Variable Interest Entities - Summary of Variable Interest Entities Information (Detail) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Total Assets	$ 209	$ 230
Total Liabilities	4	4
LIHTC Investments [Member]
Variable Interest Entity [Line Items]
Total Assets	6,176	6,003
Total Liabilities	2,971	2,943
Maximum Exposure to Loss	1,583	1,561
KCC Indirect Investments [Member]
Variable Interest Entity [Line Items]
Total Assets	22,367	26,817
Total Liabilities	426	292
Maximum Exposure to Loss	$ 137	$ 153