Debt and Credit Agreements (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Current maturities	$ (20,000)	$ 0
Long-term debt, excluding current maturities	2,017,000	1,752,000
7.33% weighted-average fixed rate notes
Debt Instrument [Line Items]
Long-term debt	20,000	20,000
Current maturities	(20,000)	0
6.51% weighted-average fixed rate notes
Debt Instrument [Line Items]
Long-term debt	425,000	425,000
9.78% fixed rate notes
Debt Instrument [Line Items]
Long-term debt	67,000	67,000
5.58% weighted-average fixed rate notes
Debt Instrument [Line Items]
Long-term debt	175,000	175,000
3.65% weighted-average fixed rate notes
Debt Instrument [Line Items]
Long-term debt	925,000	925,000
Revolving credit facility
Debt Instrument [Line Items]
Long-term debt	$ 425,000	$ 140,000