Parent Holding Company Information (Condensed) (Details 3) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Operating Activities
Net income	$ 20,758	$ 19,444	$ 18,981	$ 16,333	$ 14,348	$ (19,048)	$ 11,886	$ 10,285	$ 75,516	$ 17,471	$ 42,330
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on sale of investments									0	(346)	(4,822)
Excess tax deficiencies from stock options exercised									8	0	4
Net cash provided by operating activities									183,793	186,843	193,943
Investing Activities
Proceeds from sales, maturities and prepayments of investment securities, available-for-sale									2,041,416	1,024,508	700,182
Purchases of investment securities, available-for-sale									(2,638,054)	(1,730,244)	(1,664,341)
Net addition of premises and equipment and other real estate owned									(10,730)	(17,492)	(22,652)
Net cash used in investment activities									(579,646)	(529,096)	(822,506)
Financing Activities
Cash dividends paid									47,472	37,395	37,396
Excess tax deficiencies from stock options exercised									8	0	4
Proceeds from stock options exercised and other stock issued									(81)	0	(145,654)
Net cash provided by financing activities									(454,861)	(365,194)	(523,079)
Net increase (decrease) in cash and cash equivalents									59,008	22,941	(105,484)
Cash and cash equivalents at beginning of period				128,032				105,091	128,032	105,091	210,575
Cash and cash equivalents at end of period	187,040				128,032				187,040	128,032	105,091
Parent Company [Member]
Operating Activities
Net income									75,516	17,091	42,712
Adjustments to reconcile net income to net cash provided by operating activities:
Subsidiary income (in excess of) less than dividends distributed									(1,040)	22,825	(13,558)
Gain on sale of investments									0	0	3,013
Excess tax deficiencies from stock options exercised									8	0	4
Net (decrease) increase in other assets and other liabilities									3,684	1,215	(708)
Net cash provided by operating activities									78,168	41,131	25,437
Investing Activities
Proceeds from sales, maturities and prepayments of investment securities, available-for-sale									787	1,376	3,671
Purchases of investment securities, available-for-sale									(19,183)	0	(13,126)
Equity contribution to subsidiaries									(28,500)	(1,110)	(105,841)
Net addition of premises and equipment and other real estate owned									(2,927)	(1,920)	(2,754)
Net cash used in investment activities									(49,823)	(1,654)	(118,050)
Financing Activities
Net (decrease) increase in other borrowed funds									143	143	(4,857)
Cash dividends paid									(47,472)	(37,395)	(37,396)
Excess tax deficiencies from stock options exercised									(8)	0	(4)
Proceeds from stock options exercised and other stock issued									81	0	145,654
Net cash provided by financing activities									(47,256)	(37,252)	103,397
Net increase (decrease) in cash and cash equivalents									(18,911)	2,225	10,784
Cash and cash equivalents at beginning of period				31,338				29,113	31,338	29,113	18,329
Cash and cash equivalents at end of period	$ 12,427				$ 31,338				$ 12,427	$ 31,338	$ 29,113