Borrowings - Additional Information (Detail) - USD ($)			3 Months Ended
	Jun. 18, 2015	Mar. 05, 2015	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	Mar. 04, 2015
Long Term And Short Term Debt [Line Items]
Total long-term debt outstanding at a fixed rate			$ 13,200,000
Total long-term debt outstanding at a variable rate			103,100,000
Capital lease			200,000
Repayment of junior subordinated debt			0	$ 8,500,000
Federal Home Loan Bank Advance
Long Term And Short Term Debt [Line Items]
Total borrowings			471,300,000		$ 472,900,000
Short-term borrowings			355,000,000
Long-term borrowings			116,300,000
Maximum borrowing capacity			2,000,000,000.0
Remaining borrowing capacity			1,400,000,000
Borrowing capacity utilized			628,100,000
Letter of credit to collateralize public funds			156,800,000
Junior Subordinated Debt
Long Term And Short Term Debt [Line Items]
Repayment of junior subordinated debt	$ 5,000,000	$ 8,500,000
Integrity Bancshares, Inc. | Junior Subordinated Debt
Long Term And Short Term Debt [Line Items]
Debt assumed in acquisition						$ 13,500,000
Mortgage Backed Securities
Long Term And Short Term Debt [Line Items]
Mortgage backed securities pledged as collateral			$ 67,400,000		$ 67,000,000