Fair Value Measurement of Assets and Liabilities - Carrying Amounts and Estimated Fair Values (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024
Financial assets
Interest bearing deposits with banks	$ 994,224	$ 1,478,713
Equity securities	78,296	71,513
Total held to maturity debt securities	3,150,523	3,028,284
Net loans	8,800	8,800
Accrued interest receivable	242,861	239,941
Financial liabilities
Deposits with stated maturities	12,270,067	12,342,544
U.S. Treasury securities
Financial assets
Total held to maturity debt securities		25,461
U.S. government agency securities
Financial assets
Total held to maturity debt securities	253,280	252,302
Obligations of states and political subdivisions
Financial assets
Total held to maturity debt securities	326,133	346,361
Residential mortgage-backed securities
Financial assets
Total held to maturity debt securities	2,457,228	2,292,148
Trust preferred securities
Financial assets
Total held to maturity debt securities	30,586	29,145
Corporate and other debt securities
Financial assets
Total held to maturity debt securities	83,296	82,867
Carrying Amount
Financial assets
Total held to maturity debt securities	3,541,456	3,532,220
Carrying Amount | Level 1
Financial assets
Cash and due from banks	376,216	411,412
Interest bearing deposits with banks	994,224	1,478,713
Accrued interest receivable	242,861	239,941
Financial liabilities
Deposits without stated maturities	38,905,691	37,733,313
Accrued interest payable	104,477	150,564
Carrying Amount | Level 1 | U.S. Treasury securities
Financial assets
Total held to maturity debt securities	0	25,480
Carrying Amount | Level 2
Financial assets
FRB and FHLB stock	339,540	328,497
Financial liabilities
Deposits with stated maturities	12,270,067	12,342,544
Short-term borrowings	51,052	72,718
Long-term borrowings	2,905,898	3,174,155
Junior subordinated debentures issued to capital trusts	57,716	57,455
Carrying Amount | Level 2 | U.S. government agency securities
Financial assets
Total held to maturity debt securities	292,897	301,315
Carrying Amount | Level 2 | Obligations of states and political subdivisions
Financial assets
Total held to maturity debt securities	349,973	372,489
Carrying Amount | Level 2 | Residential mortgage-backed securities
Financial assets
Total held to maturity debt securities	2,777,381	2,710,642
Carrying Amount | Level 2 | Trust preferred securities
Financial assets
Total held to maturity debt securities	36,097	36,081
Carrying Amount | Level 2 | Corporate and other debt securities
Financial assets
Total held to maturity debt securities	85,108	86,213
Carrying Amount | Level 3
Financial assets
Equity securities	44,589	36,871
Net loans	48,687,823	48,240,861
Fair Value
Financial assets
Total held to maturity debt securities	3,150,523	3,028,284
Fair Value | Level 1
Financial assets
Cash and due from banks	376,216	411,412
Interest bearing deposits with banks	994,224	1,478,713
Accrued interest receivable	242,861	239,941
Financial liabilities
Deposits without stated maturities	38,905,691	37,733,313
Accrued interest payable	104,477	150,564
Fair Value | Level 1 | U.S. Treasury securities
Financial assets
Total held to maturity debt securities	0	25,461
Fair Value | Level 2
Financial assets
FRB and FHLB stock	339,540	328,497
Financial liabilities
Deposits with stated maturities	12,319,465	12,363,365
Short-term borrowings	48,632	68,032
Long-term borrowings	2,878,563	3,109,622
Junior subordinated debentures issued to capital trusts	53,354	54,957
Fair Value | Level 2 | U.S. government agency securities
Financial assets
Total held to maturity debt securities	253,280	252,302
Fair Value | Level 2 | Obligations of states and political subdivisions
Financial assets
Total held to maturity debt securities	326,133	346,361
Fair Value | Level 2 | Residential mortgage-backed securities
Financial assets
Total held to maturity debt securities	2,457,228	2,292,148
Fair Value | Level 2 | Trust preferred securities
Financial assets
Total held to maturity debt securities	30,586	29,145
Fair Value | Level 2 | Corporate and other debt securities
Financial assets
Total held to maturity debt securities	83,296	82,867
Fair Value | Level 3
Financial assets
Equity securities	44,589	36,871
Net loans	$ 46,731,393	$ 46,634,654