Investment Securities - Narrative (Details)	3 Months Ended			6 Months Ended
	Jun. 30, 2023 USD ($)	Jun. 30, 2022 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2023 USD ($) bond	Jun. 30, 2022 USD ($)	Mar. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)	Mar. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, transfer to held-to-maturity		$ 1,990,000,000	$ 500,800,000
Debt securities, held-to-maturity transfer, unrealized loss				$ 136,000,000
Available-for-sale, transfer to held-to-maturity, gain (loss)				0
Held to maturity mortgage backed securities	$ 3,759,968,000			3,759,968,000			$ 3,761,094,000
Available for sale mortgage backed securities	4,012,265,000			4,012,265,000			4,331,413,000
Held-to-maturity, interest receivable	20,700,000			20,700,000
Available-for-sale, interest receivable	16,600,000			16,600,000
Available-for-sale securities	3,579,758,000			3,579,758,000			3,852,854,000
Securities valued at less than historical cost, amount	$ 3,532,270,000			$ 3,532,270,000
Securities valued at less than historical cost (as percent)	98.70%			98.70%
Securities - AFS, expense (reversal)	$ (1,326,000)	0		$ 11,474,000	$ 0
Securities charged-off	0			$ 7,000,000
Number of uncollectible bonds | bond				1
Available-for-sale, net of allowance for credit losses	2,396,000			$ 2,396,000		$ 5,800,000	0
Number of nonperforming bonds | bond				2
Held-to-maturity, allowance for credit losses	3,214,000	1,381,000		$ 3,214,000	1,381,000	1,888,000	1,388,000	$ 1,377,000	$ 1,279,000
Securities pledged as collateral	3,750,000,000			3,750,000,000			3,960,000,000
Realized gains	0	0		0	37,000
Realized losses	391,000	150,000		$ 391,000	$ 240,000
Income tax benefit related to security gains (losses) (as percent)				26.135%	26.135%
Commercial Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Held to maturity mortgage backed securities	144,900,000			$ 144,900,000			149,200,000
Available for sale mortgage backed securities	898,200,000			898,200,000			1,070,000,000.00
Residential Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Held to maturity mortgage backed securities	1,050,000,000.00			1,050,000,000.00			1,040,000,000.00
Available for sale mortgage backed securities	1,380,000,000			1,380,000,000			1,470,000,000
State and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Held to maturity mortgage backed securities	1,859,956,000			1,859,956,000			1,861,102,000
Available for sale mortgage backed securities	1,029,164,000			1,029,164,000			1,056,124,000
Available-for-sale securities	885,505,000			885,505,000			871,074,000
Securities valued at less than historical cost, amount	863,343,000			863,343,000
Securities charged-off	0			0
Available-for-sale, net of allowance for credit losses	0			0		0	0
Held-to-maturity, allowance for credit losses	934,000	$ 103,000		934,000	$ 103,000	$ 362,000	$ 110,000	$ 1,285,000	$ 1,197,000
State and political subdivisions | Other Credit Enhancement or Insurance
Debt Securities, Available-for-sale [Line Items]
Held to maturity mortgage backed securities	$ 987,971,000			$ 987,971,000