Retirement Benefits - Summary of Company's Plan Assets, Benefit Obligations, and Funded Status (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in fair value of assets:
Fair value - beginning of year	$ 932.5
Gain (loss) on assets	117.1	$ 144.8	$ (57.8)
Fair value - end of year	957.0	932.5
Amounts recognized in the consolidated balance sheets:
Pension benefits, noncurrent	(405.2)	(398.9)
Net liability recognized in the consolidated balance sheets	(2,666.4)	(2,131.1)
Pension Benefits
Change in fair value of assets:
Fair value - beginning of year	932.5	894.8
Gain (loss) on assets	117.1	144.8
Employer contributions	15.6	1.4
Benefits paid	(108.2)	(108.5)
Fair value - end of year	957.0	932.5	894.8
Change in benefit obligation:
Benefit obligation - beginning of year	1,349.8	1,288.7
Interest cost	42.5	52.7	49.7
Actuarial losses (gains)	97.4	116.9
Benefits paid	(108.2)	(108.5)
Benefit obligation - end of year	1,381.5	1,349.8	1,288.7
Funded status of the plans	(424.5)	(417.3)
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	(405.2)	(398.9)
Net liability recognized in the consolidated balance sheets	(424.5)	(417.3)
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	(18.0)	(17.1)
Medical and Life Insurance Benefits
Change in fair value of assets:
Fair value - beginning of year	0.0	0.0
Gain (loss) on assets	0.0	0.0
Employer contributions	2.8	3.1
Benefits paid	(2.8)	(3.1)
Fair value - end of year	0.0	0.0	0.0
Change in benefit obligation:
Benefit obligation - beginning of year	25.9	31.3
Interest cost	0.8	1.2	1.2
Actuarial losses (gains)	1.1	(3.5)
Benefits paid	(2.8)	(3.1)
Benefit obligation - end of year	25.0	25.9	$ 31.3
Funded status of the plans	(25.0)	(25.9)
Amounts recognized in the consolidated balance sheets:
Postretirement medical and life insurance benefits, current	(3.5)	(3.6)
Postretirement medical and life insurance benefits, noncurrent	(21.5)	(22.3)
Pension benefits, noncurrent	0.0	0.0
Net liability recognized in the consolidated balance sheets	(25.0)	(25.9)
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
Medicare Part D Subsidy
Change in fair value of assets:
Benefits paid	$ (0.1)	$ (0.1)