Summary of Significant Accounting Policies (Details)					3 Months Ended												12 Months Ended
	Jan. 01, 2020 USD ($)	Jul. 02, 2019 USD ($)	Jan. 01, 2019 USD ($)	Mar. 01, 2016 USD ($)	Dec. 31, 2020 USD ($) $ / shares	Sep. 30, 2020 USD ($) $ / shares	Jun. 30, 2020 USD ($) $ / shares	Mar. 31, 2020 USD ($) $ / shares	Dec. 31, 2019 USD ($) $ / shares	Sep. 30, 2019 USD ($) $ / shares	Jun. 30, 2019 USD ($) $ / shares	Mar. 31, 2019 USD ($) $ / shares	Dec. 31, 2018 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares	Jun. 30, 2018 USD ($) $ / shares	Mar. 31, 2018 USD ($) $ / shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2019 USD ($) $ / shares shares		Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 shares	Dec. 31, 2020 USD ($) market shares	Dec. 31, 2019 USD ($) shares
Accounting Policies [Abstract]
Number of Markets in Which Entity Operates | market																						12
Goodwill [Roll Forward]
Balance at December 31, 2019	$ 1,819,811,000							$ 1,819,811,000									$ 1,819,811,000
Goodwill, Acquired During Period																	0
Balance at December 31, 2020					$ 1,819,811,000				$ 1,819,811,000								1,819,811,000	$ 1,819,811,000
Finite-lived Intangible Assets [Roll Forward]
Balance at December 31, 2019	51,130,000							51,130,000									51,130,000
Acquisitions																	1,000,000
Amortization																	(9,793,000)	(9,908,000)		$ (10,549,000)
Balance at December 31, 2020					42,336,000				51,130,000								42,336,000	51,130,000
Balance at December 31, 2019	1,870,941,000							1,870,941,000									1,870,941,000
Acquisitions																	1,000,000
Amortization																	(9,793,000)
Balance at December 31, 2020					1,862,147,000				1,870,941,000								1,862,147,000	1,870,941,000
Finite-Lived Intangible Assets, Net [Abstract]
Finite-Lived Intangible Assets, Gross																						$ 108,665,000	$ 107,665,000
Accumulated amortization																						(66,329,000)	(56,535,000)
Net book value	51,130,000				42,336,000			51,130,000	51,130,000								$ 42,336,000	51,130,000				42,336,000	51,130,000
Cash Equivalents and Cash Flows [Abstract]
Cash equivalents maturity period																	90 days
Cash Payments [Abstract]
Interest																	$ 215,888,000	287,272,000		199,464,000
Income taxes paid																	110,798,000	86,960,000		55,626,000
Noncash Transactions [Abstract]
Loans charged-off to the allowance for credit losses																	(49,333,000)	(28,467,000)		(30,400,000)
Real Estate Owned, Transfer to Real Estate Owned																	3,436,000	17,937,000		3,524,000
Loans foreclosed upon with repossessions transferred to other repossessed assets																	25,000	93,000		1,899,000
Other Real Estate Sales Financed																	0	871,000		891,000
Fixed assets transferred to other real estate																	0	8,182,000		0
Available-for-sale securities transferred to Held-to-Maturity								873,600,000						$ 179,800,000			873,613,000	0		179,763,000
Held-for-sale loans transferred to held-for-investment loan portfolios																	0	0		44,980,000
Right of Use Assets Recognized																	15,820,000	90,927,000	[1]	0
Loans [Abstract]
Loans and Leases Receivable, Deferred Income																						$ 53,200,000	13,700,000
Percentage of loan portfolio assigned specific risk rating																						81.40%
Other Assets [Abstract]
Premises and equipment, net																						$ 290,001,000	273,932,000
Amortization																	2,300,000	2,700,000		3,000,000.0
Cash surrender value of life insurance																						743,900,000	652,700,000
Bank-owned life insurance																	18,784,000	17,361,000		12,535,000
investments are included in CRA investments																						143,200,000	100,900,000
Investments as per community reinvestment act																						90,200,000	58,400,000
Variable Interest Entity [Line Items]
Federal Reserve Bank and Federal Home Loan Bank (FHLB) stock																						80,400,000	80,400,000
Other Investments																						$ 47,800,000	38,200,000
Gain (loss) due to change in fair value of investments																	1,100,000	2,800,000		2,700,000
Number of trusts investment																						12
Value of investments with trust companies																						$ 308,556,000	278,037,000
Other Real Estate Owned [Abstract]
Other real estate owned																						12,400,000	30,300,000
Valuation allowance related to other real estate owned																						0	772,000
Foreclosed real estate expense																	$ 8,600,000	$ 4,200,000		$ 723,000
Income Tax Contingency [Line Items]
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued																						$ (571,000)	$ 0
