Parent Company Financial Statements - Consolidated Balance Sheet of Parent Company Only (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 01, 2012
Cash and cash equivalents	$ 287,393	$ 213,981	$ 239,044	$ 208,953
Securities available for sale	1,534,065	1,141,650	1,172,683
Other assets	374,741	350,867
Total Assets	16,127,090	13,563,405	12,023,976
Other liabilities	140,325	130,418
Junior subordinated debt	58,246	75,205
Long-term	83,155	84,637
Total Liabilities	14,105,634	11,789,022
Stockholders' Equity	2,021,456	1,774,383	1,402,069	1,210,199
Total Liabilities and Stockholders' Equity	16,127,090	13,563,405
Parent Company [Member]
Cash and cash equivalents	129,320	145,910	114,654	166,058
Securities available for sale	1,228	2,061
Other assets	16,615	18,387
Total Assets	2,408,624	2,180,033
Other liabilities	25,772	25,225
Advances from affiliates	292,337	294,245
Junior subordinated debt	59,279	76,290
Short-term	8,351	8,439
Long-term	1,429	1,451
Total Liabilities	387,168	405,650
Stockholders' Equity	2,021,456	1,774,383
Total Liabilities and Stockholders' Equity	2,408,624	2,180,033
Parent Company [Member] | Bank Subsidiary [Member]
Investment in and advances to subsidiaries	2,006,808	1,759,551
Parent Company [Member] | Non-bank Subsidiaries [Member]
Investment in and advances to subsidiaries	$ 254,653	$ 254,124