Basis of presentation (Details) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2011	Dec. 31, 2010	Dec. 31, 2009
Basis of presentation (Details) [Abstract]
Number of jurisdictions with risk-to-capital requirements	16
Maximum permitted risk-to-capital ratio commonly applied	25 to 1
Risk-to-capital ratio at end of period	20.4 to 1
Risk-to-capital ratio under a stress loss scenario	40 to 1
Excess of policyholders position over required regulatory minimum	$ 183,000,000
Excess of policyholders position over required regulatory minimum, combined basis	260,000,000
Risk-to-capital ratio of combined insurance operations, including reinsurance affiliates, at end of period	23.4 to 1
Percentage of new insurance written in jurisdictions with risk-to-capital requirements (in hundreths)	47.00%	50.00%
Contribution to subsidiary under GSE agreement	200,000,000
Minimum number of jurisdictions that could prevent the entity from continuing to write new insurance	1
Mitigation of paid losses by rescission of policies	400,000,000	1,200,000,000	1,200,000,000
Estimated mitigation of incurred losses by impact of rescissions on loss reserving methodology		$ 200,000,000	$ 2,500,000,000
Percentage of claims received in a quarter resolved by rescission, lower range limit	18.00%
Percentage of claims received in a quarter resolved by rescission, upper range limit	24.00%
Number of loans in primary delinquency inventory	184,452
Period during which the insured may initiate legal proceedings (in years)	3