Derivatives (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012
Interest rate swaps maturing June 2017 [Member]
Summarized details of swaps outstanding and the related hedged items
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	1.44%	1.62%
Interest rate swaps maturing March 2015 [Member]
Summarized details of swaps outstanding and the related hedged items
Issuance date	Feb. 01, 2011
Maturity date	Mar. 01, 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	1.34%	1.66%
Interest Rate Swaps Maturing November Two Thousand Eighteeen [Member] [Domain]
Summarized details of swaps outstanding and the related hedged items
Issuance date	Nov. 04, 2013
Maturity date	Nov. 03, 2018
Face value of medium - term notes	300,000
Aggregate notional amount of interest rate swaps	$ 100,000
Fixed interest rate	2.45%
Weighted-average variable interest rate on hedged debt	1.19%