Parent Company Information - Schedule of Condensed Statements of Cash Flows (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Net income	$ 143,627	$ 132,511	$ 130,006
Depreciation and amortization	18,477	16,380	15,850
Stock-based compensation	4,816	3,156	4,830
Net amortization of premiums and accretion of discounts on securities	21,124	11,871	12,443
Gains on securities transactions, net	(2,587)	(32,068)	(11,598)
Fair value of borrowings carried at fair value	(2,574)	(1,256)	5,841
Other assets	(49,345)	34,977	(9,569)
Net cash provided by operating activities	323,311	191,337	279,014
Sales	257,497	552,486	425,473
Maturities, calls and principal repayments	248,721	280,046	327,316
Net cash provided by (used in) investing activities	183,708	(192,841)	429,183
Dividends paid to common shareholders	(125,870)	(116,779)	(115,190)
Common stock issued, net	7,805	8,027	8,391
Net cash used in financing activities	32,968	(14,267)	1,003,248
Net change in cash and cash equivalents	474,051	12,763	(295,051)
Cash and cash equivalents at beginning of year	379,049	366,286	661,337
Cash and cash equivalents at end of year	853,100	379,049	366,286
Parent Company [Member]
Net income	143,627	132,511	130,006
Equity in undistributed earnings of subsidiary	(7,381)	(27,131)	(78,602)
Depreciation and amortization	30	30	30
Stock-based compensation	4,816	3,156	4,830
Net amortization of premiums and accretion of discounts on securities	162	(56)	(51)
Gains on securities transactions, net			(106)
Fair value of borrowings carried at fair value	(2,574)	(1,256)	5,841
Other assets	2,977	535	(538)
Accrued expenses and other liabilities	(1,387)	132	(3,363)
Net cash provided by operating activities	140,270	107,921	58,047
Loan originations		(36,990)
Sales			94
Maturities, calls and principal repayments	1,000		1,250
Cash and cash equivalents acquired in acquisitions	3,934
Net cash provided by (used in) investing activities	4,934	(36,990)	1,344
Redemption of junior subordinated debentures	(10,000)
Dividends paid to common shareholders	(125,870)	(116,779)	(115,190)
Common stock issued, net	7,805	8,027	8,391
Net cash used in financing activities	(128,065)	(108,752)	(106,799)
Net change in cash and cash equivalents	17,139	(37,821)	(47,408)
Cash and cash equivalents at beginning of year	42,728	80,549	127,957
Cash and cash equivalents at end of year	$ 59,867	$ 42,728	$ 80,549