Fair Value Measurements - Assets and Liabilities (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Assets and liabilities measured at estimated fair value on a recurring basis
Debt	$ 742,300,000	$ 910,200,000
Assets:
Derivative assets	559,100,000	312,000,000.0
Liabilities:
Derivative liabilities	581,600,000	332,500,000
Transfers from Level 1 to Level 2, assets	0	0
Transfers from Level 2 to Level 1, assets	0	0
Transfers from Level 1 to Level 2, liabilities	0	0
Transfers from Level 2 to Level 1, liabilities	0	0
Commodity contracts
Assets:
Derivative assets	556,700,000	303,600,000
Liabilities:
Derivative liabilities	579,000,000.0	323,900,000
Foreign currency contracts
Assets:
Derivative assets	2,400,000	8,500,000
Liabilities:
Derivative liabilities	2,600,000	8,700,000
Fair value measured on recurring basis
Assets:
Total assets at fair value	565,500,000	317,700,000
Liabilities:
Total liabilities at fair value	581,600,000	332,500,000
Fair value measured on recurring basis | Cash surrender value of life insurance
Assets:
Cash surrender value of life insurance	6,400,000	5,600,000
Fair value measured on recurring basis | Commodity contracts
Assets:
Derivative assets	556,700,000	303,600,000
Liabilities:
Derivative liabilities	579,000,000.0	323,900,000
Fair value measured on recurring basis | Foreign currency contracts
Assets:
Derivative assets	2,400,000	8,500,000
Liabilities:
Derivative liabilities	2,600,000	8,700,000
Level 1 Inputs | Fair value measured on recurring basis
Assets:
Total assets at fair value	421,800,000	196,300,000
Liabilities:
Total liabilities at fair value	414,700,000	210,600,000
Level 1 Inputs | Fair value measured on recurring basis | Cash surrender value of life insurance
Assets:
Cash surrender value of life insurance	0	0
Level 1 Inputs | Fair value measured on recurring basis | Commodity contracts
Assets:
Derivative assets	421,800,000	196,300,000
Liabilities:
Derivative liabilities	414,700,000	210,600,000
Level 1 Inputs | Fair value measured on recurring basis | Foreign currency contracts
Assets:
Derivative assets	0	0
Liabilities:
Derivative liabilities	0	0
Level 2 Inputs | Fair value measured on recurring basis
Assets and liabilities measured at estimated fair value on a recurring basis
Debt	742,300,000	910,200,000
Notes receivable	26,900,000	44,900,000
Assets:
Total assets at fair value	140,200,000	120,200,000
Liabilities:
Total liabilities at fair value	164,300,000	120,500,000
Level 2 Inputs | Fair value measured on recurring basis | Cash surrender value of life insurance
Assets:
Cash surrender value of life insurance	6,400,000	5,600,000
Level 2 Inputs | Fair value measured on recurring basis | Commodity contracts
Assets:
Derivative assets	131,400,000	106,100,000
Liabilities:
Derivative liabilities	161,700,000	111,800,000
Level 2 Inputs | Fair value measured on recurring basis | Foreign currency contracts
Assets:
Derivative assets	2,400,000	8,500,000
Liabilities:
Derivative liabilities	2,600,000	8,700,000
Level 3 Inputs | Fair value measured on recurring basis
Assets:
Total assets at fair value	3,500,000	1,200,000
Liabilities:
Total liabilities at fair value	2,500,000	1,400,000
Level 3 Inputs | Fair value measured on recurring basis | Cash surrender value of life insurance
Assets:
Cash surrender value of life insurance	0	0
Level 3 Inputs | Fair value measured on recurring basis | Commodity contracts
Assets:
Derivative assets	3,500,000	1,200,000
Liabilities:
Derivative liabilities	2,500,000	1,400,000
Level 3 Inputs | Fair value measured on recurring basis | Foreign currency contracts
Assets:
Derivative assets	0	0
Liabilities:
Derivative liabilities	$ 0	$ 0