Gypsum Transportation Limited (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2017	Jun. 30, 2016
Property, Plant and Equipment [Abstract]
Recovered receivable per release and debt settlement agreement			$ 8
Recovery of receivable	$ 0	$ 0	3	$ 0	$ (3)
Recovered receivable recorded in interest income			1
Recovered receivable recorded in other income			$ 4