Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows from operating activities:
Net income	$ 58,224,000	$ 44,546,000	$ 37,356,000
Adjustments to reconcile net income to cash provided by operating activities:
Depreciation and amortization	17,112,000	17,368,000	20,136,000
Write-down of equipment pending disposal	70	0	0
Deferred income tax expense (benefit)	8,762,000	2,405,000	(17,257,000)
Stock-based compensation expense	2,485,000	2,111,000	1,764,000
Tax benefits from stock-based compensation	360,000	204,000	239,000
Excess tax benefits from stock-based compensation	(273,000)	(124,000)	(225,000)
Net cash provided by operating activities	135,822,000	143,101,000	109,258,000
Cash flows from investing activities:
Net cash used in investing activities	(141,079,000)	(125,939,000)	(387,511,000)
Cash flows from financing activities:
Repayments of long-term debt	(40,000)	(302,000)	(35,851,000)
Repayment of junior subordinated debentures held by subsidiary trusts	(41,238,000)	0	0
Proceeds from issuance of common stock	1,911,000	385,000	167,400,000
Common stock issuance costs	0	0	(13,597,000)
Excess tax benefits from stock-based compensation	273,000	124,000	225,000
Purchase and retirement of common stock	(263,000)	(248,000)	(3,699,000)
Dividends paid to common stockholders	(26,208,000)	(19,233,000)	(17,905,000)
Dividends paid to preferred stockholders	(3,300,000)	(3,422,000)	(3,422,000)
Net cash provided by (used in) financing activities	334,142,000	(230,333,000)	340,389,000
Net change in cash and cash equivalents	328,885,000	(213,171,000)	62,136,000
Cash and cash equivalents at beginning of year	472,447,000	685,618,000	623,482,000
Cash and cash equivalents at end of year	801,332,000	472,447,000	685,618,000
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Transfer of preferred stock pending redemption from equity to liability	50,000,000
Transfered accrued liabilities to common stock in conjunction with the vesting of liability-classified non-vested stock awards		216,000	59,000
Parent Company
Cash flows from operating activities:
Net income	58,224,000	44,546,000	37,356,000
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries	(28,914,000)	(25,444,000)	(32,226,000)
Stock-based compensation expense	2,485,000	2,111,000	1,764,000
Tax benefits from stock-based compensation	360,000	204,000	239,000
Excess tax benefits from stock-based compensation	(273,000)	(124,000)	(225,000)
Other, net	3,327,000	2,600,000	(4,325,000)
Net cash provided by operating activities	35,209,000	23,893,000	2,583,000
Cash flows from investing activities:
Capitalization of subsidiaries	0	0	(130,000)
Capital expenditures, net of sales	1,000	(3,000)	0
Net cash used in investing activities	1,000	(3,000)	(130,000)
Cash flows from financing activities:
Net (decrease) increase in advances from nonbank subsidiaries	(2,838,000)	(2,913,000)	5,002,000
Repayments of long-term debt	0	0	(33,929,000)
Repayment of junior subordinated debentures held by subsidiary trusts	(41,238,000)	0	0
Proceeds from issuance of common stock	1,911,000	385,000	167,400,000
Common stock issuance costs	0	0	(13,597,000)
Excess tax benefits from stock-based compensation	273,000	124,000	225,000
Purchase and retirement of common stock	(263,000)	(248,000)	(3,699,000)
Dividends paid to common stockholders	(26,208,000)	(19,233,000)	(17,905,000)
Dividends paid to preferred stockholders	(3,300,000)	(3,422,000)	(3,422,000)
Net cash provided by (used in) financing activities	(71,663,000)	(25,307,000)	100,075,000
Net change in cash and cash equivalents	(36,453,000)	(1,417,000)	102,528,000
Cash and cash equivalents at beginning of year	131,860,000	133,277,000	30,749,000
Cash and cash equivalents at end of year	95,407,000	131,860,000	133,277,000
Cash Flow, Noncash Investing and Financing Activities Disclosure [Abstract]
Transfer of preferred stock pending redemption from equity to liability	$ 50,000,000