Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows from operating activities:
Net earnings	$ 74,225	$ 68,369	$ 59,659
Adjustments to reconcile net earnings to net cash provided by operating activities:
Depreciation and amortization	7,920	7,352	7,102
Provision for loan losses	3,484	6,626	8,962
Securities premium amortization (discount accretion), net	15,757	8,251	4,460
Gain on sale of assets, net	(2,628)	(80)	(2,765)
Deferred federal income tax expense (benefit)	(1,517)	2,005	1,089
Change in loans held for sale	(827)	2,531	(8,837)
Change in other assets	(5,170)	(1,075)	4,573
Change in other liabilities	1,818	1,132	(2,415)
Total adjustments	18,837	26,742	12,169
Net cash provided by operating activities	93,062	95,111	71,828
CASH FLOWS FROM INVESTING ACTIVITIES:
Cash paid for acquisition of bank, less cash acquired			(2,463)
Net decrease (increase) in interest-bearing time deposits in banks	12,170	42,511	(47,063)
Activity in available-for-sale securities:
Sales	144,144	22,970	28,039
Maturities	1,909,635	1,886,632	1,812,241
Purchases	(2,049,275)	(2,171,404)	(2,068,639)
Activity in held-to-maturity securities - maturities	2,557	5,458	6,216
Net increase in loans	(306,412)	(108,152)	(82,823)
Purchases of bank premises and equipment and computer software	(16,180)	(14,777)	(11,240)
Proceeds from sale of other assets	8,370	5,732	9,924
Net cash used in (provided by) investing activities	(294,991)	(331,030)	(355,808)
Cash flows from financing activities:
Net increase in noninterest-bearing deposits	210,132	142,103	99,387
Net increase in interest-bearing deposits	87,654	79,394	179,237
Net increase in short-term borrowings	51,941	29,400	32,262
Common stock transactions:
Proceeds from stock issuances	824	950	789
Dividends paid	(38,719)	(29,359)	(28,346)
Net cash provided by (used in) financing activities	311,832	222,488	283,329
NET INCREASE (DECREASE) IN CASH AND CASH EQUIVALENTS	109,903	(13,431)	(651)
CASH AND CASH EQUIVALENTS, beginning of year	250,836	264,267	264,918
Cash and cash equivalents, end of year	$ 360,739	$ 250,836	$ 264,267