Long-Term Debt, Narrative (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Senior Notes	$ 500,000,000
Additional notes issued	225,000,000	275,000,000	531,646,000
Current portion of long-term debt	0	65,000,000
Total debt outstanding, Fair value	710,377,000	1,130,967,000
Commitment fee (in hundredths)	0.30%
Senior Notes Series A Due February 27, 2020 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage (in hundredths)	2.72%
Additional notes issued	675,000,000
Credit facility, amount outstanding	150,000,000	82,500,000
Senior Notes Series B Due February 27, 2023 [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage (in hundredths)	3.29%
Additional notes issued	157,500,000
Credit facility, amount outstanding	350,000,000	192,500,000
Term Loan Due July 1, 2016 [Member]
Debt Instrument [Line Items]
Credit facility, amount outstanding	208,000,000	468,000,000
Revolving Credit Facility Due November 9, 2015 [Member]
Debt Instrument [Line Items]
Credit facility, amount outstanding	41,150,000	185,000,000
Term Loan [Member]
Debt Instrument [Line Items]
Maturity date	Jul. 01, 2016
Average borrowing under revolving credit facility	401,270,000
Current portion of long-term debt	0
Term loan, duration	5 years
Weighted average interest rate (in hundredths)	1.70%
Term Loan [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Interest rate, variable rate basis	LIBOR
Basis spread on variable rate (in hundredths)	1.50%
Term Loan [Member] | Alternate Base Rate [Member]
Debt Instrument [Line Items]
Interest rate, variable rate basis	Alternate Base Rate
Basis spread on variable rate (in hundredths)	0.50%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maturity date	Nov. 09, 2015
Average borrowing under revolving credit facility	75,113,000
Commitment amount	25,000,000
Credit facility, amount outstanding	3,930,000
Weighted average interest rate (in hundredths)	1.70%
Revolving Credit Facility [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Interest rate, variable rate basis	LIBOR
Basis spread on variable rate (in hundredths)	1.50%
Revolving Credit Facility [Member] | Alternate Base Rate [Member]
Debt Instrument [Line Items]
Interest rate, variable rate basis	Alternate Base Rate
Basis spread on variable rate (in hundredths)	0.50%
Line Of Credit [Member]
Debt Instrument [Line Items]
Credit facility, amount outstanding	$ 869,000