Incurred Claims, Net of Reinsurance, Cumulative Number of Reported Delinquencies and Total of Incurred-But-Not-Reported Liabilities (Detail) $ in Millions		Dec. 31, 2017 USD ($) Claim	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 7,142
U.S. Mortgage Insurance
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance		7,142
U.S. Mortgage Insurance | Accident Year 2008
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	1,396	$ 1,391	$ 1,385	$ 1,376	$ 1,347	$ 1,353	$ 1,339	$ 1,211	$ 1,041	$ 943
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1]	$ 1
Number of reported delinquencies | Claim	[1],[2]	133,491
U.S. Mortgage Insurance | Accident Year 2009
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 1,802	1,799	1,792	1,782	1,752	1,755	1,762	1,697	1,341	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1]	$ 1
Number of reported delinquencies | Claim	[1],[2]	151,732
U.S. Mortgage Insurance | Accident Year 2010
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 1,174	1,173	1,173	1,165	1,146	1,139	1,157	977	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1]	$ 1
Number of reported delinquencies | Claim	[1],[2]	90,231
U.S. Mortgage Insurance | Accident Year 2011
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 939	939	938	929	913	931	910	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1]	$ 1
Number of reported delinquencies | Claim	[1],[2]	68,984
U.S. Mortgage Insurance | Accident Year 2012
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 668	671	673	671	675	718	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1]	$ 0
Number of reported delinquencies | Claim	[1],[2]	48,170
U.S. Mortgage Insurance | Accident Year 2013
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 384	387	392	407	475	0	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1]	$ 0
Number of reported delinquencies | Claim	[1],[2]	33,934
U.S. Mortgage Insurance | Accident Year 2014
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 261	269	288	328	0	0	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1]	$ 0
Number of reported delinquencies | Claim	[1],[2]	26,080
U.S. Mortgage Insurance | Accident Year 2015
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 187	208	235	0	0	0	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1]	$ 0
Number of reported delinquencies | Claim	[1],[2]	20,915
U.S. Mortgage Insurance | Accident Year 2016
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 160	198	0	0	0	0	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1]	$ 1
Number of reported delinquencies | Claim	[1],[2]	17,423
U.S. Mortgage Insurance | Accident Year 2017
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 171	0	0	0	0	0	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1]	$ 19
Number of reported delinquencies | Claim	[1],[2]	15,097
Canada Mortgage Insurance
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1],[3]	$ 1,225
Canada Mortgage Insurance | Accident Year 2008
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1],[3]	158	158	158	158	158	158	155	152	149	108
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1],[3],[4]	$ 0
Number of reported delinquencies | Claim	[1],[3],[5]	6,138
Canada Mortgage Insurance | Accident Year 2009
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1],[3]	$ 195	195	195	196	196	194	191	169	151	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1],[3],[4]	$ 0
Number of reported delinquencies | Claim	[1],[3],[5]	6,702
Canada Mortgage Insurance | Accident Year 2010
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1],[3]	$ 167	168	167	168	169	167	149	135	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1],[3],[4]	$ 0
Number of reported delinquencies | Claim	[1],[3],[5]	6,601
Canada Mortgage Insurance | Accident Year 2011
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1],[3]	$ 149	150	150	151	151	149	133	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1],[3],[4]	$ 0
Number of reported delinquencies | Claim	[1],[3],[5]	5,707
Canada Mortgage Insurance | Accident Year 2012
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1],[3]	$ 108	108	108	109	110	111	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1],[3],[4]	$ 0
Number of reported delinquencies | Claim	[1],[3],[5]	5,316
Canada Mortgage Insurance | Accident Year 2013
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1],[3]	$ 96	97	97	98	102	0	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1],[3],[4]	$ 0
Number of reported delinquencies | Claim	[1],[3],[5]	4,949
Canada Mortgage Insurance | Accident Year 2014
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1],[3]	$ 84	85	87	91	0	0	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1],[3],[4]	$ 0
Number of reported delinquencies | Claim	[1],[3],[5]	4,948
Canada Mortgage Insurance | Accident Year 2015
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1],[3]	$ 87	91	101	0	0	0	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1],[3],[4]	$ 0
Number of reported delinquencies | Claim	[1],[3],[5]	4,626
Canada Mortgage Insurance | Accident Year 2016
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1],[3]	$ 103	119	0	0	0	0	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1],[3],[4]	$ 0
Number of reported delinquencies | Claim	[1],[3],[5]	5,133
Canada Mortgage Insurance | Accident Year 2017
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1],[3]	$ 78	0	0	0	0	0	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[1],[3],[4]	$ 26
Number of reported delinquencies | Claim	[1],[3],[5]	3,785
Australia Mortgage Insurance
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[6]	$ 1,139
Australia Mortgage Insurance | Accident Year 2008
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[6]	178	178	177	179	176	172	155	153	158	74
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[3],[6]	$ 0
Number of reported delinquencies | Claim	[2],[3],[6]	9,197
Australia Mortgage Insurance | Accident Year 2009
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[6]	$ 136	135	133	130	127	122	101	109	71	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[3],[6]	$ 0
Number of reported delinquencies | Claim	[2],[3],[6]	8,868
Australia Mortgage Insurance | Accident Year 2010
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[6]	$ 145	143	143	142	140	138	111	74	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[3],[6]	$ 0
Number of reported delinquencies | Claim	[2],[3],[6]	8,683
Australia Mortgage Insurance | Accident Year 2011
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[6]	$ 128	127	127	127	132	142	98	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[3],[6]	$ 0
Number of reported delinquencies | Claim	[2],[3],[6]	9,321
Australia Mortgage Insurance | Accident Year 2012
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[6]	$ 87	86	88	90	108	88	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[3],[6]	$ 0
Number of reported delinquencies | Claim	[2],[3],[6]	7,595
Australia Mortgage Insurance | Accident Year 2013
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[6]	$ 62	63	69	84	82	0	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[3],[6]	$ 0
Number of reported delinquencies | Claim	[2],[3],[6]	7,087
Australia Mortgage Insurance | Accident Year 2014
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[6]	$ 71	76	92	80	0	0	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[3],[6]	$ 0
Number of reported delinquencies | Claim	[2],[3],[6]	7,463
Australia Mortgage Insurance | Accident Year 2015
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[6]	$ 96	119	88	0	0	0	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[3],[6]	$ 1
Number of reported delinquencies | Claim	[2],[3],[6]	7,606
Australia Mortgage Insurance | Accident Year 2016
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[6]	$ 141	106	0	0	0	0	0	0	0	0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[3],[6]	$ 14
Number of reported delinquencies | Claim	[2],[3],[6]	8,020
Australia Mortgage Insurance | Accident Year 2017
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[6]	$ 95	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0	$ 0
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims as of December 31, 2016	[3],[6]	$ 36
Number of reported delinquencies | Claim	[2],[3],[6]	4,001

[1] Represents the year in which first monthly mortgage payments have been missed by the borrower.

[2] Represents reported and outstanding delinquencies less actual cures as of December 31 for each respective accident year.

[3] Amounts translated into U.S. dollars at the average foreign exchange rates for the year ended December 31, 2017.

[4] Incurred-but-not-reported liabilities exist only relative to the year 2017 as lenders are required to report losses after three consecutive monthly mortgage payments have been missed by the borrower.

[5] Represents reported delinquencies as of December 31 for each respective accident year.

[6] The accident year is estimated by allowing an additional five months for development from the time the first monthly mortgage payments have been missed by the borrower.