Retirement Plans - Fair value of pension plan assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 29, 2018	Dec. 30, 2017	Dec. 31, 2016
Change in fair value of plan assets
Valuation of owned properties period	3 years
Level 1
Change in fair value of plan assets
Fair value of total pension plan assets	$ 2,742	$ 3,377
Level 2
Change in fair value of plan assets
Fair value of total pension plan assets	1,217	1,211
Unobservable Inputs Level 3
Change in fair value of plan assets
Fair value of total pension plan assets	460	460
Not Subject to Leveling
Change in fair value of plan assets
Fair value of total pension plan assets	2,703	2,829
Cash and equivalents | Level 1
Change in fair value of plan assets
Fair value of total pension plan assets	19	22
Cash and equivalents | Level 2
Change in fair value of plan assets
Fair value of total pension plan assets	19	10
Cash and equivalents | Not Subject to Leveling
Change in fair value of plan assets
Fair value of total pension plan assets	113	149
Domestic Equity Securities | Level 1
Change in fair value of plan assets
Fair value of total pension plan assets	1,256	1,404
Domestic Equity Securities | Not Subject to Leveling
Change in fair value of plan assets
Fair value of total pension plan assets	828	665
International Equity Securities | Level 1
Change in fair value of plan assets
Fair value of total pension plan assets	835	919
International Equity Securities | Not Subject to Leveling
Change in fair value of plan assets
Fair value of total pension plan assets	450	636
Mutual Funds | Level 1
Change in fair value of plan assets
Fair value of total pension plan assets	266	387
National, state and local governments debt securities | Level 1
Change in fair value of plan assets
Fair value of total pension plan assets	366	645
National, state and local governments debt securities | Level 2
Change in fair value of plan assets
Fair value of total pension plan assets	290	289
National, state and local governments debt securities | Not Subject to Leveling
Change in fair value of plan assets
Fair value of total pension plan assets	53	56
Corporate debt securities | Level 2
Change in fair value of plan assets
Fair value of total pension plan assets	908	912
Corporate debt securities | Not Subject to Leveling
Change in fair value of plan assets
Fair value of total pension plan assets	220	148
Asset-backed debt securities | Not Subject to Leveling
Change in fair value of plan assets
Fair value of total pension plan assets	104	103
Private investment partnerships | Not Subject to Leveling
Change in fair value of plan assets
Fair value of total pension plan assets	650	591
Real estate | Unobservable Inputs Level 3
Change in fair value of plan assets
Fair value of total pension plan assets	460	460	$ 494
Real estate | Not Subject to Leveling
Change in fair value of plan assets
Fair value of total pension plan assets	$ 285	284
Hedge funds | Not Subject to Leveling
Change in fair value of plan assets
Fair value of total pension plan assets		$ 197