Income Taxes (Income Taxes - Additional Information) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Taxes
Deferred tax assets related to state net operating loss carryforwards	$ 7	$ 9
State net operating loss carryforwards specific valuation allowances	$ 25	27
State net operating loss carryforwards expiration date	Dec. 31, 2037
Accrued interest liabilities, net of the related tax benefits	$ 3	3
Allocation of earnings for bad debt deductions of former thrift subsidiaries included in retained earnings	157	157
Interest expense recognized in connection with income taxes	1	2	$ 1
Proportional amortization for qualifying LIHTC investments	$ 154	$ 223	$ 153