Other Postretirement Benefit Plans - Employer Contributions to VEBA Trust (Details) - Accrued postretirement liability - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Assets:
Total Assets	$ 11.1	$ 64.4	$ 72.3
Fair Value, Recurring
Assets:
Total Assets	0.5	64.4
Fair Value, Recurring | Level 1
Assets:
Total Assets	10.6	0.0
Fair Value, Recurring | Level 2
Assets:
Total Assets	0.0	0.0
Fair Value, Recurring | Level 3
Assets:
Total Assets	0.0	0.0
Cash and cash equivalents | Fair Value, Recurring
Assets:
Total Assets	0.5	9.4
Cash and cash equivalents | Fair Value, Recurring | Level 1
Assets:
Total Assets	0.0	0.0
Cash and cash equivalents | Fair Value, Recurring | Level 2
Assets:
Total Assets	0.0	0.0
Cash and cash equivalents | Fair Value, Recurring | Level 3
Assets:
Total Assets	0.0	0.0
Common collective fund - U.S. equities | Fair Value, Recurring
Assets:
Total Assets		7.4
Common collective fund - U.S. equities | Fair Value, Recurring | Level 1
Assets:
Total Assets		0.0
Common collective fund - U.S. equities | Fair Value, Recurring | Level 2
Assets:
Total Assets		0.0
Common collective fund - U.S. equities | Fair Value, Recurring | Level 3
Assets:
Total Assets		0.0
Common collective fund - international equities | Fair Value, Recurring
Assets:
Total Assets		4.2
Common collective fund - international equities | Fair Value, Recurring | Level 1
Assets:
Total Assets		0.0
Common collective fund - international equities | Fair Value, Recurring | Level 2
Assets:
Total Assets		0.0
Common collective fund - international equities | Fair Value, Recurring | Level 3
Assets:
Total Assets		0.0
Common collective funds - fixed income | Fair Value, Recurring
Assets:
Total Assets	0.0	43.4
Common collective funds - fixed income | Fair Value, Recurring | Level 1
Assets:
Total Assets	10.6	0.0
Common collective funds - fixed income | Fair Value, Recurring | Level 2
Assets:
Total Assets	0.0	0.0
Common collective funds - fixed income | Fair Value, Recurring | Level 3
Assets:
Total Assets	$ 0.0	$ 0.0