Summary of Revenues for Segments and Corporate and Other Activities (Detail) - USD ($) $ in Millions	3 Months Ended																12 Months Ended
	Dec. 31, 2017	[1]	Sep. 30, 2017	[1]	Jun. 30, 2017	[1]	Mar. 31, 2017	[1]	Dec. 31, 2016	[2]	Sep. 30, 2016	[2]	Jun. 30, 2016	[2]	Mar. 31, 2016	[2]	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Revenues	$ 1,686		$ 2,215		$ 2,223		$ 2,171		$ 2,198		$ 2,150		$ 2,236		$ 1,785		$ 8,295	$ 8,369	$ 8,548
Segment, Continuing Operations | U.S. Mortgage Insurance
Segment Reporting Information [Line Items]
Revenues																	772	726	665
Segment, Continuing Operations | Canada Mortgage Insurance
Segment Reporting Information [Line Items]
Revenues																	780	645	564
Segment, Continuing Operations | Australia Mortgage Insurance
Segment Reporting Information [Line Items]
Revenues																	(40)	440	474
Segment, Continuing Operations | Long-term Care Insurance
Segment Reporting Information [Line Items]
Revenues																	4,062	4,037	3,752
Segment, Continuing Operations | Life Insurance
Segment Reporting Information [Line Items]
Revenues																	1,591	1,381	1,902
Segment, Continuing Operations | Fixed Annuities
Segment Reporting Information [Line Items]
Revenues																	818	832	891
Segment, Continuing Operations | U.S. Life Insurance
Segment Reporting Information [Line Items]
Revenues																	6,471	6,250	6,545
Segment, Continuing Operations | Runoff
Segment Reporting Information [Line Items]
Revenues																	339	302	259
Segment, Continuing Operations | Corporate and Other
Segment Reporting Information [Line Items]
Revenues																	$ (27)	$ 6	$ 41

[1] Our Australian mortgage insurance business completed a review of its premium earnings pattern in the fourth quarter of 2017 and recorded $468 million of lower earned premiums. The review indicated an observed and expected continuation of a longer duration between policy inception and first loss event.

[2] Our life insurance business completed its annual review of assumptions in the fourth quarter of 2016, which resulted in higher revenues of $6 million in our universal life insurance products. The updated assumptions reflected changes primarily to mortality experience in older age populations and modest updates related to long-term interest rates, lapses and other refinements.