Incurred Claims, Net of Reinsurance, Cumulative Number of Reported Delinquencies and Total of Incurred-But-Not-Reported Liabilities (Detail) $ in Millions		Dec. 31, 2018 USD ($) Claim	Dec. 31, 2017 USD ($)	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 ($)	Dec. 31, 2009 USD ($)
U.S. Mortgage Insurance
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 5,781
U.S. Mortgage Insurance | Accident Year 2009
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 1,803	$ 1,802	$ 1,799	$ 1,792	$ 1,782	$ 1,752	$ 1,755	$ 1,762	$ 1,697	$ 1,341
Number of reported delinquencies | Claim	[1],[2]	151,948
U.S. Mortgage Insurance | Accident Year 2010
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 1,174	1,174	1,173	1,173	1,165	1,146	1,139	1,157	977
Number of reported delinquencies | Claim	[1],[2]	90,403
U.S. Mortgage Insurance | Accident Year 2011
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 939	939	939	938	929	913	931	910
Number of reported delinquencies | Claim	[1],[2]	69,155
U.S. Mortgage Insurance | Accident Year 2012
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 667	668	671	673	671	675	718
Number of reported delinquencies | Claim	[1],[2]	48,392
U.S. Mortgage Insurance | Accident Year 2013
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 382	384	387	392	407	475
Number of reported delinquencies | Claim	[1],[2]	34,187
U.S. Mortgage Insurance | Accident Year 2014
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 259	261	269	288	328
Number of reported delinquencies | Claim	[1],[2]	26,434
U.S. Mortgage Insurance | Accident Year 2015
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 181	187	208	235
Number of reported delinquencies | Claim	[1],[2]	21,372
U.S. Mortgage Insurance | Accident Year 2016
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 138	160	198
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims	[1]	$ 1
Number of reported delinquencies | Claim	[1],[2]	18,427
U.S. Mortgage Insurance | Accident Year 2017
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 121	171
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims	[1]	$ 2
Number of reported delinquencies | Claim	[1],[2]	18,087
U.S. Mortgage Insurance | Accident Year 2018
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1]	$ 117
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims	[1]	$ 16
Number of reported delinquencies | Claim	[1],[2]	11,269
Australia Mortgage Insurance
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[4]	$ 1,083
Australia Mortgage Insurance | Accident Year 2009
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[4]	$ 131	131	131	130	131	131	133	113	117	68
Number of reported delinquencies | Claim	[2],[3],[4]	2,389
Australia Mortgage Insurance | Accident Year 2010
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[4]	$ 139	139	139	141	141	143	143	110	56
Number of reported delinquencies | Claim	[2],[3],[4]	2,342
Australia Mortgage Insurance | Accident Year 2011
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[4]	$ 132	133	133	135	137	143	147	77
Number of reported delinquencies | Claim	[2],[3],[4]	2,339
Australia Mortgage Insurance | Accident Year 2012
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[4]	$ 95	94	95	98	102	116	73
Number of reported delinquencies | Claim	[2],[3],[4]	1,896
Australia Mortgage Insurance | Accident Year 2013
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[4]	$ 67	67	70	77	90	69
Number of reported delinquencies | Claim	[2],[3],[4]	1,540
Australia Mortgage Insurance | Accident Year 2014
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[4]	$ 70	72	77	91	65
Number of reported delinquencies | Claim	[2],[3],[4]	1,433
Australia Mortgage Insurance | Accident Year 2015
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[4]	$ 93	96	114	76
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims		$ 1
Number of reported delinquencies | Claim	[2],[3],[4]	1,549
Australia Mortgage Insurance | Accident Year 2016
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[4]	$ 125	140	105
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims	[3],[4]	$ 3
Number of reported delinquencies | Claim	[2],[3],[4]	2,183
Australia Mortgage Insurance | Accident Year 2017
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[4]	$ 135	100
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims	[3],[4]	$ 14
Number of reported delinquencies | Claim	[2],[3],[4]	3,342
Australia Mortgage Insurance | Accident Year 2018
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[3],[4]	$ 96
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims	[3],[4]	$ 33
Number of reported delinquencies | Claim	[2],[3],[4]	3,215
Canada Mortgage Insurance [Member]
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance	[1],[3]	$ 1,156
Canada Mortgage Insurance [Member] | Accident Year 2009
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance		$ 197	197	197	198	198	199	196	193	171	$ 153
Number of reported delinquencies | Claim	[5]	6,702
Canada Mortgage Insurance [Member] | Accident Year 2010
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance		$ 169	169	170	169	170	171	169	151	$ 137
Number of reported delinquencies | Claim	[5]	6,601
Canada Mortgage Insurance [Member] | Accident Year 2011
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance		$ 150	150	151	152	15	153	151	$ 134
Number of reported delinquencies | Claim	[5]	5,707
Canada Mortgage Insurance [Member] | Accident Year 2012
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance		$ 108	109	109	110	110	111	$ 112
Number of reported delinquencies | Claim	[5]	5,316
Canada Mortgage Insurance [Member] | Accident Year 2013
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance		$ 97	98	98	98	100	$ 103
Number of reported delinquencies | Claim	[5]	4,949
Canada Mortgage Insurance [Member] | Accident Year 2014
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance		$ 85	86	86	88	$ 92
Number of reported delinquencies | Claim	[5]	4,948
Canada Mortgage Insurance [Member] | Accident Year 2015
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance		$ 87	88	92	$ 103
Number of reported delinquencies | Claim	[5]	4,626
Canada Mortgage Insurance [Member] | Accident Year 2016
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance		$ 94	105	$ 121
Number of reported delinquencies | Claim	[5]	5,133
Canada Mortgage Insurance [Member] | Accident Year 2017
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance		$ 84	$ 78
Number of reported delinquencies | Claim	[5]	3,785
Canada Mortgage Insurance [Member] | Accident Year 2018
Claims Development [Line Items]
Incurred claims and allocated claim adjustment expenses, net of reinsurance		$ 85
Total of Incurred-But-Not-Reported liabilities including expected development on reported claims	[6]	$ 24
Number of reported delinquencies | Claim	[5]	3,688

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

[2] Represents outstanding delinquencies plus paid claims as of December 31, 2018 for each respective accident year.

[3] Amounts translated into U.S. dollars at the average foreign exchange rates for the year ended December 31, 2018.

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

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

[6] Incurred-but-not-reported liabilities exist only relative to the year 2018 as lenders are required to report losses after three consecutive monthly mortgage payments have been missed by the borrower.