Assets and Liabilities Measured at Fair Value (Carrying Value of Assets and Liabilities Recorded at Fair Value on Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Commodity derivative contracts		$ 89,358
Assets, Total		89,358
Liabilities, Nonqualified employee savings plan	$ 13,904	12,971
Liabilities, Commodity derivative contracts	48,864
Liabilities, Foreign currency exchange derivative contracts	73	29
Liabilities, Total	62,841	13,000
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Nonqualified employee savings plan	13,904	12,971
Liabilities, Total	13,904	12,971
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Commodity derivative contracts		89,358
Assets, Total		89,358
Liabilities, Commodity derivative contracts	48,864
Liabilities, Foreign currency exchange derivative contracts	73	29
Liabilities, Total	$ 48,937	$ 29