Retirement Benefits - Narrative (Details) - USD ($) shares in Millions, $ in Millions	1 Months Ended		12 Months Ended
	Mar. 31, 2017	Jul. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Projected Retirement Benefit Expense In Next 12 months				$ 52.0
Other postretirement benefit costs for a defined contribution plan				35.5	$ 34.5	$ 41.2
Other Postretirement Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions				0.0	$ 0.0
Projected Retirement Benefit Expense In Next 12 months				23.0
Other postretirement benefit costs for a defined contribution plan				$ 1.7
Assumed increase in per capita cost of covered benefits in next 12 months				6.80%
Assumed ultimate health care cost trend rate				4.50%
Year that ultimate trend rate is reached				2038
Expected long-term rate of return on assets				4.00%	4.00%	4.00%
Fair value of plan assets				$ 0.6	$ 0.7	$ 1.8
Other Postretirement Benefits [Member] | Fair Value, Inputs, Level 3 [Member] | Private equity [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets				0.6
Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions				143.4	125.1
Projected Retirement Benefit Expense In Next 12 months				29.0
Accumulated benefit obligation				$ 2,810.5	$ 2,710.7
Expected long-term rate of return on assets				7.75%	8.00%	8.00%
Actual return on plan assets				16.90%	5.30%	(1.20%)	6.50%	14.30%
Fair value of plan assets				$ 2,129.6	$ 1,894.6	$ 1,962.3
Defined Benefit Plan, Weighted Average Life Expectancy				17 years
Pension Benefits [Member] | Private equity [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets				$ 137.5	142.6
Pension Benefits [Member] | Hedge funds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets				307.4	283.9
Pension Benefits [Member] | Real estate and other [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets				93.9	96.5
Pension Benefits [Member] | Fair Value, Inputs, Level 3 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets				0.0	0.2	$ 194.6
Pension Benefits [Member] | Fair Value, Inputs, Level 3 [Member] | Private equity [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets				0.0	0.0
Pension Benefits [Member] | Fair Value, Inputs, Level 3 [Member] | Hedge funds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets				0.0	0.0
Pension Benefits [Member] | Fair Value, Inputs, Level 3 [Member] | Real estate and other [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets				0.0	$ 0.0
Pension Benefits [Member] | Forecast [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Expected long-term rate of return on assets			7.75%
Pension Benefits [Member] | MultiemployerPlan1 [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Percent of Current Year Contributions to Plan that Company Contributions Exceed					5.00%
Pension Benefits [Member] | Nonqualified and U.K. [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Estimated employer contributions in next 12 months				9.0
Pension Benefits [Member] | U.S. [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employer contributions	$ 135.0	$ 115.0
Estimated employer contributions in next 12 months				$ 40.0
Percentage of total assets				96.00%
Number of shares of equity securities issued by employer included in plan assets				3.0	3.0
Fair value of employer equity securities included in plan assets				$ 71.3	$ 47.1
Dividends on employer stock received by plan					0.7
Pension Benefits [Member] | U.S. [Member] | Global debt securities and cash [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Unfunded commitments				84.0
Pension Benefits [Member] | U.S. [Member] | Private equity [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Unfunded commitments				72.0
Pension Benefits [Member] | U.S. [Member] | Hedge funds [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Unfunded commitments				11.0
Pension Benefits [Member] | U.S. [Member] | Real estate and other [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Unfunded commitments				10.0
Voluntary lump sum cash out [Member] | Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefits paid				$ 22.0
Small annuity buy out [Member] | Pension Benefits [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Benefits paid					$ 47.0