Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net income	$ 86,795	$ 84,401	$ 86,136
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	18,313	16,855	16,245
Write-down of equipment pending disposal	806	102	616
Deferred income tax expense (benefit)	12,449	5,345	11,276
Stock-based compensation expense	3,959	3,634	2,936
Tax benefits from stock-based compensation	1,443	2,193	1,898
Excess tax benefits from stock-based compensation	(1,184)	(2,205)	(2,031)
Net cash provided by operating activities	115,400	122,008	150,225
Cash flows from investing activities:
Payments to Acquire Businesses, Net of Cash Acquired	(1,636)	35,556	0
Net cash used in investing activities	(95,339)	(195,842)	(204,937)
Cash flows from financing activities:
Redemption of preferred stock	(16,538)	(48)	(243)
Repayment of junior subordinated debentures held by subsidiary trusts	0	(20,439)	0
Proceeds from issuance of common stock, net of stock issuance costs	3,369	6,299	9,814
Excess tax benefits from stock-based compensation	1,184	2,205	2,031
Purchase and retirement of common stock	(20,647)	(9,739)	(448)
Dividends paid to common stockholders	(36,290)	(28,626)	(17,909)
Net cash used in financing activities	(38,274)	337,677	(211,793)
Net change in cash and cash equivalents	(18,213)	263,843	(266,505)
Cash and cash equivalents at beginning of year	798,670	534,827	801,332
Cash and cash equivalents at end of year	780,457	798,670	534,827
Parent Company
Cash flows from operating activities:
Net income	86,795	84,401	86,136
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries	(27,439)	(37,684)	(23,748)
Stock-based compensation expense	3,959	3,634	2,936
Tax benefits from stock-based compensation	1,443	2,193	1,898
Excess tax benefits from stock-based compensation	(1,184)	(2,205)	(2,031)
Other, net	(11,333)	8,991	(5,804)
Net cash provided by operating activities	52,241	59,330	59,387
Cash flows from investing activities:
Payments to Acquire Businesses, Net of Cash Acquired	(7,207)	(37,891)	0
Net cash used in investing activities	(7,207)	(37,891)	0
Cash flows from financing activities:
Net (decrease) increase in advances from nonbank subsidiaries	(2,021)	(10,632)	6,992
Redemption of preferred stock	0	0	(50,000)
Repayments of Long-term Debt	(950)	0	0
Repayment of junior subordinated debentures held by subsidiary trusts	0	(20,439)	0
Proceeds from issuance of common stock, net of stock issuance costs	3,369	6,001	9,814
Excess tax benefits from stock-based compensation	1,184	2,205	2,031
Purchase and retirement of common stock	(20,647)	(9,739)	(448)
Dividends paid to common stockholders	(36,290)	(28,626)	(17,909)
Net cash used in financing activities	(55,355)	(61,230)	(49,520)
Net change in cash and cash equivalents	(10,321)	(39,791)	9,867
Cash and cash equivalents at beginning of year	65,483	105,274	95,407
Cash and cash equivalents at end of year	$ 55,162	$ 65,483	$ 105,274