Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Available-for-Sale:
Amortized Cost	$ 6,575,100,000	$ 4,859,300,000
Gross Unrealized Gains	1,800,000	20,600,000
Gross Unrealized Losses	(293,100,000)	(59,400,000)
Estimated Fair Value	6,283,800,000	4,820,500,000
Held-to-Maturity:
Amortized Cost	3,220,300,000	1,687,600,000
Allowance for Credit Losses	(1,600,000)	0	$ 0	$ 0
Net Carrying Amount	3,218,700,000	1,687,600,000
Gross Unrealized Gains	19,200,000	15,600,000
Gross Unrealized Losses	(208,800,000)	(35,700,000)
Estimated Fair Value	3,029,100,000	1,667,500,000
U.S. Treasury notes
Available-for-Sale:
Amortized Cost	698,200,000	697,600,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(53,300,000)	(12,900,000)
Estimated Fair Value	644,900,000	684,700,000
Held-to-Maturity:
Amortized Cost	296,200,000
Allowance for Credit Losses	0
Net Carrying Amount	296,200,000
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Value	296,200,000
State, county, and municipal securities
Available-for-Sale:
Amortized Cost	320,300,000	434,700,000
Gross Unrealized Gains	200,000	2,100,000
Gross Unrealized Losses	(27,800,000)	(9,300,000)
Estimated Fair Value	292,700,000	427,500,000
Held-to-Maturity:
Amortized Cost	197,300,000	67,600,000
Allowance for Credit Losses	(100,000)	0
Net Carrying Amount	197,200,000	67,600,000
Gross Unrealized Gains	800,000	2,000,000.0
Gross Unrealized Losses	(21,400,000)	(400,000)
Estimated Fair Value	176,600,000	69,200,000
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	144,600,000	356,000,000.0
Gross Unrealized Gains	0	100,000
Gross Unrealized Losses	(6,700,000)	(9,200,000)
Estimated Fair Value	137,900,000	346,900,000
Held-to-Maturity:
Amortized Cost	340,800,000
Allowance for Credit Losses	0
Net Carrying Amount	340,800,000
Gross Unrealized Gains	0
Gross Unrealized Losses	(32,300,000)
Estimated Fair Value	308,500,000
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Amortized Cost	3,816,400,000	2,027,300,000
Gross Unrealized Gains	1,100,000	14,100,000
Gross Unrealized Losses	(171,800,000)	(23,300,000)
Estimated Fair Value	3,645,700,000	2,018,100,000
Held-to-Maturity:
Amortized Cost	2,302,000,000	1,609,000,000
Allowance for Credit Losses	0	0
Net Carrying Amount	2,302,000,000	1,609,000,000
Gross Unrealized Gains	18,200,000	13,200,000
Gross Unrealized Losses	(152,400,000)	(35,300,000)
Estimated Fair Value	2,167,800,000	1,586,900,000
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain	17,900,000	20,100,000
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss	(21,800,000)
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	288,400,000	174,400,000
Gross Unrealized Gains	0	100,000
Gross Unrealized Losses	(13,600,000)	(1,100,000)
Estimated Fair Value	274,800,000	173,400,000
Collateralized Loan Obligations
Available-for-Sale:
Amortized Cost	1,075,400,000	898,200,000
Gross Unrealized Gains	0	1,200,000
Gross Unrealized Losses	(4,500,000)	0
Estimated Fair Value	1,070,900,000	899,400,000
Corporate Debt Securities
Available-for-Sale:
Amortized Cost	231,800,000	271,100,000
Gross Unrealized Gains	500,000	3,000,000.0
Gross Unrealized Losses	(15,400,000)	(3,600,000)
Estimated Fair Value	216,900,000	270,500,000
Held-to-Maturity:
Amortized Cost	84,000,000.0	11,000,000.0
Allowance for Credit Losses	(1,500,000)	0
Net Carrying Amount	82,500,000	11,000,000.0
Gross Unrealized Gains	200,000	400,000
Gross Unrealized Losses	(2,700,000)	0
Estimated Fair Value	$ 80,000,000.0	$ 11,400,000