Variable Interest Entities - Variable Interest Entities Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Total Assets	$ 230	$ 178
Total Liabilities	4	4
LIHTC Investments [Member]
Variable Interest Entity [Line Items]
Total Assets	6,003	4,814
Total Liabilities	2,943	2,003
Maximum Exposure to Loss	1,561	1,465
KCC Indirect Investments [Member]
Variable Interest Entity [Line Items]
Total Assets	26,817	32,755
Total Liabilities	292	201
Maximum Exposure to Loss	$ 153	$ 195