Parent Company Only Financial Information (Details) - USD ($)	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Investment in unconsolidated subsidiaries:
Other assets																$ 1,220,435,000	$ 904,359,000
Total assets																27,805,496,000	25,031,044,000
Liabilities and Equity [Abstract]
Subordinated debt and other borrowings																749,080,000	485,130,000
Other Liabilities																288,569,000	158,951,000
Stockholders' equity																4,355,748,000	3,965,940,000	$ 3,707,952,000	$ 1,496,696,000
Total liabilities and stockholders' equity																27,805,496,000	25,031,044,000
Expenses:
Interest expense - subordinated debentures													$ 301,794,000	$ 210,375,000	$ 92,832,000
Personnel expense, including stock compensation													313,359,000	271,673,000	209,662,000
Income tax expense (benefit)													96,656,000	90,508,000	124,007,000
Net income	$ 96,079,000	$ 110,521,000	$ 100,321,000	$ 93,960,000	$ 95,318,000	$ 93,747,000	$ 86,865,000	$ 83,510,000	$ 26,798,000	$ 64,442,000	$ 43,086,000	$ 39,653,000	400,881,000	359,440,000	173,979,000
CONDENSED STATEMENTS OF CASH FLOWS [Abstract]
Net income	96,079,000	$ 110,521,000	$ 100,321,000	93,960,000	95,318,000	$ 93,747,000	$ 86,865,000	83,510,000	26,798,000	$ 64,442,000	$ 43,086,000	39,653,000	400,881,000	359,440,000	173,979,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, amortization and accretion													8,342,000	(23,604,000)	(23,618,000)
Stock-based compensation expense													21,226,000	17,636,000	19,538,000
Deferred tax (expense) benefit													14,696,000	11,765,000	59,651,000
Income from equity method investment													(90,111,000)	(51,222,000)	(37,958,000)
Dividends received from equity method investment													51,312,000	33,651,000	21,650,000
Excess tax benefit from stock compensation													(1,011,000)	(2,966,000)	(5,366,000)
(Increase) decrease in other assets													(57,499,000)	(24,471,000)	(37,023,000)
Increase (decrease) in other liabilities													79,264,000	60,617,000	(17,700,000)
Net cash provided by operating activities													434,288,000	470,757,000	165,864,000
Investing activities:
Increase in other investments													(55,926,000)	(56,918,000)	(7,804,000)
Net cash used in investing activities													(2,619,061,000)	(2,969,084,000)	(2,115,528,000)
Financing activities:
Repayments of other borrowings													184,175,000	620,000	220,000
Proceeds from common stock issuance													0	0	192,194,000
Share repurchase													(61,416,000)	(20,694,000)	0
Payments of Ordinary Dividends, Common Stock													49,828,000	45,454,000	35,907,000
Common dividends paid													(49,828,000)	(45,454,000)	(35,907,000)
Net cash provided by financing activities													1,989,788,000	2,440,423,000	2,545,615,000
Cash and Cash Equivalents, Period Increase (Decrease)													(194,985,000)	(57,904,000)	595,951,000
Investment Income, Dividend													114,000,000.0	83,100,000	63,100,000
Parent Company
Assets [Abstract]
Cash and cash equivalents	182,091,000			94,043,000	94,043,000			64,851,000	64,851,000			36,984,000	182,091,000	64,851,000	36,984,000	182,091,000	94,043,000	$ 64,851,000	$ 36,984,000
Investment in unconsolidated subsidiaries:
Unconsolidated subsidiaries																3,995,000	3,995,000
Other investments																97,459,000	79,646,000
Current income tax receivable																11,696,000	19,032,000
Other assets																29,566,000	26,668,000
Total assets																4,985,933,000	4,326,244,000
Liabilities and Equity [Abstract]
Taxes Payable																2,467,000	0
Subordinated debt and other borrowings																620,256,000	357,153,000
Other Liabilities																7,462,000	3,151,000
Stockholders' equity																4,355,748,000	3,965,940,000
Total liabilities and stockholders' equity																4,985,933,000	4,326,244,000
CONDENSED STATEMENTS OF OPERATIONS [Abstract]
