Debt Narratives (Details) (USD $)	3 Months Ended		6 Months Ended			6 Months Ended			3 Months Ended	6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 May 2019	Jun. 30, 2014 April 2019	Jun. 30, 2014 April 2015	Sep. 30, 2014 July 2018	Jun. 30, 2014 July 2018	Jun. 30, 2014 July 2014	Jun. 30, 2014 July 2016	Jun. 30, 2014 February 2013	Jun. 30, 2014 Unsecured Debt May 2019	Dec. 31, 2013 Unsecured Debt May 2019	Jun. 30, 2014 Unsecured Debt April 2019	Dec. 31, 2013 Unsecured Debt April 2019	Jun. 30, 2014 Unsecured Debt April 2015	Dec. 31, 2013 Unsecured Debt April 2015
Debt Instrument [Line Items]
Reclassification of debt					$ 30,000,000
Senior Notes														0	250,000,000	0	200,000,000	0	136,465,000
Debt, Weighted Average Interest Rate	5.11%		5.11%		5.25%
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	72.00%		72.00%		76.00%
Line of Credit Facility, Maximum Borrowing Capacity	500,000,000		500,000,000
Long-term Line of Credit	465,000,000		465,000,000		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	31,600,000		31,600,000
Repayments of Debt and Capital Lease Obligations			898,200,000
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.35%
Line of Credit Facility, Remaining Borrowing Capacity	233,400,000		233,400,000
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			14,491,000	12,967,000
Repayments of Debt			5,200,000	400,000		250,000,000	200,000,000	136,500,000	13,000,000	135,000,000	86,400,000	45,000,000	4,800,000
Repayments of Long-term Debt			135,371,000	3,951,000			9,100,000				21,700,000	14,200,000
Repayments of Other Debt			300,000
Gains (Losses) on Extinguishment of Debt	(29,158,000)	468,000	(29,158,000)	468,000
Capital Lease Obligations Incurred			$ 19,000,000	$ 28,500,000