Securities - Amortized Cost and Fair Value of Securities (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Securities available-for-sale [Abstract]
Amortized Cost	$ 4,579,141,000	$ 3,487,296,000
Gross Unrealized Gains	84,879,000	117,222,000
Gross Unrealized Losses	29,367,000	17,837,000
Fair Value	4,634,653,000	3,586,681,000
Available-for-sale, Amortized Cost [Abstract]
Due in one year or less	106,128,000
Due in one year to five years	56,921,000
Due in five years to ten years	508,208,000
Due after ten years	1,748,950,000
Mortgage-backed securities	1,938,218,000
Asset-backed securities	220,716,000
Amortized Cost	4,579,141,000	3,487,296,000
Available-for-sale, Fair Value [Abstract]
Due in one year or less	106,120,000
Due in one year to five years	56,821,000
Due in five years to ten years	517,678,000
Due after ten years	1,765,856,000
Mortgage-backed securities	1,967,981,000
Asset-backed securities	220,197,000
Fair Value	4,634,653,000	3,586,681,000
Securities held-to-maturity [Abstract]
Amortized Cost	989,398,000	1,028,550,000
Gross Unrealized Gains	25,817,000	38,272,000
Gross Unrealized Losses	808,000	291,000
Fair Value	1,014,407,000	1,066,531,000
Allowance for credit losses - securities held-to-maturity	(161,000)	(191,000)
HeldToMaturitySecurities, net of allowance for credit losses	989,237,000	1,028,359,000
Held-to-maturity, Amortized Cost [Abstract]
Due in one year or less	0
Due in one year to five years	1,409,000
Due in five years to ten years	5,676,000
Due after ten years	982,313,000
Mortgage-backed securities	0
Asset-backed securities	0
Amortized Cost	989,398,000	1,028,550,000
Held-to-maturity, Fair Value [Abstract]
Due in one year or less	0
Due in one year to five years	1,472,000
Due in five years to ten years	5,734,000
Due after ten years	1,007,201,000
Mortgage-backed securities	0
Asset-backed securities	0
Fair Value	1,014,407,000	1,066,531,000
U.S. Treasury securities
Securities available-for-sale [Abstract]
Amortized Cost	144,387,000	82,199,000
Gross Unrealized Gains	0	10,000
Gross Unrealized Losses	230,000	0
Fair Value	144,157,000	82,209,000
Available-for-sale, Amortized Cost [Abstract]
Amortized Cost	144,387,000	82,199,000
Available-for-sale, Fair Value [Abstract]
Fair Value	144,157,000	82,209,000
U.S. government agency securities
Securities available-for-sale [Abstract]
Amortized Cost	423,481,000	74,916,000
Gross Unrealized Gains	1,566,000	1,547,000
Gross Unrealized Losses	2,992,000	60,000
Fair Value	422,055,000	76,403,000
Available-for-sale, Amortized Cost [Abstract]
Amortized Cost	423,481,000	74,916,000
Available-for-sale, Fair Value [Abstract]
Fair Value	422,055,000	76,403,000
Mortgage-backed securities
Securities available-for-sale [Abstract]
Amortized Cost	1,938,218,000	1,623,759,000
Gross Unrealized Gains	41,663,000	67,759,000
Gross Unrealized Losses	11,900,000	2,327,000
Fair Value	1,967,981,000	1,689,191,000
Available-for-sale, Amortized Cost [Abstract]
Amortized Cost	1,938,218,000	1,623,759,000
Available-for-sale, Fair Value [Abstract]
Fair Value	1,967,981,000	1,689,191,000
State and municipal securities
Securities available-for-sale [Abstract]
Amortized Cost	1,733,063,000	1,411,288,000
Gross Unrealized Gains	38,130,000	44,559,000
Gross Unrealized Losses	11,545,000	12,484,000
Fair Value	1,759,648,000	1,443,363,000
Available-for-sale, Amortized Cost [Abstract]
Amortized Cost	1,733,063,000	1,411,288,000
Available-for-sale, Fair Value [Abstract]
Fair Value	1,759,648,000	1,443,363,000
Securities held-to-maturity [Abstract]
Amortized Cost	989,398,000	1,028,550,000
Gross Unrealized Gains	25,817,000	38,272,000
Gross Unrealized Losses	808,000	291,000
Fair Value	1,014,407,000	1,066,531,000
Held-to-maturity, Amortized Cost [Abstract]
Amortized Cost	989,398,000	1,028,550,000
Held-to-maturity, Fair Value [Abstract]
Fair Value	1,014,407,000	1,066,531,000
Asset-backed securities
Securities available-for-sale [Abstract]
Amortized Cost	220,716,000	177,878,000
Gross Unrealized Gains	214,000	715,000
Gross Unrealized Losses	733,000	657,000
Fair Value	220,197,000	177,936,000
Available-for-sale, Amortized Cost [Abstract]
Amortized Cost	220,716,000	177,878,000
Available-for-sale, Fair Value [Abstract]
Fair Value	220,197,000	177,936,000
Corporate notes and other
Securities available-for-sale [Abstract]
Amortized Cost	119,276,000	117,256,000
Gross Unrealized Gains	3,306,000	2,632,000
Gross Unrealized Losses	1,967,000	2,309,000
Fair Value	120,615,000	117,579,000
Available-for-sale, Amortized Cost [Abstract]
Amortized Cost	119,276,000	117,256,000
Available-for-sale, Fair Value [Abstract]
Fair Value	$ 120,615,000	$ 117,579,000