Commitments and contingencies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Significant Commitments [Line Items]
Maximum credit risk for recourse associated with loans sold under Federal National Mortgage Association Delegated Underwriting and Servicing program	$ 2,000,000,000	$ 1,800,000,000
Period of noncancelable operating lease in years	26 years
Outstanding loan principal balances reinsured	67,000,000
Assets of subsidiaries providing reinsurance to satisfy claims	41,000,000
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