Legal proceedings and contingencies (Details)				3 Months Ended	6 Months Ended
	Mar. 31, 2016 party	Feb. 04, 2013 USD ($)	Dec. 05, 2012 USD ($)	Jan. 01, 2016 USD ($)	Dec. 30, 2016 USD ($)	Jan. 01, 2016 USD ($)
Loss Contingencies [Line Items]
Loss on sale of discontinued operations					$ 0	$ 21,000,000
Broadcast Communications
Loss Contingencies [Line Items]
Asset sale agreement			$ 225,000,000
Cash consideration received on disposal of discontinued operations		$ 160,000,000
Asset sale agreement, promissory note		15,000,000
Asset sale agreement, earnout		$ 50,000,000
Loss on sale of discontinued operations				$ 21,000,000
Loss on sale of discontinued operations, net of income taxes				$ 17,000,000
Exelis | Passaic River Alaska
Loss Contingencies [Line Items]
Estimated cost for all participating parties of EPA's preferred alternative					$ 1,380,000,000
Number of responsible parties notified | party	100