Investment Securities - Additional Information on Securities in a Continuous Loss Position (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015 Securities	Dec. 31, 2014 Securities
Schedule of Available-for-sale Securities [Line Items]
Number of securities, available for sale securities	42	71
Number of securities, amortized costs	$ 278,300	$ 574,840
Number of securities, unrealized loss	4,002	10,390
Fannie Mae, Freddie Mac, or Ginnie Mae [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities, available for sale securities	40	66
Number of securities, amortized costs	274,005	566,113
Number of securities, unrealized loss	3,950	10,254
Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of securities, available for sale securities	2	5
Number of securities, amortized costs	4,295	8,727
Number of securities, unrealized loss	$ 52	$ 136