New Accounting Pronouncements Revenue Recognition (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017
Sales	$ 3,625.1	$ 3,552.9	$ 10,313.6	$ 10,259.8
Excise and Sales Taxes	(690.9)	(669.7)	(1,962.7)	(1,836.6)
Net Sales	2,934.2	2,883.2	8,350.9	8,423.2
Cost of Goods Sold	(1,714.0)	(1,589.1)	(4,988.8)	(4,716.9)
Gross Profit	1,220.2	1,294.1	3,362.1	3,706.3
Selling, General and Administrative Expense	(713.9)	(783.8)	(2,139.7)	(2,271.5)
Special items	(36.6)	(4.1)	267.7	(27.3)
Operating Income (Loss)	469.7	506.2	1,490.1	1,407.5
Interest income (expense), net	(67.4)	(72.6)	(227.3)	(258.4)
Other pension and postretirement benefits (costs), net		9.6	27.5	32.3
Other pension and postretirement benefits (costs), net	7.6	9.6	27.5	32.3
Other income (expense), net	0.2	(2.7)	0.2	0.2
Income (loss) before income taxes	410.1	440.5	1,290.5	1,181.6
Income tax benefit (expense)	(64.5)	(147.4)	(231.6)	(338.5)
Income (Loss) Net of Tax, Including Portion Attributable to Noncontrolling Interest	345.6	293.1	1,058.9	843.1
Net income (loss) including noncontrolling interests	345.6	293.1	1,058.9	843.1
Net Income (Loss) Attributable to Noncontrolling Interest	(7.3)	(6.1)	(18.4)	(17.7)
Net income (loss) attributable to Molson Coors Brewing Company	$ 338.3	$ 287.0	$ 1,040.5	$ 825.4
Income (Loss) from Continuing Operations, Per Basic Share	$ 1.57	$ 1.33	$ 4.82	$ 3.83
Income (Loss) from Continuing Operations, Per Diluted Share	$ 1.56	$ 1.33	$ 4.80	$ 3.81
Accounts receivable, net	$ 933.4		$ 933.4			$ 733.8
Other current assets, net	312.7		312.7			277.6
Accounts payable and other current liabilities	2,820.6		2,820.6			2,684.5
Deferred tax liabilities	1,853.6		1,853.6			1,648.6
Retained earnings	7,953.2		7,953.2			7,206.1
Accumulated Other Comprehensive Income (Loss), Net of Tax	(996.4)		(996.4)			$ (860.0)
Initial Application Period Cumulative Effect Transition for Accounting Standards Update 2014-09			(27.8)
Calculated Under Revenue Guidance In Effect Before Topic 606 Member [Member]
Sales	3,637.8		10,367.1
Excise and Sales Taxes	(690.9)		(1,962.7)
Net Sales	2,946.9		8,404.4
Cost of Goods Sold	(1,714.0)		(4,988.8)
Gross Profit	1,232.9		3,415.6
Selling, General and Administrative Expense	(733.3)		(2,185.2)
Special items	(36.6)		267.7
Operating Income (Loss)	463.0		1,498.1
Interest income (expense), net	(66.6)		(224.8)
Other pension and postretirement benefits (costs), net	7.6		27.5
Other income (expense), net	0.2		0.2
Income (loss) before income taxes	404.2		1,301.0
Income tax benefit (expense)	(63.6)		(233.7)
Income (Loss) Net of Tax, Including Portion Attributable to Noncontrolling Interest	340.6		1,067.3
Net Income (Loss) Attributable to Noncontrolling Interest	(7.3)		(18.4)
Net income (loss) attributable to Molson Coors Brewing Company	$ 333.3		$ 1,048.9
Income (Loss) from Continuing Operations, Per Basic Share	$ 1.54		$ 4.86
Income (Loss) from Continuing Operations, Per Diluted Share	$ 1.54		$ 4.84
Accounts receivable, net	$ 933.2		$ 933.2
Other current assets, net	306.3		306.3
Accounts payable and other current liabilities	2,766.8		2,766.8
Deferred tax liabilities	1,864.7		1,864.7
Accumulated Other Comprehensive Income (Loss), Net of Tax	(996.5)		(996.5)
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Retained earnings	7,989.4		7,989.4
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Restatement Adjustment [Member]
Sales	(12.7)		(53.5)
Excise and Sales Taxes	0.0		0.0
Net Sales	(12.7)		(53.5)
Cost of Goods Sold	0.0		0.0
Gross Profit	(12.7)		(53.5)
Selling, General and Administrative Expense	19.4		45.5
Special items	0.0		0.0
Operating Income (Loss)	6.7		(8.0)
Interest income (expense), net	(0.8)		(2.5)
Other pension and postretirement benefits (costs), net	0.0		0.0
Other income (expense), net	0.0		0.0
Income (loss) before income taxes	5.9		(10.5)
Income tax benefit (expense)	(0.9)		2.1
Income (Loss) Net of Tax, Including Portion Attributable to Noncontrolling Interest	5.0		(8.4)
Net Income (Loss) Attributable to Noncontrolling Interest	0.0		0.0
Net income (loss) attributable to Molson Coors Brewing Company	$ 5.0		$ (8.4)
Income (Loss) from Continuing Operations, Per Basic Share	$ 0.03		$ (0.04)
Income (Loss) from Continuing Operations, Per Diluted Share	$ 0.02		$ (0.04)
Accounts receivable, net	$ 0.2		$ 0.2
Other current assets, net	6.4		6.4
Accounts payable and other current liabilities	53.8		53.8
Deferred tax liabilities	(11.1)		(11.1)
Retained earnings	(36.2)		(36.2)
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ 0.1		0.1
Retained earnings
Net income (loss) including noncontrolling interests			1,040.5	$ 825.4
Initial Application Period Cumulative Effect Transition for Accounting Standards Update 2014-09			$ (27.8)
Forecast [Member] | Minimum [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification					$ 60.0
Forecast [Member] | Maximum [Member]
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification					$ 65.0