Debt and Credit Agreements (Details 2) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Debt Instrument [Line Items]
Long-term debt	$ 1,995,000		$ 1,995,000		$ 1,752,000
Revolving credit facility
Debt Instrument [Line Items]
Long-term debt	$ 383,000		$ 383,000		$ 140,000
Long-term debt, weighted average interest rate	2.00%		2.00%
Availability under the credit facility	$ 1,400,000		$ 1,400,000
Weighted-average effective interest rate	2.20%	2.10%	2.30%	2.20%