Share-Based Compensation & Deferred Compensation Arrangements - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period for performance-based awards to executives and key non-officer employees	3 years
Measurement time frame for grants to executives and key nonofficer employees	3 years
Upper range of share based awards that may potentially be taken in cash (in units)	100,000	100,000	100,000
Total unrecognized compensation cost related to performance based awards	$ 14.2
Weighted-average life of unrecognized compensation cost	1 year 1 month 6 days
Deferred Compensation Plans [Abstract]
Liability associated with deferred compensation plans	$ 61.4	$ 40.9	$ 40.9
Portion of liability classified in Accrued liabilities	1.2	0.9
Impact of deferred compensation plans on Other operating expenses	13.1	4.3	0.1
Amount recorded in earnings related to the investment activities in phantom stock associated with deferred compensation plans	10.1	3.8	(0.4)
Cash surrender value of life insurance policies	42.2	33.1
Earnings (loss) from investments in corporate-owned life insurance policies	$ 5.9	3.5	(2.1)
Liability Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vesting period for awards to non-employee directors	1 year
Total fair value of liability awards vested	$ 25.1	23.7	16.6
Equity Awards
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Total fair value of equity awards vested	$ 0.5	$ 0.6	$ 1.1