Parent Company Information - Schedule of Condensed Statements of Financial Condition (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Interest bearing deposits with banks	$ 367,838	$ 134,915
Investment securities available for sale	886,970	829,692
Other assets	539,392	576,919
Total Assets	18,793,855	16,156,541
Long-term borrowings	2,526,408	2,792,306
Junior subordinated debentures issued to capital trusts	41,252	41,089
Shareholders’ equity	1,863,017	1,541,040	1,502,377	1,254,836
Total Liabilities and Shareholders’ Equity	18,793,855	16,156,541
Parent Company [Member]
Cash	78,163	33,592
Interest bearing deposits with banks	135	135
Investment securities available for sale	833	4,373
Investment in subsidiaries	1,968,736	1,682,093
Other assets	18,086	13,741
Total Assets	2,065,953	1,733,934
Dividends payable to shareholders	25,512	21,957
Long-term borrowings	131,308	121,757
Junior subordinated debentures issued to capital trusts	41,252	41,089
Accrued expenses and other liabilities	4,864	8,091
Shareholders’ equity	1,863,017	1,541,040
Total Liabilities and Shareholders’ Equity	$ 2,065,953	$ 1,733,934