Significant Inputs Considered in Determining the Measurement of Credit Losses Recognized in Earnings for Securities (Detail)		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Investment [Line Items]
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Portion Recognized in Earnings, Net, Qualitative Disclosures, Default Rate	[1]	9.00%	2.00%
Inputs Considered in Determining Measurement of AFS Securities Credit Losses Recognized in Earnings, Prepayment Rate	[1]	13.00%	16.00%
Inputs Considered In Determining Measurement Of AFS Securities Credit Losses Recognized in Earnings Loss Severity	[1]	56.00%	46.00%
Maximum [Member]
Investment [Line Items]
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Portion Recognized in Earnings, Net, Qualitative Disclosures, Default Rate	[1]			9.00%
Inputs Considered in Determining Measurement of AFS Securities Credit Losses Recognized in Earnings, Prepayment Rate	[1]			21.00%
Inputs Considered In Determining Measurement Of AFS Securities Credit Losses Recognized in Earnings Loss Severity	[1]			56.00%
Minimum [Member]
Investment [Line Items]
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities, Portion Recognized in Earnings, Net, Qualitative Disclosures, Default Rate	[1]			2.00%
Inputs Considered in Determining Measurement of AFS Securities Credit Losses Recognized in Earnings, Prepayment Rate	[1]			7.00%
Inputs Considered In Determining Measurement Of AFS Securities Credit Losses Recognized in Earnings Loss Severity	[1]			40.00%

[1] During the year ended December 31, 2015, all OTTI credit losses recognized in earnings related to one private MBS security with a fair value of $20 million at December 31, 2015. During the year ended December 31, 2014, OTTI credit losses recognized in earnings related to one private MBS security with a fair value of $16 million at December 31, 2014.