Retirement Benefits - Summary of Company's Plan Assets, Benefit Obligations, and Funded Status (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Nov. 30, 2015
Change in fair value of assets:
Fair value - beginning of period		$ 925.1
Gain (loss) on assets	$ (22.2)	96.8	$ 93.7	$ (64.2)
Fair value - end of period		931.2	925.1
Amounts recognized in the consolidated balance sheets:
Postretirement medical and life insurance benefits, current		(4.8)	(5.2)
Postretirement medical and life insurance benefits, noncurrent		(32.7)	(37.4)
Pension benefits, noncurrent		(492.8)	(548.2)
Net liability recognized in the consolidated balance sheets		(2,156.3)	(2,213.9)
Pension Benefits
Change in fair value of assets:
Fair value - beginning of period		925.1	931.4
Gain (loss) on assets		96.8	93.7
Employer contributions		77.2	34.1
Benefits paid		(133.2)	(134.1)
Settlement		(34.7)	0.0
Fair value - end of period	931.4	931.2	925.1
Change in benefit obligation:
Benefit obligation - beginning of period		1,492.1	1,531.0
Service cost	1.1	15.0	14.0	10.8
Interest cost	5.3	57.6	64.1	63.6
Actuarial losses (gains)		46.1	17.1
Benefits paid		(133.2)	(134.1)
Settlement		(34.7)	0.0
Benefit obligation - end of year	1,531.0	1,442.9	1,492.1
Funded status of the plans		(511.7)	(567.0)
Amounts recognized in the consolidated balance sheets:
Postretirement medical and life insurance benefits, current		0.0	0.0
Postretirement medical and life insurance benefits, noncurrent		0.0	0.0
Pension benefits, noncurrent		(492.8)	(548.2)
Net liability recognized in the consolidated balance sheets		(511.7)	(567.0)
Pension Benefits | Other Current Liabilities
Amounts recognized in the consolidated balance sheets:
Pension liability, non-qualified current (component of other current liabilities)		(1.3)	(1.3)
Pension Benefits | Other Noncurrent Liabilities
Amounts recognized in the consolidated balance sheets:
Pension benefits, noncurrent		(17.6)	(17.5)
Medical and Life Insurance Benefits
Change in fair value of assets:
Fair value - beginning of period		0.0	0.0
Gain (loss) on assets		0.0	0.0
Employer contributions		4.2	4.3
Benefits paid		(4.2)	(4.3)
Settlement		0.0	0.0
Fair value - end of period	0.0	0.0	0.0
Change in benefit obligation:
Benefit obligation - beginning of period		42.6	50.8
Service cost	0.0	0.0	0.0	0.0
Interest cost	0.2	1.5	1.9	$ 1.9
Actuarial losses (gains)		(2.4)	(5.8)
Benefits paid		(4.2)	(4.3)
Settlement		0.0	0.0
Benefit obligation - end of year	$ 50.8	37.5	42.6
Funded status of the plans		(37.5)	(42.6)
Amounts recognized in the consolidated balance sheets:
Postretirement medical and life insurance benefits, current		(4.8)	(5.2)
Postretirement medical and life insurance benefits, noncurrent		(32.7)	(37.4)
Pension benefits, noncurrent		0.0	0.0
Net liability recognized in the consolidated balance sheets		(37.5)	(42.6)
Medical and Life Insurance Benefits | Other Current Liabilities
Amounts recognized in the consolidated balance sheets:
Pension liability, non-qualified current (component of other current liabilities)		0.0	0.0
Medical and Life Insurance Benefits | Other Noncurrent Liabilities
Amounts recognized in the consolidated balance sheets:
Pension benefits, noncurrent		$ 0.0	$ 0.0