Summary of Significant Accounting Policies (Revenue Recognition) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Balance Sheet
Other assets	$ 417.8				$ 335.2				$ 417.8	$ 335.2		$ 379.0
Deferred income taxes and other long-term tax liabilities, net	1,028.3				796.4				1,028.3	796.4		806.8
Retained earnings	4,750.5				4,152.5				4,750.5	4,152.5		4,185.9
Income Statement
Revenue	2,530.0	$ 2,565.7	$ 2,517.8	$ 2,427.5	2,560.0	$ 2,562.0	$ 2,526.7	$ 2,392.8	10,040.9	10,041.5	$ 9,387.7
Cost of operations									6,150.0	6,214.6	5,764.0
Depreciation, amortization and depletion									1,033.4	1,036.3	991.1
Selling, general and administrative									1,059.5	1,057.4	969.8
Operating income	$ 483.3	$ 440.3	$ 408.2	$ 404.2	406.8	$ 448.1	$ 425.5	$ 388.1	1,735.8	1,668.5	$ 1,537.5
As Computed Excluding the Adoption of ASU 2014-09
Income Statement
Revenue									10,383.8
Cost of operations									6,487.0
Depreciation, amortization and depletion									1,039.3
Selling, general and administrative									1,059.9
Operating income									1,735.4
Accounting Standards Update 2014-09 | Effect of Change
Income Statement
Revenue									342.9
Cost of operations									337.0
Depreciation, amortization and depletion									5.9
Selling, general and administrative									0.4
Operating income									$ (0.4)
Calculated under Revenue Guidance in Effect before Topic 606
Balance Sheet
Other assets					335.2					335.2
Deferred income taxes and other long-term tax liabilities, net					796.4					796.4
Retained earnings					$ 4,152.5					$ 4,152.5
Difference between Revenue Guidance in Effect before and after Topic 606 | Accounting Standards Update 2014-09
Balance Sheet
Other assets												43.8
Deferred income taxes and other long-term tax liabilities, net												10.4
Retained earnings												$ 33.4