Investment Securities - Additional Information on Securities in a Continuous Loss Position (Detail) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) Security	Dec. 31, 2014 USD ($) Security
Schedule of Available-for-sale Securities
Number of securities | Security	42	71
Amortized cost basis	$ 246,694	$ 574,840
Unrealized loss	$ 3,419	$ 10,390
Issued by Fannie Mae, Freddie Mac, or Ginnie Mae
Schedule of Available-for-sale Securities
Number of securities | Security	39	66
Amortized cost basis	$ 241,918	$ 566,113
Unrealized loss	$ 3,347	$ 10,254
Issued by Political Subdivisions
Schedule of Available-for-sale Securities
Number of securities | Security	2	5
Amortized cost basis	$ 4,267	$ 8,727
Unrealized loss	$ 68	$ 136
Others
Schedule of Available-for-sale Securities
Number of securities | Security	1	0
Amortized cost basis	$ 509	$ 0
Unrealized loss	$ 4	$ 0