XML 141 R106.htm IDEA: XBRL DOCUMENT v3.20.4
Commitments and Contingencies - Changes in Asset Retirement Obligations (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]    
Balance at beginning of year $ 143.9 $ 121.8
Accretion expense 5.9 5.6
Liabilities incurred and liabilities assumed in business combinations 0.3 0.6
Liabilities settled (10.3) (1.2)
Revisions in estimated cash flows 14.0 17.1
Balance at end of year $ 153.8 $ 143.9