Debt Narratives (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Debt Instrument [Line Items]
Reclassification of debt					$ 30,000,000
Debt, Weighted Average Interest Rate	5.19%		5.19%		5.25%
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	74.00%		74.00%		76.00%
Line of Credit Facility, Maximum Borrowing Capacity	500,000,000		500,000,000
Long-term Line of Credit	377,500,000		377,500,000		600,000,000
Letters of Credit, Maximum Borrowing Capacity	175,000,000		175,000,000
Line of Credit Facility, Amount Outstanding	235,000,000		235,000,000		30,000,000
Letters of Credit Outstanding, Amount	31,700,000		31,700,000
Repayments of Debt and Capital Lease Obligations			22,100,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.35%
Line of Credit Facility, Remaining Borrowing Capacity	233,300,000		233,300,000
Proceeds from Issuance of Senior Long-term Debt			550,000,000
Line of Credit Facility, Increase (Decrease), Net			95,000,000
Repayments of Long-term Capital Lease Obligations			21,979,000	19,585,000
Repayments of Debt			985,800,000	19,700,000
Repayments of Long-term Debt			222,958,000	90,435,000
Repayments of Other Debt			400,000	100,000
Gains (Losses) on Extinguishment of Debt	0	0	(29,158,000)	468,000
Capital Lease Obligations Incurred			31,100,000	34,000,000
May 2019
Debt Instrument [Line Items]
Repayments of Debt			250,000,000
April 2019
Debt Instrument [Line Items]
Repayments of Debt			200,000,000
Repayments of Long-term Debt			9,100,000
April 2015
Debt Instrument [Line Items]
Repayments of Debt			136,500,000
July 2018
Debt Instrument [Line Items]
Repayments of Debt			222,500,000
July 2014
Debt Instrument [Line Items]
Repayments of Debt			86,400,000
Repayments of Long-term Debt			21,700,000
July 2016
Debt Instrument [Line Items]
Repayments of Debt			45,000,000
Repayments of Long-term Debt			14,200,000
February 2013
Debt Instrument [Line Items]
Repayments of Debt and Capital Lease Obligations			19,600,000
Unsecured Debt | May 2019
Debt Instrument [Line Items]
Senior Notes	0		0		250,000,000
Unsecured Debt | April 2019
Debt Instrument [Line Items]
Senior Notes	0		0		200,000,000
Unsecured Debt | April 2015
Debt Instrument [Line Items]
Senior Notes	$ 0		$ 0		$ 136,465,000