Loss reserves (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Loss Reserve [Roll Forward]
Reserve at beginning of period	$ 5,884,171,000		$ 6,704,990,000		$ 4,775,552,000
Less reinsurance recoverable	275,290,000		332,227,000		232,988,000
Net reserve at beginning of year	5,608,881,000	[1],[2]	6,372,763,000	[1],[2]	4,542,564,000	[1]
Adjustment to reserves	0	[3]	(92,000,000)	[3]	0	[3]
Adjusted beginning reserves	5,608,881,000		6,280,763,000		4,542,564,000
Losses and LAE incurred in respect of default notices received in [Abstract]
Current year	1,814,035,000		1,874,449,000		2,912,679,000
Prior years	(99,328,000)	[4]	(266,908,000)	[4]	466,765,000	[4]
Subtotal	1,714,707,000	[5]	1,607,541,000	[5]	3,379,444,000	[5]
Losses and LAE paid in respect of default notices received in [Abstract]
Current year	121,383,000		60,897,000		62,491,000
Prior years	2,838,069,000		2,256,206,000		1,605,668,000
Reinsurance terminations	(38,769,000)	[6]	(37,680,000)	[6]	(118,914,000)	[6]
Subtotal	2,920,683,000	[7]	2,279,423,000	[7]	1,549,245,000	[7]
Net reserve at end of period	4,402,905,000	[2]	5,608,881,000	[1],[2]	6,372,763,000	[1],[2]
Plus reinsurance recoverables	154,607,000		275,290,000		332,227,000
Reserve at end of period	4,557,512,000		5,884,171,000		6,704,990,000
Losses incurred [Abstract]
Number of new default notices received, net of cures	86,592		108,701		161,081
Losses paid [Abstract]
Historical average period for uncured default to develop into paid claim (in months)			12M
Premium refund liability, expected claim payments	114,000,000		113,000,000
Premium refund liability, expected future rescissions	58,000,000		101,000,000
Aging of the Primary Default Inventory [Abstract]
3 months or less	31,456		37,640		48,252
3 months or less (in hundredths)	18.00%		18.00%		19.00%
4 - 11 months	46,352		58,701		98,210
4 - 11 months (in hundredths)	26.00%		27.00%		39.00%
12 months or more	97,831		118,383		103,978
12 months or more (in hundredths)	56.00%		55.00%		42.00%
Total primary default inventory	175,639		214,724		250,440
Total primary default inventory (in hundredths)	100.00%		100.00%		100.00%
Primary claims received inventory included in ending default inventory	12,610		20,898		16,389
Primary claims received inventory included in ending default inventory (in hundredths)	7.00%		10.00%		7.00%
Number of payments delinquent [Abstract]
3 payments or less	42,804		51,003		60,970
3 payments or less (in hundredths)	24.00%		24.00%		24.00%
4 - 11 payments	47,864		65,797		105,208
4 - 11 payments (in hundredths)	27.00%		31.00%		42.00%
12 payments or more	84,971		97,924		84,262
12 payments or more (in hundredths)	49.00%		45.00%		34.00%
Total primary default inventory	175,639		214,724		250,440
Total primary default inventory (in hundredths)	100.00%		100.00%		100.00%
Claims resolved by rescissions [Abstract]
Percentage of claims received in a quarter resolved by rescission, lower range limit (in hundredths)	17.00%
Percentage of claims received in a quarter resolved by rescission, upper range limit (in hundredths)	20.00%				28.00%
Estimated Rescission Reduction - Loss Reserve [Roll Forward]
Estimated rescission reduction - beginning reserve	1,300,000,000		2,100,000,000		500,000,000
Estimated rescission reduction - losses incurred	0		200,000,000		2,500,000,000
Rescission reduction - paid claims	600,000,000		1,200,000,000		1,200,000,000
Amounts that may have been applied to a deductible	0		(200,000,000)		(300,000,000)
Net rescission reduction - paid claims	600,000,000		1,000,000,000		900,000,000
Estimated rescission reduction - ending reserve	700,000,000		1,300,000,000		2,100,000,000
Statute of limitations to bring legal proceedings disputing right to rescind coverage (in years)	3Y
Primary Default Inventory [Roll Forward]
Default inventory at beginning of period	214,724		250,440		182,188
Plus: New Notices	169,305		205,069		259,876
Less: Cures	(149,643)		(183,017)		(149,251)
Less: Paids (including those charged to a deductible or captive)	(51,138)		(43,826)		(29,732)
Less: Rescissions and denials	(7,609)		(13,942)		(12,641)
Default inventory at end of period	175,639		214,724		250,440
Pool insurance notice inventory [Abstract]
Pool insurance notice inventory (in number of loans)	32,971		43,329		44,231
Increase (Decrease) in Severity, Primary Defaults [Member]
Losses incurred [Abstract]
Change in loss reserves	(165,000,000)
Percentage of prior year default inventory resolved (in hundredths)	57.00%
Decrease in Estimated Loss Adjustment Expenses [Member]
Losses incurred [Abstract]
Change in loss reserves	(114,000,000)
Increase (Decrease) in Expected Claim Rate [Member]
Losses incurred [Abstract]
Change in loss reserves	200,000,000		(432,000,000)		337,000,000
Percentage of prior year default inventory resolved (in hundredths)			55.00%		50.00%
Decrease in Severity and Claim Rates, Pool Defaults [Member]
Losses incurred [Abstract]
Change in loss reserves	(20,000,000)
Increase (Decrease) in Severity, Pool Defaults [Member]
Losses incurred [Abstract]
Change in loss reserves			185,000,000
Decrease Related to LAE Reserves and Reinsurance [Member]
Losses incurred [Abstract]
Change in loss reserves			(20,000,000)		(7,000,000)
Increase in Severity [Member]
Losses incurred [Abstract]
Change in loss reserves					$ 137,000,000

[1] At December 31, 2010, 2009 and 2008 the estimated reduction in loss reserves related to rescissions approximated $1.3 billion, $2.1 billion and $0.5 billion, respectively.

[2] At December 31, 2011, 2010 and 2009 the estimated reduction in loss reserves related to rescissions approximated $0.7 billion, $1.3 billion and $2.1 billion, respectively.

[3] In periods prior to 2010 an estimate of premium to be refunded in conjunction with claim payments was included in Loss Reserves. In 2010, we separately stated portions of this liability in Other liabilities and Premium deficiency reserve on the consolidated balance sheet.

[4] A negative number for prior year losses incurred indicates a redundancy of prior year loss reserves, and a positive number for prior year losses incurred indicates a deficiency of prior year loss reserves.

[5] Rescissions did not have a significant impact on incurred losses in 2011. Rescissions mitigated our incurred losses by an estimated $0.2 billion and $2.5 billion in 2010 and 2009, respectively.

[6] In a termination, the reinsurance agreement is cancelled, with no future premium ceded and funds for any incurred but unpaid losses transferred to us. The transferred funds result in an increase in our investment portfolio (including cash and cash equivalents) and a decrease in net losses paid (reduction to losses incurred). In addition, there is an offsetting decrease in the reinsurance recoverable (increase in losses incurred), and thus there is no net impact to losses incurred. (See Note 11 - "Reinsurance")

[7] Rescissions mitigated our paid losses by an estimated $0.6 billion, $1.0 billion and $0.9 billion in 2011, 2010 and 2009, respectively, which excludes amounts that may have been applied to a deductible.