Retirement Benefit Plans - Assets Measured at Fair Value (Details) - Accrued pension cost - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets:
Fair value of plan assets at end of year	$ 866.1	$ 794.9
Level 1 | Fair Value, Recurring
Assets:
Fair value of plan assets at end of year	242.5	202.5
Level 1 | Fair Value, Recurring | Cash and cash equivalents
Assets:
Fair value of plan assets at end of year	37.1	17.1
Level 1 | Fair Value, Recurring | Government and agency securities
Assets:
Fair value of plan assets at end of year	45.1	35.8
Level 1 | Fair Value, Recurring | Corporate bonds - investment grade
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 1 | Fair Value, Recurring | Equity securities - U.S. companies
Assets:
Fair value of plan assets at end of year	0.0	0.1
Level 1 | Fair Value, Recurring | Common collective funds - fixed income
Assets:
Fair value of plan assets at end of year	44.5	42.0
Level 1 | Fair Value, Recurring | Mutual funds - fixed income
Assets:
Fair value of plan assets at end of year	55.4	66.9
Level 1 | Fair Value, Recurring | Mutual funds - international equity
Assets:
Fair value of plan assets at end of year	60.4	36.0
Level 1 | Fair Value, Recurring | Mutual funds - domestic equity
Assets:
Fair value of plan assets at end of year	0.0	3.2
Level 1 | Fair Value, Recurring | Mutual funds - other assets
Assets:
Fair value of plan assets at end of year	0.0	1.4
Level 2 | Fair Value, Recurring
Assets:
Fair value of plan assets at end of year	102.5	82.5
Level 2 | Fair Value, Recurring | Cash and cash equivalents
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 2 | Fair Value, Recurring | Government and agency securities
Assets:
Fair value of plan assets at end of year	3.0	3.0
Level 2 | Fair Value, Recurring | Corporate bonds - investment grade
Assets:
Fair value of plan assets at end of year	99.5	79.5
Level 2 | Fair Value, Recurring | Equity securities - U.S. companies
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 2 | Fair Value, Recurring | Common collective funds - fixed income
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 2 | Fair Value, Recurring | Mutual funds - fixed income
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 2 | Fair Value, Recurring | Mutual funds - international equity
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 2 | Fair Value, Recurring | Mutual funds - domestic equity
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 2 | Fair Value, Recurring | Mutual funds - other assets
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 3 | Fair Value, Recurring
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 3 | Fair Value, Recurring | Cash and cash equivalents
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 3 | Fair Value, Recurring | Government and agency securities
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 3 | Fair Value, Recurring | Corporate bonds - investment grade
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 3 | Fair Value, Recurring | Equity securities - U.S. companies
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 3 | Fair Value, Recurring | Common collective funds - fixed income
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 3 | Fair Value, Recurring | Mutual funds - fixed income
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 3 | Fair Value, Recurring | Mutual funds - international equity
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 3 | Fair Value, Recurring | Mutual funds - domestic equity
Assets:
Fair value of plan assets at end of year	0.0	0.0
Level 3 | Fair Value, Recurring | Mutual funds - other assets
Assets:
Fair value of plan assets at end of year	0.0	0.0
Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Recurring
Assets:
Fair value of plan assets at end of year	345.0	285.0
Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Recurring | Cash and cash equivalents
Assets:
Fair value of plan assets at end of year	37.1	17.1
Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Recurring | Government and agency securities
Assets:
Fair value of plan assets at end of year	48.1	38.8
Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Recurring | Corporate bonds - investment grade
Assets:
Fair value of plan assets at end of year	99.5	79.5
Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Recurring | Equity securities - U.S. companies
Assets:
Fair value of plan assets at end of year	0.0	0.1
Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Recurring | Common collective funds - fixed income
Assets:
Fair value of plan assets at end of year	44.5	42.0
Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Recurring | Mutual funds - fixed income
Assets:
Fair value of plan assets at end of year	55.4	66.9
Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Recurring | Mutual funds - international equity
Assets:
Fair value of plan assets at end of year	60.4	36.0
Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Recurring | Mutual funds - domestic equity
Assets:
Fair value of plan assets at end of year	0.0	3.2
Fair Value, Inputs, Level 1, 2 and 3 | Fair Value, Recurring | Mutual funds - other assets
Assets:
Fair value of plan assets at end of year	0.0	1.4
Fair Value Measured at Net Asset Value Per Share | Cash and cash equivalents
Assets:
Fair value of plan assets at end of year	0.0	0.2
Fair Value Measured at Net Asset Value Per Share | Common collective funds - fixed income
Assets:
Fair value of plan assets at end of year	203.8	202.5
Fair Value Measured at Net Asset Value Per Share | Equity securities - international companies
Assets:
Fair value of plan assets at end of year	0.3	1.0
Fair Value Measured at Net Asset Value Per Share | Common collective fund - U.S. equities
Assets:
Fair value of plan assets at end of year	63.2	76.3
Fair Value Measured at Net Asset Value Per Share | Common collective fund - international equities
Assets:
Fair value of plan assets at end of year	42.6	31.9
Fair Value Measured at Net Asset Value Per Share | Common collective funds - diversified growth
Assets:
Fair value of plan assets at end of year	20.1	17.9
Fair Value Measured at Net Asset Value Per Share | Limited partnerships
Assets:
Fair value of plan assets at end of year	13.2	18.7
Fair Value Measured at Net Asset Value Per Share | Real estate partnerships
Assets:
Fair value of plan assets at end of year	7.7	11.2
Fair Value Measured at Net Asset Value Per Share | Other liability-driven investments
Assets:
Fair value of plan assets at end of year	144.4	128.2
Fair Value Measured at Net Asset Value Per Share | Other assets
Assets:
Fair value of plan assets at end of year	$ 25.8	$ 22.0