Loans and Allowance for Credit Losses - Summary of Loans by Portfolio Type and Company's Internal Credit Quality Rating (Parenthetical) (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Trouble Debt Restructure [Member] | Government National Mortgage Association [Member]
Financing Receivable, Recorded Investment [Line Items]
Restructured GNMA loans whose repayments are insured by the Federal Housing Administration or guaranteed by the Department of Veterans Affairs, were classified with a pass rating	$ 2,200,000,000	$ 2,200,000,000
GNMA loans 90 days or more past due whose repayments are insured by the Federal Housing Administration or guaranteed by the Department of Veterans Affairs, were classified with a pass rating	3,000,000,000	3,100,000,000
Special Mention or Classified Rating [Member] | Energy Related Businesses [Member]
Financing Receivable, Recorded Investment [Line Items]
Loans to Customers in Energy Related Businesses	$ 946,000,000	$ 122,000,000