Parent Company Financial Statements - Consolidated Balance Sheet of Parent Company Only (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash and cash equivalents	$ 489,119	$ 287,393	$ 213,981	$ 239,044
Securities available for sale	1,630,567	1,534,065	1,141,650
Other assets	401,704	374,741
Total Assets	17,557,662	16,127,090	13,563,405
Other liabilities	147,641	140,325
Long-term borrowings	641,480	541,443
Long-term	84,668	83,155
Total Liabilities	15,461,480	14,105,634
Stockholders' Equity	2,096,182	2,021,456	1,774,383	1,402,069
Total Liabilities and Stockholders' Equity	17,557,662	16,127,090
Parent Company [Member]
Cash and cash equivalents	227,554	129,320	$ 145,910	$ 114,654
Securities available for sale	1,168	1,228
Other assets	17,206	16,615
Total Assets	2,579,621	2,408,624
Other liabilities	26,831	25,772
Advances from affiliates	289,540	292,337
Long-term borrowings	157,777	59,279
Short-term	8,216	8,351
Long-term	1,075	1,429
Total Liabilities	483,439	387,168
Stockholders' Equity	2,096,182	2,021,456
Total Liabilities and Stockholders' Equity	2,579,621	2,408,624
Parent Company [Member] | Bank Subsidiary [Member]
Investment in and advances to subsidiaries	2,073,352	2,006,808
Parent Company [Member] | Non-bank Subsidiaries [Member]
Investment in and advances to subsidiaries	$ 260,341	$ 254,653