Commitments and Contingencies - Additional Information (Detail) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Significant Commitments [Line Items]
Maximum credit risk for recourse associated with loans sold under Federal National Mortgage Association Delegated Underwriting and Servicing program	$ 2,100,000,000	$ 2,000,000,000
Outstanding loan principal balances reinsured	54,000,000
Assets of subsidiaries providing reinsurance to satisfy claims	39,000,000
Range of reasonably possible losses, minimum	0
Range of reasonably possible losses, maximum	70,000,000
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