Investment Securities - Additional Information (Detail) - USD ($)	6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Trading securities	$ 0
Securities pledged as collateral	374,900,000		$ 470,400,000
FHLB pledged investment securities	3,000,000		0
Proceeds from sale of investment securities	88,200,000	$ 140,900,000
Gains from sale of investment securities	928,000	943,000
Losses from sale of investment securities	0	$ 0
Unamortized premiums	21,000,000		22,400,000
Unaccreted discounts	$ 136,000		188,000
Held-to-maturity securities, unrealized loss			0
Weighted average duration of MBS portfolio	4 years 7 months 6 days
Owned investment securities	$ 555,900,000		330,754,000
Total unrealized losses on securities	7,364,000		3,594,000
Mortgage-Backed Securities ("MBS") [Member]
Schedule of Available-for-sale Securities [Line Items]
OTTI on evaluation of securities	$ 0		0
Bond [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair market value of investment			$ 1,300,000