Fair Values of Assets and Liabilities - Losses Recognized Related to Nonrecurring Fair Value Measurements of Individual Assets or Portfolios (Detail) - Fair Value, Measurements, Nonrecurring [Member] - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Loans Receivable [Member]
Fair Value Assets Measured On Nonrecurring Basis Losses Recognized [Line Items]
Losses recognized related to nonrecurring fair value measurements	$ 23	$ 37
Other Assets [Member]
Fair Value Assets Measured On Nonrecurring Basis Losses Recognized [Line Items]
Losses recognized related to nonrecurring fair value measurements	$ 5	$ 7