XML 253 R207.htm IDEA: XBRL DOCUMENT v3.20.4
Employee benefits - Additional information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Fair Value, Transfers Between Level 1 and Level 2, Description and Policy [Abstract]      
Transfers in or out of Level 1 $ 0 $ 0  
Transfers in or out of Level 2 0 0  
Transfers in or out of Level 3 0 0  
Savings Plans      
Fair Value, Transfers Between Level 1 and Level 2, Description and Policy [Abstract]      
Savings Plans 77,000,000 81,000,000  
Savings Plans Contributions $ 14,000,000.0 $ 15,100,000 $ 12,700,000
Savings Plans Stocks Held 1,362,593 1,378,048  
OPEB Plan      
Fair Value, Transfers Between Level 1 and Level 2, Description and Policy [Abstract]      
Service cost $ 713,000 $ 759,000 1,028,000
Termination benefit cost $ 0 $ 0 $ 1,790,000