PARENT COMPANY STATEMENTS (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended	12 Months Ended
	Jan. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Assets:
Cash and cash equivalents		$ 324,878	$ 211,837	$ 361,876
Other assets		107,584	83,129
Total assets		6,488,275	5,444,506	5,402,813
Liabilities and shareholders' equity:
Accrued interest and other liabilities		88,118	72,719
Borrowings		102,048	102,024
Shareholders' equity		774,583	607,258	565,697	544,651
Total liabilities and shareholders' equity		6,488,275	5,444,506
Revenues:
Other income		89,222	88,792	83,535
Expenses:
Interest on long-term notes and debentures		5,801	5,960	6,153
Other expenses		12,869	12,436	11,508
Income (Loss) before income taxes		103,527	86,385	53,067
Income tax benefit		(30,385)	(23,065)	(11,622)
Net income		73,142	63,320	41,445
Operating activities:
Net income		73,142	63,320	41,445
Adjustments to reconcile net income to net cash provided by operating activities
Net change in other assets and other liabilities		14,269	3,554	37,566
Investing activities:
Net cash (used in)/provided by investing activities		(26,134)	(207,250)	(86,572)
Financing activities:
Issuance of common stock	57,500
Net cash provided by / (used in) financing activities		41,358	(39,877)	190,561
Change in cash and cash equivalents		113,041	(150,039)	148,123
Cash and cash equivalents at beginning of year	324,878	211,837	361,876	213,753
Cash and cash equivalents at end of year		324,878	211,837	361,876
Supplemental disclosures of cash flow information:
Cash paid for interest		61,564	67,485	85,011
Supplemental disclosures of noncash financing activities
Dividends declared and unpaid		9,609	7,976	7,211
Parent Company [Member]
Assets:
Cash and cash equivalents		38,853	35,969	57,097
Investment securities		3,572	3,612
Investment in and advances to subsidiaries		838,130	674,516
Other assets		8,145	8,243
Total assets		888,700	722,340
Liabilities and shareholders' equity:
Accrued interest and other liabilities		12,069	13,058
Borrowings		102,048	102,024
Shareholders' equity		774,583	607,258
Total liabilities and shareholders' equity		888,700	722,340
Revenues:
Dividends from subsidiaries		52,251	3,500	36,000
Interest and dividends on investments		87	345	656
Other income		0	2	45
Total revenues		52,338	3,847	36,701
Expenses:
Interest on long-term notes and debentures		5,815	5,960	6,153
Other expenses		273	14	101
Total expenses		6,088	5,974	6,254
Income (Loss) before income taxes		46,250	(2,127)	30,447
Income tax benefit		2,237	2,131	1,529
Income before equity in undistributed net income (loss) of subsidiaries		48,487	4	31,976
Equity in undistributed net income (loss) of subsidiaries		24,655	63,316	9,469
Net income		73,142	63,320	41,445
Operating activities:
Net income		73,142	63,320	41,445
Gain on sale of fixed assets and investment securities		0	0	(7)
Adjustments to reconcile net income to net cash provided by operating activities
Equity in undistributed net (income)/loss of subsidiaries		(24,655)	(63,316)	(9,469)
Net change in other assets and other liabilities		(504)	(2,397)	1,190
Net cash provided by (used in) operating activities		47,983	(2,393)	33,159
Investing activities:
Purchase of investment securities		0	(10)	0
Proceeds from sale of investment securities		2	0	26
Repayments from/(advances to) subsidiaries		51	1,712	1,656
Capital contributions to subsidiaries		(102,462)	0	0
Net cash (used in)/provided by investing activities		(102,409)	1,702	1,682
Financing activities:
Issuance of common stock		91,714	9,957	4,341
Cash dividends paid		(34,404)	(30,394)	(28,789)
Net cash provided by / (used in) financing activities		57,310	(20,437)	(24,448)
Change in cash and cash equivalents		2,884	(21,128)	10,393
Cash and cash equivalents at beginning of year		35,969	57,097	46,704
Cash and cash equivalents at end of year		38,853	35,969	57,097
Supplemental disclosures of cash flow information:
Cash paid for interest		5,959	5,960	6,283
Supplemental disclosures of noncash financing activities
Dividends declared and unpaid		$ 9,609	$ 7,976	$ 7,211