Investment Securities - Amortized Cost and Approximate Fair Values of Investment Securities (Details) - USD ($)	3 Months Ended	12 Months Ended
	Jun. 30, 2022	Dec. 31, 2021	Mar. 31, 2022	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020
Available-for-Sale:
Amortized Cost	$ 8,193,800,000	$ 4,859,300,000
Gross Unrealized Gains	3,500,000	20,600,000
Gross Unrealized Losses	(456,200,000)	(59,400,000)
Estimated Fair Value	7,741,100,000	4,820,500,000
Held-to-Maturity:
Amortized Cost	3,131,600,000	1,687,600,000
Allowance for Credit Losses	(1,600,000)	0	$ (1,600,000)	$ 0	$ 0	$ 0
Net Carrying Amount	3,130,000,000	1,687,600,000
Gross Unrealized Gains	500,000	15,600,000
Gross Unrealized Losses	(270,000,000.0)	(35,700,000)
Estimated Fair Value	2,860,500,000	1,667,500,000
U.S. Treasury notes
Available-for-Sale:
Amortized Cost	871,000,000.0	697,600,000
Gross Unrealized Gains	1,400,000	0
Gross Unrealized Losses	(62,600,000)	(12,900,000)
Estimated Fair Value	809,800,000	684,700,000
Held-to-Maturity:
Amortized Cost	296,700,000
Allowance for Credit Losses	0
Net Carrying Amount	296,700,000
Gross Unrealized Gains	0
Gross Unrealized Losses	(3,600,000)
Estimated Fair Value	293,100,000
State, county, and municipal securities
Available-for-Sale:
Amortized Cost	325,000,000.0	434,700,000
Gross Unrealized Gains	100,000	2,100,000
Gross Unrealized Losses	(42,900,000)	(9,300,000)
Estimated Fair Value	282,200,000	427,500,000
Held-to-Maturity:
Amortized Cost	186,600,000	67,600,000
Allowance for Credit Losses	(100,000)	0
Net Carrying Amount	186,500,000	67,600,000
Gross Unrealized Gains	500,000	2,000,000.0
Gross Unrealized Losses	(24,000,000.0)	(400,000)
Estimated Fair Value	163,000,000.0	69,200,000
Obligations of U.S. government agencies
Available-for-Sale:
Amortized Cost	235,300,000	356,000,000.0
Gross Unrealized Gains	0	100,000
Gross Unrealized Losses	(9,400,000)	(9,200,000)
Estimated Fair Value	225,900,000	346,900,000
Held-to-Maturity:
Amortized Cost	328,100,000
Allowance for Credit Losses	0
Net Carrying Amount	328,100,000
Gross Unrealized Gains	0
Gross Unrealized Losses	(28,900,000)
Estimated Fair Value	299,200,000
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Amortized Cost	5,054,700,000	2,027,300,000
Gross Unrealized Gains	2,000,000.0	14,100,000
Gross Unrealized Losses	(241,300,000)	(23,300,000)
Estimated Fair Value	4,815,400,000	2,018,100,000
Held-to-Maturity:
Amortized Cost	2,236,000,000	1,609,000,000
Allowance for Credit Losses	0	0
Net Carrying Amount	2,236,000,000	1,609,000,000
Gross Unrealized Gains	0	13,200,000
Gross Unrealized Losses	(208,700,000)	(35,300,000)
Estimated Fair Value	2,027,300,000	1,586,900,000
Debt Securities, Held-To-Maturity, Transfer, Unamortized Gain	15,900,000	20,100,000
Debt Securities, Held-To-Maturity, Transfer, Unamortized Loss	(21,100,000)
Private mortgage-backed securities
Available-for-Sale:
Amortized Cost	278,100,000	174,400,000
Gross Unrealized Gains	0	100,000
Gross Unrealized Losses	(25,100,000)	(1,100,000)
Estimated Fair Value	253,000,000.0	173,400,000
Collateralized Loan Obligations
Available-for-Sale:
Amortized Cost	1,145,700,000	898,200,000
Gross Unrealized Gains	0	1,200,000
Gross Unrealized Losses	(51,700,000)	0
Estimated Fair Value	1,094,000,000	899,400,000
Corporate Debt Securities
Available-for-Sale:
Amortized Cost	284,000,000.0	271,100,000
Gross Unrealized Gains	0	3,000,000.0
Gross Unrealized Losses	(23,200,000)	(3,600,000)
Estimated Fair Value	260,800,000	270,500,000
Held-to-Maturity:
Amortized Cost	84,200,000	11,000,000.0
Allowance for Credit Losses	(1,500,000)	0
Net Carrying Amount	82,700,000	11,000,000.0
Gross Unrealized Gains	0	400,000
Gross Unrealized Losses	(4,800,000)	0
Estimated Fair Value	$ 77,900,000	$ 11,400,000