Income from equity method investment													24,298,000	13,731,000	10,126,000
Other income (loss)													3,485,000	1,310,000	576,000
Expenses:
Interest expense - subordinated debentures													18,425,000	17,703,000	13,583,000
Personnel expense, including stock compensation													21,226,000	17,636,000	16,629,000
Other expense													1,496,000	1,312,000	4,349,000
Income before income taxes and equity in undistributed income of subsidiaries													100,796,000	61,650,000	39,342,000
Income tax expense (benefit)													(4,457,000)	(8,570,000)	(12,748,000)
Income before equity in undistributed income of subsidiaries													105,253,000	70,220,000	52,090,000
Equity in undistributed income of bank subsidiaries													294,354,000	288,728,000	121,341,000
Equity in undistributed income (loss) of nonbank subsidiaries													1,274,000	492,000	548,000
Net income													400,881,000	359,440,000	173,979,000
CONDENSED STATEMENTS OF CASH FLOWS [Abstract]
Net income													400,881,000	359,440,000	173,979,000
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation, amortization and accretion													(3,094,000)	105,000	48,000
Stock-based compensation expense													21,226,000	17,636,000	19,538,000
Increase (Decrease) in Income Taxes Payable, Net of Income Taxes Receivable													2,467,000	(24,000)	24,000
Deferred tax (expense) benefit													(2,857,000)	(549,000)	5,919,000
Income from equity method investment													(24,298,000)	(13,731,000)	(10,126,000)
Dividends received from equity method investment													8,953,000	5,872,000	5,655,000
Excess tax benefit from stock compensation													(1,011,000)	(2,966,000)	(5,365,000)
(Increase) decrease in other assets													7,295,000	4,390,000	(3,990,000)
Increase (decrease) in other liabilities													5,322,000	2,758,000	(9,194,000)
Equity in undistributed (income) loss of bank subsidiaries													(294,354,000)	(288,728,000)	(121,341,000)
Equity in undistributed (income) loss of nonbank subsidiaries													(1,274,000)	(492,000)	(548,000)
Net cash provided by operating activities													118,199,000	83,502,000	54,249,000
Investing activities:
Investment in consolidated banking subsidiaries													(180,000,000)	0	(182,288,000)
Increase in other investments													(1,411,000)	(2,321,000)	(815,000)
Net cash used in investing activities													(181,411,000)	(2,321,000)	(183,103,000)
Financing activities:
Net (decrease) increase in subordinated debt and other borrowings													316,078,000	19,850,000	0
Repayments of other borrowings													49,880,000	620,000	60,000
Proceeds from common stock issuance													0	0	192,194,000
Exercise of common stock options and stock appreciation rights, net of shares surrendered for taxes													(3,694,000)	(5,071,000)	494,000
Share repurchase													(61,416,000)	(20,694,000)	0
Payments of Ordinary Dividends, Common Stock													(49,828,000)	(45,454,000)	(35,907,000)
Net cash provided by financing activities													151,260,000	(51,989,000)	156,721,000
Cash and Cash Equivalents, Period Increase (Decrease)													88,048,000	29,192,000	27,867,000
Cash and cash equivalents, beginning of year				$ 94,043,000				$ 64,851,000				$ 36,984,000	94,043,000	64,851,000	36,984,000
Cash and cash equivalents, end of year	$ 182,091,000				$ 94,043,000				$ 64,851,000				182,091,000	94,043,000	64,851,000
Equity Securities, FV-NI, Unrealized Gain (Loss)													1,057,000	209,000	350,000
Banking
Assets [Abstract]
Investments in consolidated subsidiaries																4,653,310,000	4,096,349,000
CONDENSED STATEMENTS OF OPERATIONS [Abstract]
Income from bank subsidiaries													113,982,000	83,090,000	63,100,000
Nonbank subsidiaries
Assets [Abstract]
Investments in consolidated subsidiaries																$ 7,816,000	$ 6,511,000
CONDENSED STATEMENTS OF OPERATIONS [Abstract]
Income from bank subsidiaries													$ 178,000	$ 170,000	$ 101,000