Securities (Summary of Book Values and Fair Values of Investment Securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Fair Value
Amortized Cost	$ 2,622,066	$ 2,758,556
Fair Value	2,100,406	2,314,493
Unrealized gains	0	4
Unrealized (losses)	(521,660)	(444,067)
Securities held to maturity:
Amortized Cost	535,460	541,700
Fair Value	410,321	432,528
Unrealized gains	0	7
Unrealized (losses)	(125,139)	(109,179)
U.S. Treasuries
Fair Value
Amortized Cost	174,693	174,420
Fair Value	170,859	168,758
Unrealized gains	0	0
Unrealized (losses)	(3,834)	(5,662)
Government-sponsored enterprise securities
Fair Value
Amortized Cost	71,962	71,957
Fair Value	56,904	57,456
Unrealized gains	0	0
Unrealized (losses)	(15,058)	(14,501)
Mortgage-backed securities
Fair Value
Amortized Cost	2,355,736	2,467,839
Fair Value	1,854,448	2,045,000
Unrealized gains	0	4
Unrealized (losses)	(501,288)	(422,843)
Securities held to maturity:
Amortized Cost	12,755	15,150
Fair Value	11,674	14,221
Unrealized gains	0	0
Unrealized (losses)	(1,081)	(929)
Corporate bonds
Fair Value
Amortized Cost	19,675	44,340
Fair Value	18,195	43,279
Unrealized gains	0	0
Unrealized (losses)	(1,480)	(1,061)
State and local governments
Securities held to maturity:
Amortized Cost	522,705	526,550
Fair Value	398,647	418,307
Unrealized gains	0	7
Unrealized (losses)	$ (124,058)	$ (108,250)

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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