Commitments and contingencies (Details Textual) (USD $)	Jun. 30, 2012	Dec. 31, 2011
Commitments and contingencies (Textual) [Abstract]
Maximum credit risk for recourse associated with loans sold under Federal National Mortgage Association Delegated Underwriting and Servicing program	$ 1,800,000,000	$ 1,800,000,000
Outstanding loan principal balances reinsured	71,000,000
Assets of subsidiaries providing reinsurance to satisfy claims	42,000,000
Range of reasonably possible losses, minimum	0
Range of reasonably possible losses, maximum	40,000,000
2011 - 2013 [Member]
Significant Commitments [Line Items]
Naming rights of football stadium per year	5,000,000
2014 - 2017 [Member]
Significant Commitments [Line Items]
Naming rights of football stadium per year	$ 6,000,000