Parent Company Information - Schedule of Condensed Statements of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net Income	$ 116,172	$ 131,961	$ 143,627
Depreciation and amortization	19,465	19,712	18,477
Stock-based compensation	7,489	6,055	4,816
Net amortization of premiums and accretion of discounts on securities	26,949	16,229	21,124
Gains on securities transactions, net	(745)	(14,678)	(2,587)
Fair value of borrowings carried at fair value	0	(881)	(2,574)
Other assets	17,231	(17,587)	49,345
Net cash provided by operating activities	183,976	285,364	323,311
Sales	62,025	57,003	257,497
Purchases	(28,415)	(289,181)	(455,601)
Maturities, calls and principal repayments	153,673	183,006	248,721
Cash and cash equivalents acquired in acquisitions	102,025	0	117,587
Net cash (used in) provided by investing activities	(511,567)	(786,026)	183,708
Issuance of long-term borrowings	0	125,000	0
Dividends paid to common shareholders	(88,119)	(129,271)	(125,870)
Common stock issued, net	3,408	7,071	7,805
Net cash provided by (used in) financing activities	788,830	16,730	(32,968)
Net change in cash and cash equivalents	461,239	(483,932)	474,051
Cash and cash equivalents at beginning of year	369,168	853,100	379,049
Cash and cash equivalents at end of year	830,407	369,168	853,100
Parent Company [Member]
Net Income	116,172	131,961	143,627
Adjustments to reconcile net income to net cash provided by operating activities:	(13,663)	(21,494)	(7,381)
Depreciation and amortization	18	27	30
Stock-based compensation	7,489	6,055	4,816
Net amortization of premiums and accretion of discounts on securities	163	162	162
Gains on securities transactions, net	0	(14)	0
Fair value of borrowings carried at fair value	0	(881)	(2,574)
Other assets	7,947	(4,210)	2,977
Accrued expenses and other liabilities	(3,166)	1,124	(1,387)
Net cash provided by operating activities	114,960	112,730	140,270
Sales	46	643	0
Purchases	(500)	0	0
Maturities, calls and principal repayments	0	0	1,000
Cash and cash equivalents acquired in acquisitions	14,776	0	3,934
Net cash (used in) provided by investing activities	14,322	643	4,934
Issuance of long-term borrowings	0	125,000	0
Redemption of junior subordinated debentures	0	(142,313)	(10,000)
Dividends paid to common shareholders	(88,119)	(129,271)	(125,870)
Common stock issued, net	3,408	7,071	7,805
Net cash provided by (used in) financing activities	(84,711)	(139,513)	(128,065)
Net change in cash and cash equivalents	44,571	(26,140)	17,139
Cash and cash equivalents at beginning of year	33,727	59,867	42,728
Cash and cash equivalents at end of year	$ 78,298	$ 33,727	$ 59,867