Derivative Instruments (Details) - USD ($) $ in Thousands		1 Months Ended					12 Months Ended
	Oct. 03, 2011	Dec. 31, 2011	Sep. 30, 2011	Jul. 31, 2011	Dec. 31, 2010	Apr. 30, 2010	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 09, 2011	Sep. 29, 2011	Jul. 12, 2011	Dec. 06, 2010	Apr. 15, 2010
Derivative [Line Items]
Interest rate contract with notional amount											$ 350,000
Future amortization interest expense over 10 year periods							$ 1,295	$ 1,612	$ 1,723
Interest rate exchange agreement expiration date			Feb. 28, 2012
Additional interest rate contract settlement		$ 4,000
Expense reclassified into interest expense for interest rate contracts							6,900	$ 7,000	$ 7,200
Amount to be recognized from hedged transactions within 12 months, approximate							$ 6,700
4.2% Senior Notes, due December 2021
Derivative [Line Items]
Interest rate on senior notes							4.20%			4.20%
4.5% Senior Notes, due December 2020
Derivative [Line Items]
Interest rate on senior notes							4.50%						4.50%
Interest Rate Exchange Agreement Expiring 2010
Derivative [Line Items]
Interest rate contract with notional amount														$ 300,000
Interest rate contract settlement amount					$ 31,000
Future amortization interest expense over 10 year periods					$ 31,000
Term of amortized interest expense, years					10 years
Interest rate									5.80%
Interest rate exchange agreement expiration date						Dec. 31, 2010
Interest Rate Exchange Agreement Expiring 2011
Derivative [Line Items]
Interest rate contract with notional amount												$ 350,000
Interest rate contract settlement amount	$ 34,700
Interest rate exchange agreement expiration date				Sep. 30, 2011
Interest Rate Exchange Agreements Expiring 2011 And 2012
Derivative [Line Items]
Future amortization interest expense over 10 year periods		37,900
Interest rate								5.30%
Settlement amount interest rate contract								$ 38,700
Interest rate settlement recognized as other expense		$ 800