loan Facility [Member]	Jun. 30, 2011 Revolving Credit Facility [Member]	Mar. 31, 2011 Revolving Credit Facility [Member]
Credit facility commitment															$ 100,000,000											$ 250,000,000		$ 350,000,000
Floating rate senior note repaid																		242,140,000
Outstanding borrowing under credit facility														100,000,000		0	25,000,000										60,000,000
Long-term debt		1,085,915,000		1,056,085,000	1,085,915,000	1,056,085,000	1,030,759,000													242,129,000	242,109,000	298,380,000	298,288,000	298,198,000
Line of credit facility, interest rate description			one-month LIBOR plus 1.35%		one-month LIBOR plus 1%																	6.6% Senior Notes in 2018
Additional borrowings under AR credit facility													100,000,000
Annual principal payment of debt			5,000,000
Repayment of debt	111,750,000
Debt instrument interest rate					Federal Funds rate plus 0.5%
Debt instrument maturity period	March 31, 2015																								2012
Interest rate on notes																		6.875%	6.875%			6.60%
Debt covenant										Additionally, if no amounts are outstanding under both the Revolving Facility and the AR Credit Facility, consolidated debt, including debt guaranteed by the Corporation, may be reduced by the Corporation's unrestricted cash and cash equivalents in excess of $50,000,000, such reduction not to exceed $200,000,000, for purposes of the covenant calculation.	3.5x	3.75x
Reduction of consolidated debt in debt ratio calculation								50,000,000	200,000,000
Additional interest expense		239,000		223,000	474,000	441,000
Increase in annual interest expense due to ongoing amortization of the terminated value of the swap agreements																							$ 1,000,000