Profit Sharing Plans - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Profit Sharing Plans [Abstract]
Minimum percentage amount of operating margin the company must attain for full calendar year for any profit sharing per individual to be paid (in hundredths)	10.00%
Amount of gross compensation eligible for profit sharing deferral per individual in calendar year when minimum operating margin is attained by company (in hundredths)	2.00%
Maximum amount of gross compensation eligible for profit sharing per individual in calendar year when 35% operating margin is attained by company (in hundredths)	20.00%
Percentage of operating margin the company must attain for full calendar year for maximum amount of profit sharing per individual to be paid (in hundredths)	35.00%
Profit sharing expense	$ 355	$ 346	$ 309