Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Tax Disclosure [Abstract]
Income tax expense	$ 13	$ (19)	$ 13	$ (2,300)
Built in gain tax	$ 4		$ 4
Deferred tax Benefit		$ 24		$ 2,312