Parent Company Financial Statements - Statements of Cash Flows of Parent Company Only (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Supplemental Cash Flow Information [Line Items]
Net income	$ 117,804	$ 110,410	$ 87,047
Other, net	14,280	34,848	34,989
Net cash flows provided by operating activities	386,084	544,030	289,530
Proceeds from sale of securities available for sale	22,047	87,101	101,973
Net cash flows used in investing activities	(644,410)	(793,503)	(284,030)
Decrease in long-term debt	(113,967)	(197,568)	(166,144)
Increase in long-term debt	92,583	40,315	52,827
Decrease in junior subordinated debt	(134,021)
Net proceeds from issuance of preferred stock	106,882
Net proceeds from issuance of common stock	62,092	8,895	71,053
Tax benefit (expense) of stock-based compensation	1,326	386	(61)
Cash dividends paid	(71,246)	(67,646)	(60,686)
Net cash flows (used in) provided by financing activities	233,263	279,564	71,882
Net Increase (Decrease) in Cash and Cash Equivalents	(25,063)	30,091	77,382
Cash and cash equivalents at beginning of year	239,044	208,953	131,571
Cash and Cash Equivalents at End of Year	213,981	239,044	208,953
Interest	46,337	56,306	75,178
Parent Company [Member]
Supplemental Cash Flow Information [Line Items]
Net income	117,804	110,410	87,047
Undistributed earnings from subsidiaries	(43,804)	(39,489)	(23,681)
Other, net	(6,218)	109	8,666
Net cash flows provided by operating activities	67,782	71,030	72,032
Proceeds from sale of securities available for sale	128	201	389
(Increase) decrease in property, plant and equipment		4,193	(243)
Net decrease (increase) in advances to subsidiaries	1,080	(1,349)	(7,551)
Net increase in investment in subsidiaries	(1,688)	(79,450)	(16,611)
Net cash flows used in investing activities	(480)	(76,405)	(24,016)
Net (decrease) increase in advance from affiliate	(854)	12,944	16,900
Net increase (decrease) in short-term borrowings	84	(2)	(316)
Decrease in long-term debt	(808)	(1,277)	(1,206)
Increase in long-term debt	499	671	798
Decrease in junior subordinated debt	(134,021)
Net proceeds from issuance of preferred stock	106,882
Net proceeds from issuance of common stock	62,092	8,895	71,053
Tax benefit (expense) of stock-based compensation	1,326	386	(61)
Cash dividends paid	(71,246)	(67,646)	(60,686)
Net cash flows (used in) provided by financing activities	(36,046)	(46,029)	26,482
Net Increase (Decrease) in Cash and Cash Equivalents	31,256	(51,404)	74,498
Cash and cash equivalents at beginning of year	114,654	166,058	91,560
Cash and Cash Equivalents at End of Year	145,910	114,654	166,058
Interest	$ 14,345	$ 15,690	$ 16,768