Derivatives (Details) (USD $)			9 Months Ended		9 Months Ended		9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Interest rate swaps maturing March 2015 [Member]	Sep. 30, 2013 Interest rate swaps maturing March 2015 [Member]	Sep. 30, 2014 Interest rate swaps maturing June 2017 [Member]	Sep. 30, 2013 Interest rate swaps maturing June 2017 [Member]	Sep. 30, 2014 Interest rate swaps maturing November 2018 [Member]	Sep. 30, 2014 Interest rate swaps maturing June 2019 [Member]	Sep. 30, 2014 Interest rate swaps maturing September 2019 [Member]
Summary Of Derivative Instruments And Hedged Items [Abstract]
Issuance date			Feb. 24, 2011		May 24, 2011		Nov. 12, 2013	Feb. 25, 2014	May 06, 2014
Maturity date			Mar. 02, 2015		Jun. 01, 2017		Nov. 15, 2018	Jun. 01, 2019	Sep. 03, 2019
Face value of medium - term notes			$ 350,000,000		$ 350,000,000		$ 300,000,000	$ 350,000,000	$ 400,000,000
Aggregate notional amount of interest rate swaps	$ 600,000,000	$ 400,000,000	$ 150,000,000		$ 150,000,000		$ 100,000,000	$ 100,000,000	$ 100,000,000
Fixed interest rate			3.15%		3.50%		2.45%	2.55%	2.45%
Weighted-average variable interest rate on hedged debt			1.28%	1.34%	1.42%	1.51%	1.18%	1.10%	0.86%