Debt and Financing Arrangements (Financing Structure) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Components of financing structure:
Oustanding, total	$ 1,816,445	[1]	$ 1,616,987	[1]
Balance, debt	798,215	[2]	797,949	[2]
$1,000,000 secured letter of credit facility - uncommitted [Member]
Components of financing structure:
Credit facility, commitment	1,000,000
Credit facility, outstanding	907,592	[1]
$400,000 secured letter of credit facility (pre-amendment) [Member]
Components of financing structure:
Credit facility, commitment			400,000
Credit facility, outstanding			141,351	[1]
$450,000 secured letter of credit facility [Member]
Components of financing structure:
Credit facility, commitment	450,000
Credit facility, outstanding	108,853	[1]
$900,000 secured letter of credit facility - uncommitted
Components of financing structure:
Credit facility, commitment			900,000
Credit facility, outstanding			675,636	[1]
$400,000 unsecured letter of credit facility [Member]
Components of financing structure:
Credit facility, commitment			400,000
Credit facility, outstanding			0	[1]
5.5% Senior notes due 2020 [Member]
Components of financing structure:
Outstanding, debt	300,000	[1]	300,000	[1]
Balance, debt	298,868	[2]	298,755	[2]
7.5% Senior notes due 2016 [Member]
Components of financing structure:
Outstanding, debt	500,000	[1]	500,000	[1]
Balance, debt	$ 499,347	[2]	$ 499,194	[2]

[1] Indicates utilization of commitment amount, not drawn borrowings.

[2] Represents the principal amount borrowed, net of unamortized discount.