Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2022	Dec. 31, 2021	Jun. 30, 2022	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020
Available-for-Sale:
Amortized Cost	$ 7,448,200,000	$ 4,859,300,000
Gross Unrealized Gains	200,000	20,600,000
Gross Unrealized Losses	(665,000,000.0)	(59,400,000)
Estimated Fair Value	6,783,400,000	4,820,500,000
Held-to-Maturity:
Amortized Cost	3,487,600,000	1,687,600,000
Allowance for Credit Losses	(1,900,000)	0	$ (1,600,000)	$ 0	$ 0	$ 0
Net Carrying Amount	3,485,700,000	1,687,600,000
Gross Unrealized Gains	200,000	15,600,000
Gross Unrealized Losses	(422,400,000)	(35,700,000)
Estimated Fair Value	3,063,500,000	1,667,500,000
U.S. Treasury notes
Available-for-Sale:
Amortized Cost	373,500,000	697,600,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(36,700,000)	(12,900,000)
Estimated Fair Value	336,800,000	684,700,000
Held-to-Maturity:
Amortized Cost	396,100,000
Allowance for Credit Losses	0
Net Carrying Amount	396,100,000
Gross Unrealized Gains	0
Gross Unrealized Losses	(10,600,000)
Estimated Fair Value	385,500,000
State, county, and municipal securities
Available-for-Sale:
Amortized Cost	317,800,000	434,700,000
Gross Unrealized Gains	0	2,100,000
Gross Unrealized Losses	(57,300,000)	(9,300,000)
Estimated Fair Value	260,500,000	427,500,000
Held-to-Maturity:
Amortized Cost	184,000,000.0	67,600,000
Allowance for Credit Losses	(100,000)	0
Net Carrying Amount	183,900,000	67,600,000
Gross Unrealized Gains	200,000	2,000,000.0
Gross Unrealized Losses	(33,200,000)	(400,000)
Estimated Fair Value	150,900,000	69,200,000
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	216,300,000	356,000,000.0
Gross Unrealized Gains	0	100,000
Gross Unrealized Losses	(17,300,000)	(9,200,000)
Estimated Fair Value	199,000,000.0	346,900,000
Held-to-Maturity:
Amortized Cost	350,800,000
Allowance for Credit Losses	0
Net Carrying Amount	350,800,000
Gross Unrealized Gains	0
Gross Unrealized Losses	(51,200,000)
Estimated Fair Value	299,600,000
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Amortized Cost	4,852,200,000	2,027,300,000
Gross Unrealized Gains	200,000	14,100,000
Gross Unrealized Losses	(442,700,000)	(23,300,000)
Estimated Fair Value	4,409,700,000	2,018,100,000
Held-to-Maturity:
Amortized Cost	2,476,600,000	1,609,000,000
Allowance for Credit Losses	0	0
Net Carrying Amount	2,476,600,000	1,609,000,000
Gross Unrealized Gains	0	13,200,000
Gross Unrealized Losses	(320,000,000.0)	(35,300,000)
Estimated Fair Value	2,156,600,000	1,586,900,000
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain	15,200,000	20,100,000
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss	(20,800,000)
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	270,300,000	174,400,000
Gross Unrealized Gains	0	100,000
Gross Unrealized Losses	(37,100,000)	(1,100,000)
Estimated Fair Value	233,200,000	173,400,000
Collateralized Loan Obligations
Available-for-Sale:
Amortized Cost	1,145,500,000	898,200,000
Gross Unrealized Gains	0	1,200,000
Gross Unrealized Losses	(40,400,000)	0
Estimated Fair Value	1,105,100,000	899,400,000
Corporate Debt Securities
Available-for-Sale:
Amortized Cost	272,600,000	271,100,000
Gross Unrealized Gains	0	3,000,000.0
Gross Unrealized Losses	(33,500,000)	(3,600,000)
Estimated Fair Value	239,100,000	270,500,000
Held-to-Maturity:
Amortized Cost	80,100,000	11,000,000.0
Allowance for Credit Losses	(1,800,000)	0
Net Carrying Amount	78,300,000	11,000,000.0
Gross Unrealized Gains	0	400,000
Gross Unrealized Losses	(7,400,000)	0
Estimated Fair Value	$ 70,900,000	$ 11,400,000