Fair Value Measurements - Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Assets and liabilities measured at estimated fair value on a recurring basis
Debt	$ 1,186.3	$ 772.2
Total derivative assets	347.2	529.6
Gross Amounts Recognized	332.5	468.5
Commodity contracts
Assets and liabilities measured at estimated fair value on a recurring basis
Total derivative assets	331.2	517.3
Gross Amounts Recognized	326.1	463.4
Foreign currency contracts
Assets and liabilities measured at estimated fair value on a recurring basis
Total derivative assets	16.0	12.4
Gross Amounts Recognized	6.4	5.0
Fair value measured on recurring basis
Assets and liabilities measured at estimated fair value on a recurring basis
Total assets	351.2	532.1
Total liabilities	332.5	468.5
Fair value measured on recurring basis | Cash surrender value of life insurance
Assets and liabilities measured at estimated fair value on a recurring basis
Cash surrender value of life insurance	4.0	2.4
Fair value measured on recurring basis | Commodity contracts
Assets and liabilities measured at estimated fair value on a recurring basis
Total derivative assets	331.2	517.3
Gross Amounts Recognized	326.1	463.4
Fair value measured on recurring basis | Foreign currency contracts
Assets and liabilities measured at estimated fair value on a recurring basis
Total derivative assets	16.0	12.4
Gross Amounts Recognized	6.4	5.0
Level 1
Assets and liabilities measured at estimated fair value on a recurring basis
Total assets	273.6	262.0
Total liabilities	236.6	339.8
Level 1 | Cash surrender value of life insurance
Assets and liabilities measured at estimated fair value on a recurring basis
Cash surrender value of life insurance	0.0	0.0
Level 1 | Commodity contracts
Assets and liabilities measured at estimated fair value on a recurring basis
Total derivative assets	273.6	262.0
Gross Amounts Recognized	236.6	339.8
Level 1 | Foreign currency contracts
Assets and liabilities measured at estimated fair value on a recurring basis
Total derivative assets	0.0	0.0
Gross Amounts Recognized	0.0	0.0
Level 2
Assets and liabilities measured at estimated fair value on a recurring basis
Total assets	75.3	266.8
Total liabilities	95.2	128.4
Level 2 | Cash surrender value of life insurance
Assets and liabilities measured at estimated fair value on a recurring basis
Cash surrender value of life insurance	4.0	2.4
Level 2 | Commodity contracts
Assets and liabilities measured at estimated fair value on a recurring basis
Total derivative assets	55.3	252.0
Gross Amounts Recognized	88.8	123.4
Level 2 | Foreign currency contracts
Assets and liabilities measured at estimated fair value on a recurring basis
Total derivative assets	16.0	12.4
Gross Amounts Recognized	6.4	5.0
Level 3
Assets and liabilities measured at estimated fair value on a recurring basis
Total assets	2.3	3.3
Total liabilities	0.7	0.2
Level 3 | Cash surrender value of life insurance
Assets and liabilities measured at estimated fair value on a recurring basis
Cash surrender value of life insurance	0.0	0.0
Level 3 | Commodity contracts
Assets and liabilities measured at estimated fair value on a recurring basis
Total derivative assets	2.3	3.3
Gross Amounts Recognized	0.7	0.2
Level 3 | Foreign currency contracts
Assets and liabilities measured at estimated fair value on a recurring basis
Total derivative assets	0.0	0.0
Gross Amounts Recognized	0.0	0.0
Level 3 | Fair value measured on recurring basis
Assets and liabilities measured at estimated fair value on a recurring basis
Debt	1,200.0	772.2
Notes receivable	$ 16.9	$ 12.7