Gypsum Transportation Limited (Details) $ in Millions	3 Months Ended									12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Jun. 30, 2015 USD ($) ocean_vessel	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Self-unloading ocean vessels | ocean_vessel									2
Proceeds from sale of assets										$ 2	$ 12	$ 58
Gain on asset disposition										(1)	9	13
Cost to exit shipping operations									$ 6
Gain on disposal of shipping operations, net										0	0	1
Contract termination and (recovery) loss on receivable										0	3	6
Recovered receivable per release and debt settlement agreement											8
Recovered receivable in interest income											1
Recovered receivable in other income											4
Operating profit	$ 87	$ 93	$ 96	$ 91	$ 59	$ 97	$ 122	$ 116		367	394	355
Gypsum Transportation Limited
Property, Plant and Equipment [Line Items]
Operating profit										$ 0	$ 3	$ 7
Self-unloading ocean vessels
Property, Plant and Equipment [Line Items]
Proceeds from sale of assets									42
Gain on asset disposition									$ 7