Share-Based Compensation - Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Options
Shares, Granted		0	0
Stock Option
Options
Shares, Outstanding at beginning of year	415,960	956,502	1,532,569
Shares, Granted	0	0	0
Shares, Exercised	(209,976)	(540,542)	(543,922)
Shares, Forfeited	0	0	0
Shares, Expired	(1,550)	0	(32,145)
Shares, Outstanding at end of period	204,434	415,960	956,502
Shares, Exercisable at end of period	204,434	415,960	956,502
Weighted-Average Exercise Price, Outstanding at beginning of year	$ 40.19	$ 42.77	$ 41.69
Weighted-Average Exercise Price, Granted	0.00	0.00	0.00
Weighted-Average Exercise Price, Exercised	43.53	44.76	39.54
Weighted-Average Exercise Price, Forfeited	0.00	0.00	0.00
Weighted-Average Exercise Price, Expired	51.62	0.00	45.81
Weighted-Average Exercise Price, Outstanding at end of year	36.66	40.19	42.77
Weighted-Average Exercise Price, Exercisable at end of year	$ 36.66	$ 40.19	$ 42.77
Intrinsic value of exercised options	$ 10	$ 15	$ 4