Summary of Significant Policies (Summary of Significant Accounting and Reporting Policies - Additional Information) (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Significant Accounting Policies
Established threshold impaired loans on accrual status	$ 1
Larger commercial loans, subject to impairment review	1
Established threshold commercial loans not subject to specific allowance calculations	1
Unamortized debt issuance expense	$ 33				$ 34	$ 36
Net tax deficiencies reclassified to applicable income tax expense		$ 0	$ 5	$ 1