Shareholders' Equity - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended			36 Months Ended	212 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2012	Dec. 31, 2017	Dec. 31, 2014	Dec. 31, 2000
Shareholders' Equity [Line Items]
Number of shares outstanding	86,972,854				86,972,854
Maximum number of shares that may yet be purchased	2,986,288				2,986,288
Stock repurchase program, number of shares authorized to be repurchased						47,500,000	10,000,000
Shares repurchased	1,400,000	0	900,000
Aggregate number of shares repurchased					44,500,000
Aggregate value of shares repurchased					$ 2,498
Repurchased shares remain in treasury stock	15,800,000	14,600,000			15,800,000
Equity Offering
Shareholders' Equity [Line Items]
Common shares sold from treasury stock				23,600,000
Stock Incentive Plan
Shareholders' Equity [Line Items]
Common shares sold from treasury stock	200,000	100,000	300,000		5,100,000