Debt and Credit Agreements (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Long-term debt	$ 1,995,000	$ 1,752,000
7.33% weighted-average fixed rate notes
Debt Instrument [Line Items]
Long-term debt	20,000	20,000
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	$ 383,000	$ 140,000