Business Segments (Summary of Information by Business Segment) (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting [Abstract]
Number of operating segments | segment									1
Segment Reporting Information [Line Items]
Segment income (loss)	$ 47,600	$ 69,200	$ 32,700	$ 5,300	$ 77,500	$ 45,000	$ 51,800	$ 39,300	$ 154,800	$ 213,600	$ 245,300
Revenues from external customers									14,034,600	14,362,900	12,826,600
Interest income									3,200	1,500	1,300
Interest expense									(54,900)	(52,900)	(46,700)
Loss on early debt extinguishment									(14,800)	0	0
Income tax expense (benefit)									47,600	60,300	(5,200)
Significant noncash charges (credits)
Depreciation and amortization									152,200	134,000	116,900
Accretion of asset retirement obligations									2,100	2,000	1,800
Debt extinguishment costs									4,400
Deferred and noncurrent income taxes (benefits)									23,700	37,900	(50,400)
Additions to property, plant and equipment									214,600	193,800	273,700
Total assets at year-end	2,687,200				2,360,800				2,687,200	2,360,800	2,331,000
Operating Segment | Marketing
Segment Reporting Information [Line Items]
Segment income (loss)									215,000	214,200	295,300
Revenues from external customers									14,034,300	14,362,300	12,826,200
Interest income									0	0	0
Interest expense									(100)	(100)	(100)
Loss on early debt extinguishment									0
Income tax expense (benefit)									66,300	69,500	(2,900)
Significant noncash charges (credits)
Depreciation and amortization									138,900	124,500	110,500
Accretion of asset retirement obligations									2,100	2,000	1,800
Debt extinguishment costs									0
Deferred and noncurrent income taxes (benefits)									32,900	39,000	(61,300)
Additions to property, plant and equipment									155,500	169,200	234,000
Total assets at year-end	2,304,700				2,012,000				2,304,700	2,012,000	2,023,400
Corporate and Other Assets
Segment Reporting Information [Line Items]
Segment income (loss)									(60,200)	(600)	(50,000)
Revenues from external customers									300	600	400
Interest income									3,200	1,500	1,300
Interest expense									(54,800)	(52,800)	(46,600)
Loss on early debt extinguishment									(14,800)
Income tax expense (benefit)									(18,700)	(9,200)	(2,300)
Significant noncash charges (credits)
Depreciation and amortization									13,300	9,500	6,400
Accretion of asset retirement obligations									0	0	0
Debt extinguishment costs									4,400
Deferred and noncurrent income taxes (benefits)									(9,200)	(1,100)	10,900
Additions to property, plant and equipment									59,100	24,600	39,700
Total assets at year-end	$ 382,500				$ 348,800				$ 382,500	$ 348,800	$ 307,600