Income Tax Expense - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Tax Disclosure [Abstract]
Effective income tax rate before elimination of unfunded future income tax and sale of equity investment	30.10%	32.90%
Increase in income tax expense due to unfunded future income tax	$ 14
Statutory federal income tax rate	35.00%	35.00%