Borrowings - Additional Information (Detail) - USD ($)			6 Months Ended
	Jun. 18, 2015	Mar. 05, 2015	Jun. 30, 2015	Jun. 30, 2014	Mar. 04, 2015	Dec. 31, 2014
Long Term And Short Term Debt [Line Items]
Total long-term debt outstanding at a fixed rate			$ 15,000,000
Total long-term debt outstanding at a variable rate			148,800,000
Capital lease			200,000
Repayment of junior subordinated debt			13,500,000	$ 0
Total borrowings			349,000,000			$ 309,300,000
Short-term borrowings			231,000,000
Long-term borrowings			118,000,000
Federal Home Loan Bank Advances [Member]
Long Term And Short Term Debt [Line Items]
Maximum borrowing capacity			1,900,000,000.0
Remaining borrowing capacity			1,400,000,000
Borrowing capacity utilized			500,000,000
Letter of credit to collateralize public funds			168,500,000
Junior Subordinated Debt [Member]
Long Term And Short Term Debt [Line Items]
Repayment of junior subordinated debt	$ 5,000,000	$ 8,500,000
Borrowings					$ 13,500,000
Mortgage Backed Securities [Member]
Long Term And Short Term Debt [Line Items]
Mortgage backed securities pledged as collateral			$ 50,500,000			$ 35,600,000