Derivatives (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Interest rate swaps maturing June 2017 [Member]
Summary Of Derivative Instruments And Hedged Items [Abstract]
Issuance date	May 01, 2011
Maturity date	Jun. 01, 2017
Face value of medium - term notes	$ 350,000
Aggregate notional amount of interest rate swaps	150,000
Fixed interest rate	3.50%
Weighted-average variable interest rate on hedged debt	1.51%	1.83%
Interest rate swaps maturing March 2015 [Member]
Summary Of Derivative Instruments And Hedged Items [Abstract]
Issuance date	Feb. 01, 2011
Maturity date	Mar. 02, 2015
Face value of medium - term notes	350,000
Aggregate notional amount of interest rate swaps	$ 150,000
Fixed interest rate	3.15%
Weighted-average variable interest rate on hedged debt	1.41%	1.70%