Parent Company Financial Statements - Statements of Cash Flows of Parent Company Only (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Cash Flow Information [Line Items]
Net income	$ 144,050	$ 117,804	$ 110,410
Other, net	(34,746)	14,280	34,848
Net cash flows provided by operating activities	446,819	346,093	544,030
Proceeds from sale of securities available for sale	175,872	22,047	87,101
Net cash received (paid) in business combinations	59,980	141,637	203,538
Net cash flows used in investing activities	(1,738,256)	(604,419)	(793,503)
Decrease in long-term debt	(62,884)	(113,967)	(197,568)
Increase in long-term debt	385,656	92,583	40,315
Decrease in junior subordinated debt	(34,022)	(134,021)
Net proceeds from issuance of preferred stock		106,882
Net proceeds from issuance of common stock	12,857	62,092	8,895
Tax benefit of stock-based compensation	2,714	1,326	386
Preferred stock	(8,352)
Common stock	(81,220)	(71,246)	(67,646)
Net cash flows provided by financing activities	1,364,849	233,263	279,564
Net (Decrease) Increase in Cash and Cash Equivalents	73,412	(25,063)	30,091
Cash and cash equivalents at beginning of year	213,981	239,044	208,953
Cash and Cash Equivalents at End of Year	287,393	213,981	239,044
Interest	43,057	46,337	56,306
Parent Company [Member]
Supplemental Cash Flow Information [Line Items]
Net income	144,050	117,804	110,410
Undistributed earnings from subsidiaries	(55,631)	(43,804)	(39,489)
Other, net	(637)	(6,218)	109
Net cash flows provided by operating activities	87,782	67,782	71,030
Proceeds from sale of securities available for sale	934	128	201
Decrease in property, plant and equipment			4,193
Net decrease (increase) in advances to subsidiaries	2,018	1,080	(1,349)
Net (increase) decrease in investment in subsidiaries	(2,877)	1,845	1,535
Net cash received (paid) in business combinations	5,594	(3,533)	(80,985)
Net cash flows used in investing activities	5,669	(480)	(76,405)
Net (decrease) increase in advance from affiliate	(1,908)	(854)	12,944
Net (decrease) increase in short-term borrowings	(88)	84	(2)
Decrease in long-term debt	(843)	(808)	(1,277)
Increase in long-term debt	821	499	671
Decrease in junior subordinated debt	(34,022)	(134,021)
Net proceeds from issuance of preferred stock		106,882
Net proceeds from issuance of common stock	12,857	62,092	8,895
Tax benefit of stock-based compensation	2,714	1,326	386
Preferred stock	(8,352)
Common stock	(81,220)	(71,246)	(67,646)
Net cash flows provided by financing activities	(110,041)	(36,046)	(46,029)
Net (Decrease) Increase in Cash and Cash Equivalents	(16,590)	31,256	(51,404)
Cash and cash equivalents at beginning of year	145,910	114,654	166,058
Cash and Cash Equivalents at End of Year	129,320	145,910	114,654
Interest	$ 9,112	$ 14,351	$ 15,701