Investment Securities - Additional Information (Detail) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013 Security	Dec. 31, 2012 Security	Dec. 31, 2011
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Pledged securities, carrying values	$ 1,500,441,000	$ 1,712,860,000
Number of debt securities, held	54	49
Unrealized losses on debt securities, percent of amortized cost	0.78%	0.85%
Unrealized losses on debt securities, percent of Company's total amortized cost	0.15%	0.12%
Number of debt securities having unrealized losses	54	49
Number of securities recorded with an aggregate amortized cost	2	3
Number of securities, continuous unrealized loss position, twelve months or longer, fair value	2	3
Available-for-sale securities, amortized cost basis	3,953,000	5,200,000
Unrealized loss on securities	$ 180,000	$ 187,000
Unrated revenue municipal bond		1
Impairment of par value bond, percentage			50.00%
Fannie Mae And Freddie Mac Securities Rated AAA [Member] | Maximum
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Number of securities issued	2	3
Fannie Mae And Freddie Mac Securities Rated AAA [Member] | Minimum
Schedule of Securities Available-for-sale And Held To Maturity [Line Items]
Number of securities issued	1	2