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Schedule of Details Supporting Computation of Earnings Per Share (Detail) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Net income available for common stockholders	$ 145us-gaap_NetIncomeLoss	$ 290us-gaap_NetIncomeLoss	$ 344us-gaap_NetIncomeLoss	$ 368us-gaap_NetIncomeLoss	$ (30)us-gaap_NetIncomeLoss	$ 368us-gaap_NetIncomeLoss	$ 420us-gaap_NetIncomeLoss	$ 473us-gaap_NetIncomeLoss	$ 1,147us-gaap_NetIncomeLoss	$ 1,231us-gaap_NetIncomeLoss	$ 1,222us-gaap_NetIncomeLoss
Weighted-average outstanding shares of common stock used to compute basic earnings per common share									154,187	157,503	161,484
Shares used to compute diluted earnings per common share									155,874	159,151	163,457
Basic earnings per common share (in dollars per share)	$ 0.96us-gaap_EarningsPerShareBasic	$ 1.87us-gaap_EarningsPerShareBasic	$ 2.22us-gaap_EarningsPerShareBasic	$ 2.37us-gaap_EarningsPerShareBasic	$ (0.19)us-gaap_EarningsPerShareBasic	$ 2.34us-gaap_EarningsPerShareBasic	$ 2.66us-gaap_EarningsPerShareBasic	$ 2.97us-gaap_EarningsPerShareBasic	$ 7.44us-gaap_EarningsPerShareBasic	$ 7.81us-gaap_EarningsPerShareBasic	$ 7.56us-gaap_EarningsPerShareBasic
Diluted earnings per common share (in dollars per share)	$ 0.94us-gaap_EarningsPerShareDiluted	$ 1.85us-gaap_EarningsPerShareDiluted	$ 2.19us-gaap_EarningsPerShareDiluted	$ 2.35us-gaap_EarningsPerShareDiluted	$ (0.19)us-gaap_EarningsPerShareDiluted	$ 2.31us-gaap_EarningsPerShareDiluted	$ 2.63us-gaap_EarningsPerShareDiluted	$ 2.95us-gaap_EarningsPerShareDiluted	$ 7.36us-gaap_EarningsPerShareDiluted	$ 7.73us-gaap_EarningsPerShareDiluted	$ 7.47us-gaap_EarningsPerShareDiluted
Number of antidilutive stock options and restricted stock awards excluded from computation									320	704	754
Employee stock options
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of employee stock options and restricted stock									230	322	576
Restricted stock
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Dilutive effect of employee stock options and restricted stock									1,457	1,326	1,397