Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Cash flows from operating activities:
Net income	$ 95,636	$ 86,795	$ 84,401
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	19,480	18,313	16,855
Write-down of equipment pending disposal	171	806	102
Deferred income tax expense (benefit)	3,369	12,449	5,345
Stock-based compensation expense	4,376	3,959	3,634
Tax benefits from stock-based compensation	2,146	1,443	2,193
Excess tax benefits from stock-based compensation	(1,566)	(1,184)	(2,205)
Net cash provided by operating activities	118,070	115,400	122,008
Cash flows from investing activities:
Payments to Acquire Businesses, Net of Cash Acquired	18,554	(1,636)	35,556
Net cash used in investing activities	(143,092)	(95,339)	(195,842)
Cash flows from financing activities:
Redemption of preferred stock	(65)	(16,538)	(48)
Repayment of junior subordinated debentures held by subsidiary trusts	0	0	(20,439)
Proceeds from issuance of common stock, net of stock issuance costs	4,683	3,369	6,299
Excess tax benefits from stock-based compensation	1,566	1,184	2,205
Purchase and retirement of common stock	(26,854)	(20,647)	(9,739)
Dividends paid to common stockholders	(39,353)	(36,290)	(28,626)
Net cash used in financing activities	26,588	(38,274)	337,677
Net change in cash and cash equivalents	1,566	(18,213)	263,843
Cash and cash equivalents at beginning of year	780,457	798,670	534,827
Cash and cash equivalents at end of year	782,023	780,457	798,670
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Payments of Capital Distribution	1,986	0	0
Parent Company
Cash flows from operating activities:
Net income	95,636	86,795	84,401
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries	32,351	(27,439)	(37,684)
Stock-based compensation expense	4,376	3,959	3,634
Tax benefits from stock-based compensation	2,146	1,443	2,193
Excess tax benefits from stock-based compensation	(1,566)	(1,184)	(2,205)
Other, net	(801)	(11,333)	8,991
Net cash provided by operating activities	132,142	52,241	59,330
Cash flows from investing activities:
Payments to Acquire Businesses, Net of Cash Acquired	0	(7,207)	37,891
Net cash used in investing activities	1,987	(7,207)	(37,891)
Cash flows from financing activities:
Net (decrease) increase in advances from nonbank subsidiaries	9,052	(2,021)	(10,632)
Redemption of preferred stock	0	0	0
Repayments of Long-term Debt	0	(950)	0
Repayment of junior subordinated debentures held by subsidiary trusts	0	0	(20,439)
Proceeds from issuance of common stock, net of stock issuance costs	4,683	3,369	6,001
Excess tax benefits from stock-based compensation	1,566	1,184	2,205
Purchase and retirement of common stock	(26,854)	(20,647)	(9,739)
Dividends paid to common stockholders	(39,353)	(36,290)	(28,626)
Net cash used in financing activities	(50,906)	(55,355)	(61,230)
Net change in cash and cash equivalents	83,223	(10,321)	(39,791)
Cash and cash equivalents at beginning of year	55,162	65,483	105,274
Cash and cash equivalents at end of year	138,385	55,162	65,483
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Payments to Acquire Productive Assets	$ (1)	$ 0	$ 0