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stock options outstanding (in shares) | shares													178,591							178,591	276,468	101,769	119,274
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements | shares																	277,896	399,600		338,545
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount | shares																	394,593	160,492		253,193
Basic earnings per share calculation [Abstract]
Numerator - Net income (loss) available to common stockholders					$ 107,078,000	$ 106,847,000	$ 62,444,000	$ 28,356,000	$ 96,079,000	$ 110,521,000	$ 100,321,000	$ 93,960,000	$ 95,318,000	$ 93,747,000	$ 86,865,000	$ 83,510,000	$ 304,725,000	$ 400,881,000		$ 359,440,000
Denominator - Weighted average common shares outstanding (in shares) | shares																	75,376,489	76,364,303		77,111,372
Basic net income per common share (in dollars per share) | $ / shares					$ 1.42	$ 1.42	$ 0.83	$ 0.37	$ 1.26	$ 1.45	$ 1.31	$ 1.22	$ 1.24	$ 1.22	$ 1.13	$ 1.08	$ 4.04	$ 5.25		$ 4.66
Diluted net income per share calculation [Abstract]
Numerator - Net income (loss) available to common stockholders					$ 107,078,000	$ 106,847,000	$ 62,444,000	$ 28,356,000	$ 96,079,000	$ 110,521,000	$ 100,321,000	$ 93,960,000	$ 95,318,000	$ 93,747,000	$ 86,865,000	$ 83,510,000	$ 304,725,000	$ 400,881,000		$ 359,440,000
Denominator - Weighted average common shares outstanding (in shares) | shares																	75,376,489	76,364,303		77,111,372
Dilutive shares (in shares) | shares																	277,896	399,600		338,545
Weighted average diluted common shares outstanding (in shares) | shares																	75,654,385	76,763,903		77,449,917
Diluted net income per common share (in dollars per share) | $ / shares					$ 1.42	$ 1.42	$ 0.83	$ 0.37	$ 1.26	$ 1.44	$ 1.31	$ 1.22	$ 1.23	$ 1.21	$ 1.12	$ 1.08	$ 4.03	$ 5.22		$ 4.64
New Accounting Pronouncements or Change in Accounting Principle
Right of Use Assets Recognized																	$ 15,820,000	$ 90,927,000	[1]	$ 0
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease)																	$ 38,102,000
Premises and Equipment | Minimum
Premises and Equipment and Leaseholds [Abstract]
Premises and equipment, useful life																	3 years
Premises and Equipment | Maximum
Premises and Equipment and Leaseholds [Abstract]
Premises and equipment, useful life																	30 years
Computer Software, Intangible Asset
Other Assets [Abstract]
Premises and equipment, net																						$ 4,500,000	$ 6,000,000.0
Computer Software, Intangible Asset | Minimum
Premises and Equipment and Leaseholds [Abstract]
Premises and equipment, useful life																	3 years
Computer Software, Intangible Asset | Maximum
Premises and Equipment and Leaseholds [Abstract]
Premises and equipment, useful life																	7 years
Bankers Healthcare Group, LLC
Schedule of Equity Method Investments [Line Items]
Equity Method Investment, Ownership Percentage																						49.00%
Business Acquisition [Line Items]
Cash paid to redeem common stock				$ 74,100,000
Finite-lived Intangible Assets [Roll Forward]
Amortization																	$ (1,200,000)	$ (1,900,000)		$ (2,800,000)
Internal Revenue Service (IRS) | Minimum
Income Tax Contingency [Line Items]
Tax year open to audit under the statute of limitation																					2017
Internal Revenue Service (IRS) | Maximum
Income Tax Contingency [Line Items]
Tax year open to audit under the statute of limitation																	2020
Variable Interest Entity, Primary Beneficiary
Variable Interest Entity [Line Items]
Value of investments with trust companies																						$ 4,000,000.0
Advocate Capital
Business Acquisition [Line Items]
Cash paid to redeem common stock		$ 59,000,000.0
Loans, net of allowance for loan losses		45,600,000
Borrowings		$ 134,300,000
Employee Stock Option
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Stock options outstanding (in shares) | shares																						101,769
Accounting Standards Update 2016-02
Noncash Transactions [Abstract]
Right of Use Assets Recognized			$ 79,900,000
New Accounting Pronouncements or Change in Accounting Principle
Right of Use Assets Recognized			$ 79,900,000
Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle
Cumulative effect of change in accounting principle								$ 31,800,000
Financing Receivable, Allowance for Credit Loss, Period Increase (Decrease)	34,300,000
Off-Balance Sheet, Credit Loss, Liability	8,800,000
Deferred Taxes Recorded due to Cumulative Change in Accounting Principle	11,300,000
Financing Receivable, Allowance for Credit Loss, Purchased with Credit Deterioration, Increase	$ 3,800,000

[1] Includes $79.9 million recognized upon initial adoption of ASU 2016-02 on January 1, 2019.