Mergers and Acquisitions - Schedule of Expected Fair Value of Acquired Identifiable Assets and Liabilities Assumed (Detail) - USD ($) $ in Thousands	Jun. 03, 2016	Dec. 31, 2016	Dec. 31, 2015
Business Acquisition [Line Items]
Preliminary resulting goodwill		$ 863,767	$ 710,252
Bank of Georgetown [Member]
Business Acquisition [Line Items]
Value of common shares issued (6,527,746 shares)	$ 253,799
Fair value of stock options assumed	10,696
Cash for fractional shares	10
Total purchase price	264,505
Cash and cash equivalents	29,340
Investment securities	219,783
Loans	968,197
Premises and equipment	5,574
Other assets	31,605
Total identifiable assets	1,263,557
Deposits	971,685
Short-term borrowings	101,021
Long-term borrowings	67,659
Other liabilities	11,532
Total identifiable liabilities	1,151,897
Preliminary fair value of net assets acquired including identifiable intangible assets	111,660
Preliminary resulting goodwill	152,845
Bank of Georgetown [Member] | Core Deposit Intangible Assets [Member]
Business Acquisition [Line Items]
Cash for fractional shares	10
Core deposit intangibles	$ 9,058