PARENT COMPANY STATEMENTS (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended	12 Months Ended
	Jan. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Assets [Abstract]
Cash and cash equivalents		$ 228,558	$ 324,878	$ 211,837
Other assets		117,650	107,584
Total assets		7,496,800	6,488,275	5,444,506
Liabilities and shareholders' equity [Abstract]
Accrued interest and other liabilities		135,849	88,118
Borrowings		102,073	102,048
Shareholders' equity		902,778	774,583	607,258	565,697
Total liabilities and shareholders' equity		7,496,800	6,488,275
Revenues [Abstract]
Other income		99,246	89,222	88,792
Expenses [Abstract]
Interest on long-term notes and debentures		2,716	5,801	5,960
Other expenses		16,459	12,869	12,941
Income before income taxes and equity in undistributed net income of subsidiaries		108,805	103,527	86,385
Income tax benefit		(31,737)	(30,385)	(23,065)
Net income		77,068	73,142	63,320
Operating activities [Abstract]
Net income		77,068	73,142	63,320
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Net change in other assets and other liabilities		292	14,268	3,554
Investing activities [Abstract]
Net cash used in investing activities		(264,260)	(22,695)	(207,250)
Financing activities [Abstract]
Issuance of common stock	57,500	64,145	5,351	6,433
Net cash provided by (used in) financing activities		59,516	41,358	(39,877)
Change in cash and cash equivalents		(96,320)	113,041	(150,039)
Cash and cash equivalents at beginning of year	324,878	324,878	211,837	361,876
Cash and cash equivalents at end of year		228,558	324,878	211,837
Supplemental disclosures of cash flow information [Abstract]
Cash paid for interest		51,541	61,564	67,485
Supplemental disclosures of noncash financing activities [Abstract]
Dividends declared and unpaid		10,699	9,609	7,976
Parent Company [Member]
Assets [Abstract]
Cash and cash equivalents		43,800	38,853	35,969
Investment securities		3,587	3,572
Investment in and advances to subsidiaries		961,718	838,130
Other assets		8,613	8,145
Total assets		1,017,718	888,700
Liabilities and shareholders' equity [Abstract]
Accrued interest and other liabilities		12,867	12,069
Borrowings		102,073	102,048
Shareholders' equity		902,778	774,583
Total liabilities and shareholders' equity		1,017,718	888,700
Revenues [Abstract]
Dividends from subsidiaries		0	52,251	3,500
Interest and dividends on investments		98	87	345
Other income		198	0	2
Total revenues		296	52,338	3,847
Expenses [Abstract]
Interest on long-term notes and debentures		2,716	5,815	5,960
Other expenses		155	273	14
Total expenses		2,871	6,088	5,974
Income before income taxes and equity in undistributed net income of subsidiaries		(2,575)	46,250	(2,127)
Income tax benefit		1,274	2,237	2,131
(Loss) Income before equity in undistributed net income of subsidiaries		(1,301)	48,487	4
Equity in undistributed net income of subsidiaries		78,369	24,655	63,316
Net income		77,068	73,142	63,320
Operating activities [Abstract]
Net income		77,068	73,142	63,320
Adjustments to reconcile net income to net cash provided by operating activities [Abstract]
Equity in undistributed net income of subsidiaries		(78,369)	(24,655)	(63,316)
Net change in other assets and other liabilities		(746)	(504)	(2,397)
Net cash provided by (used in) operating activities		(2,047)	47,983	(2,393)
Investing activities [Abstract]
Purchase of investment securities		(3)	0	(10)
Proceeds from sale of investment securities		30	2	0
Repayments from subsidiaries, net		0	51	1,712
Capital contributions to subsidiaries		(20,081)	(102,462)	0
Net cash used in investing activities		(20,054)	(102,409)	1,702
Financing activities [Abstract]
Issuance of common stock		67,813	91,714	9,957
Cash dividends paid		(40,765)	(34,404)	(30,394)
Net cash provided by (used in) financing activities		27,048	57,310	(20,437)
Change in cash and cash equivalents		4,947	2,884	(21,128)
Cash and cash equivalents at beginning of year	38,853	38,853	35,969	57,097
Cash and cash equivalents at end of year		43,800	38,853	35,969
Supplemental disclosures of cash flow information [Abstract]
Cash paid for interest		2,738	5,959	5,960
Supplemental disclosures of noncash financing activities [Abstract]
Dividends declared and unpaid		$ 10,699	$ 9,609	$ 7,976