Income Tax Expense - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Tax Disclosure [Abstract]
Effective income tax rate before elimination of unfunded future income tax and sale of equity investment	32.70%		50.90%	43.80%	37.60%
Statutory federal income tax rate				35.00%
Unfunded future income tax expense		$ 126		$ 126