Condensed Parent Company Financial Statements - Statements of Cash Flows (Parent Company Only) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating Activities
Net income		$ 98,387	$ 92,349	$ 82,064
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation of premises, furniture, and equipment		13,995	12,804	13,367
Net securities losses (gains)		1,876	(1,420)	(2,373)
Share-based compensation expense	[1]	11,223	7,879	7,242
Tax benefit (expense) related to share-based compensation		349	(197)	(406)
Net decrease (increase) in other assets		121,577	(76,902)	10,023
Net (decrease) increase in other liabilities		(55,762)	47,118	(1,042)
Investing Activities
Purchases of securities available-for-sale		(733,440)	(933,317)	(509,481)
Purchases of premises, furniture, and equipment		(16,123)	(19,083)	(11,269)
Net cash paid for acquisitions		41,717	57,347	(16,047)
Net cash used in investing activities		(145,780)	(1,049,338)	(496,834)
Financing Activities
Net proceeds from the issuance of subordinated debt		0	146,484	0
Payments for the retirement of senior and subordinated debt		0	(153,500)	0
Treasury stock activity		0	0	120
Cash dividends paid		(37,129)	(29,198)	(27,036)
Restricted stock activity		(3,952)	(2,476)	(2,890)
Net cash used in financing activities		(4,357)	808,750	114,023
Net (decrease) increase in cash and cash equivalents		84,422	(119,054)	(225,060)
Cash and cash equivalents at beginning of year		262,148	381,202	606,262
Cash and cash equivalents at end of year		346,570	262,148	381,202
Supplemental Disclosures of Cash Flow Information:
Common stock issued for acquisitions, net of issuance costs		534,090	54,896	0
Parent Company
Operating Activities
Net income		98,387	92,349	82,064
Adjustments to reconcile net income to net cash provided by operating activities:
Equity in undistributed (income) loss of subsidiaries		(51,138)	(25,968)	26,462
Depreciation of premises, furniture, and equipment		9	2	7
Net securities losses (gains)		1,523	0	(1)
Share-based compensation expense		11,223	7,879	7,242
Tax benefit (expense) related to share-based compensation		349	(197)	(406)
Net decrease (increase) in other assets		18,667	(75,104)	23,699
Net (decrease) increase in other liabilities		(52,377)	74,571	(17,132)
Net cash provided by operating activities		26,643	73,532	121,935
Investing Activities
Purchases of securities available-for-sale		0	(14)	0
Proceeds from sales and maturities of securities available-for-sale		42,516	601	310
Purchases of premises, furniture, and equipment		(119)	0	(5)
Net cash paid for acquisitions		(47,364)	(14,905)	(16,047)
Net cash used in investing activities		(4,967)	(14,318)	(15,742)
Financing Activities
Net proceeds from the issuance of subordinated debt		0	146,484	0
Payments for the retirement of senior and subordinated debt		0	(153,500)	0
Treasury stock activity		0	0	(120)
Cash dividends paid		(37,129)	(29,198)	(27,036)
Restricted stock activity		(3,952)	(2,476)	(2,890)
Net cash used in financing activities		(41,081)	(38,690)	(30,046)
Net (decrease) increase in cash and cash equivalents		(19,405)	20,524	76,147
Cash and cash equivalents at beginning of year		140,217	119,693	43,546
Cash and cash equivalents at end of year		120,812	140,217	119,693
Supplemental Disclosures of Cash Flow Information:
Common stock issued for acquisitions, net of issuance costs		$ 534,090	$ 54,896	$ 0

[1] Comprised of restricted stock, restricted stock unit, and performance share awards expense.