Income Taxes - Total Income Tax Expense (Benefit) and Resulting Effective Tax Rate (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Income (loss) before income taxes times 35% statutory federal tax rate, Amount	$ 445	$ 399	$ 376
Amortization of tax-advantaged investments, Amount	69	63	64
Foreign tax adjustments, Amount	10	(4)	1
Reduced tax rate on lease financing income, Amount	(3)	(13)	(50)
Tax-exempt interest income, Amount	(16)	(15)	(16)
Corporate-owned life insurance income, Amount	(41)	(42)	(43)
Interest refund (net of federal tax benefit), Amount	(1)	(1)
State income tax, net of federal tax benefit, Amount	15	10	8
Tax credits, Amount	(134)	(130)	(119)
Other, Amount	(18)	4	10
Total income tax (benefit) expense	$ 326	$ 271	$ 231
Federal income tax rate	35.00%	35.00%	35.00%
Amortization of tax-advantaged investments, Rate	5.40%	5.50%	6.00%
Foreign tax adjustments, Rate	0.80%	(0.30%)	0.10%
Reduced tax rate on lease financing income, Rate	(0.20%)	(1.20%)	(4.70%)
Tax-exempt interest income, Rate	(1.30%)	(1.30%)	(1.50%)
Corporate-owned life insurance income, Rate	(3.20%)	(3.70%)	(4.00%)
Interest refund (net of federal tax benefit), Rate	(0.10%)	(0.10%)
State income tax, net of federal tax benefit, Rate	1.10%	0.90%	0.70%
Tax credits, Rate	(10.50%)	(11.40%)	(11.10%)
Other, Rate	(1.40%)	0.30%	0.90%
Total income tax expense (benefit), Rate	25.60%	23.70%	21.40%