Borrowings - Additional Information (Detail) (USD $)	3 Months Ended		0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 05, 2015	Dec. 31, 2014
Long Term And Short Term Debt [Line Items]
Total long-term debt outstanding at a fixed rate	$ 15,600,000
Total long-term debt outstanding at a variable rate	53,700,000
Capital lease	200,000
Repayment of junior subordinated debt	8,500,000	0
Total borrowings	218,200,000			309,300,000
Short-term borrowings	199,600,000
Long-term borrowings	18,700,000
Federal Home Loan Bank Advances [Member]
Long Term And Short Term Debt [Line Items]
Maximum borrowing capacity	2,000,000,000.0
Remaining borrowing capacity	1,600,000,000
Junior Subordinated Debt [Member]
Long Term And Short Term Debt [Line Items]
Repayment of junior subordinated debt			$ 8,500,000