Derivatives (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
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	1.62%	1.84%
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	1.66%	1.43%
Interest rate swaps maturing March 2013 [Member]
Summarized details of swaps outstanding and the related hedged items
Issuance date	Feb. 01, 2008
Maturity date	Mar. 01, 2013
Face value of medium - term notes	250,000
Aggregate notional amount of interest rate swaps	$ 250,000
Fixed interest rate	6.00%
Weighted-average variable interest rate on hedged debt	2.84%	2.61%