Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Cash flows from operating activities:
Net income	$ 84,401	$ 86,136	$ 58,224
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	16,855	16,245	17,112
Write-down of equipment pending disposal	102	616	70
Deferred income tax expense (benefit)	5,345	11,276	8,762
Stock-based compensation expense	3,014	2,936	2,485
Tax benefits from stock-based compensation	2,193	1,898	360
Excess tax benefits from stock-based compensation	(2,205)	(2,031)	(273)
Net cash provided by operating activities	121,388	150,225	135,822
Cash flows from investing activities:
Payments to Acquire Businesses, Net of Cash Acquired	35,556	0	0
Net cash used in investing activities	(195,842)	(204,937)	(141,079)
Cash flows from financing activities:
Redemption of preferred stock	(48)	(243)	(40)
Repayment of junior subordinated debentures held by subsidiary trusts	(20,439)	0	(41,238)
Proceeds from issuance of common stock, net of stock issuance costs	6,919	9,814	1,911
Excess tax benefits from stock-based compensation	2,205	2,031	273
Purchase and retirement of common stock	(9,739)	(448)	(263)
Dividends paid to common stockholders	(28,626)	(17,909)	(26,208)
Dividends paid to preferred stockholders	0	0	(3,300)
Net cash used in financing activities	338,297	(211,793)	334,142
Net change in cash and cash equivalents	263,843	(266,505)	328,885
Cash and cash equivalents at beginning of year	534,827	801,332	472,447
Cash and cash equivalents at end of year	798,670	534,827	801,332
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Transfer of preferred stock pending redemption from equity to liability			50,000
Parent Company
Cash flows from operating activities:
Net income	84,401	86,136	58,224
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries	(37,684)	(23,748)	(28,914)
Stock-based compensation expense	3,014	2,936	2,485
Tax benefits from stock-based compensation	2,193	1,898	360
Excess tax benefits from stock-based compensation	(2,205)	(2,031)	(273)
Other, net	8,991	(5,804)	3,327
Net cash provided by operating activities	58,710	59,387	35,209
Cash flows from investing activities:
Capital expenditures, net of sales	0	0	1
Payments to Acquire Businesses, Net of Cash Acquired	(37,891)	0	0
Net cash used in investing activities	(37,891)	0	1
Cash flows from financing activities:
Net (decrease) increase in advances from nonbank subsidiaries	(10,632)	6,992	(2,838)
Redemption of preferred stock	0	(50,000)	0
Repayment of junior subordinated debentures held by subsidiary trusts	(20,439)	0	(41,238)
Proceeds from issuance of common stock, net of stock issuance costs	6,621	9,814	1,911
Excess tax benefits from stock-based compensation	2,205	2,031	273
Purchase and retirement of common stock	(9,739)	(448)	(263)
Dividends paid to common stockholders	(28,626)	(17,909)	(26,208)
Dividends paid to preferred stockholders	0	0	(3,300)
Net cash used in financing activities	(60,610)	(49,520)	(71,663)
Net change in cash and cash equivalents	(39,791)	9,867	(36,453)
Cash and cash equivalents at beginning of year	105,274	95,407	131,860
Cash and cash equivalents at end of year	65,483	105,274	95,407
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Transfer of preferred stock pending redemption from equity to liability	$ 50,000