Mortgage Servicing Rights - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Mortgage Servicing Rights [Line Items]
Aggregate fair value of mortgage servicing rights	$ 6.5	$ 4.2
Minimum [Member]
Mortgage Servicing Rights [Line Items]
Range of fair value of mortgage servicing rights using discount rates	5.00%	5.00%
Maximum [Member]
Mortgage Servicing Rights [Line Items]
Range of fair value of mortgage servicing rights using discount rates	10.00%	10.00%