Mergers and Acquisitions - Preliminary Allocation of Purchase Price of Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Thousands		12 Months Ended
	Sep. 09, 2016	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Goodwill recognized		$ 573,800		$ 480,600
Purchase Price Allocation Adjustment [Member]
Business Acquisition [Line Items]
Goodwill recognized		92,889	$ 90,594
Loans	$ (1,505)
Accrued income and other assets	31
Liabilities
Borrowings	184
Accrued expenses and other liabilities	637
Fair value of net assets acquired	(2,295)
Goodwill acquired	2,295
Your Community Bankshares, Inc [Member]
Business Acquisition [Line Items]
Cash and due from banks	48,212
Securities	173,223
Loans	1,013,566
Goodwill and other intangible assets	104,846
Accrued income and other assets	211,807
Total assets acquired	1,551,654
Liabilities
Deposits	1,193,010
Borrowings	123,001
Accrued expenses and other liabilities	15,145
Total liabilities assumed	1,331,156
Fair value of net assets acquired	127,609
Net assets acquired	220,498
Goodwill acquired	$ 92,889	$ 92,900