Parent Company Information - Schedule of Condensed Statements of Financial Condition (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Interest bearing deposits with banks	$ 172,800	$ 171,710
Investment securities available for sale	1,493,905	1,297,373
Other assets	542,839	583,654
Total Assets	24,002,306	22,864,439
Liabilities and Shareholders’ Equity
Long-term borrowings	2,315,819	1,433,906
Junior subordinated debentures issued to capital trusts	41,774	41,577
Shareholders’ equity	2,533,165	2,377,156	$ 2,207,091	$ 1,863,017
Total Liabilities and Shareholders’ Equity	24,002,306	22,864,439
Parent Company
Assets
Cash	90,807	68,927
Interest bearing deposits with banks	0	82
Investment securities available for sale	254	239
Investments in and receivables due from subsidiaries	2,738,700	2,591,982
Other assets	36,277	36,188
Total Assets	2,866,038	2,697,418
Liabilities and Shareholders’ Equity
Dividends payable to shareholders	33,100	29,477
Long-term borrowings	235,153	236,731
Junior subordinated debentures issued to capital trusts	41,774	41,577
Accrued expenses and other liabilities	22,846	12,477
Shareholders’ equity	2,533,165	2,377,156
Total Liabilities and Shareholders’ Equity	$ 2,866,038	$ 2,697,418