Long-Term Debt, Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Current portion of long-term debt	$ 65,000,000	$ 39,005,000
Total debt outstanding, Fair value	1,133,842,000
Commitment fee (in hundredths)	0.30%
Interest rate, variable rate basis	The variable interest rate spread is currently 1.5% over LIBOR or 0.5% over the Alternate Base Rate.
Senior Notes Series A Due February 27, 2020 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage (in hundredths)	2.72%
Credit facility, amount outstanding	82,500,000	0
Senior Notes Series B Due February 27, 2023 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage (in hundredths)	3.29%
Credit facility, amount outstanding	192,500,000	0
Term Loan Due May 31, 2016 [Member]
Debt Instrument [Line Items]
Credit facility, amount outstanding	468,000,000	507,000,000
Revolving Credit Facility Due November 9, 2015 [Member]
Debt Instrument [Line Items]
Credit facility, amount outstanding	185,000,000	95,000,000
Credit Line Due June June 30, 2013 [Member]
Debt Instrument [Line Items]
Credit facility, amount outstanding	7,110,000	0
Term Loan [Member]
Debt Instrument [Line Items]
Average borrowing under revolving credit facility	495,517,000
Current portion of long-term debt	65,000,000
Term loan, duration	5 years
Interest rate terms	The interest rate spread varies with the Company's senior debt rating and is currently 1.5% over LIBOR or 0.5% over the Alternate Base Rate.
Weighted average interest rate (in hundredths)	1.80%
Senior Notes [Member]
Debt Instrument [Line Items]
Debt average interest rate (in hundredths)	1.00%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Average borrowing under revolving credit facility	90,388,000
Commitment amount	25,000,000
Credit facility, amount outstanding	3,936,000
Weighted average interest rate (in hundredths)	1.80%
Line Of Credit [Member]
Debt Instrument [Line Items]
Credit facility, amount outstanding	$ 858,000