Investment Securities (Amortization Cost, Gross Unrealized Gains/Losses, and Fair Value) (Details 1) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Available-for-Sale:
Available-for-Sale: Amortized Cost	$ 2,613,203	$ 2,607,636
Available-for-Sale: Gross Unrealized Gains	76,678	21,143
Available-for-Sale: Gross Unrealized Losses	(8,832)	(9,776)
Available-for-Sale	2,681,049	2,619,003
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Held-to-Maturity: Amortized Cost	3,004,139	3,042,294
Held-to-Maturity: Gross Unrealized Gains	100,227	29,990
Held-to-Maturity: Gross Unrealized Losses	(346)	(9,402)
Held-to-Maturity: Fair Value	3,104,020	3,062,882
Debt Securities Issued by Government-Sponsored Enterprises
Available-for-Sale:
Available-for-Sale: Amortized Cost	936	22,128
Available-for-Sale: Gross Unrealized Gains	76	19
Available-for-Sale: Gross Unrealized Losses	0	0
Available-for-Sale	1,012	22,147
Residential - Government Agencies
Available-for-Sale:
Available-for-Sale: Amortized Cost	1,159,266	1,164,466
Available-for-Sale: Gross Unrealized Gains	41,360	11,627
Available-for-Sale: Gross Unrealized Losses	(1,412)	(3,267)
Available-for-Sale	1,199,214	1,172,826
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Held-to-Maturity: Amortized Cost	994,405	1,067,416
Held-to-Maturity: Gross Unrealized Gains	38,678	13,247
Held-to-Maturity: Gross Unrealized Losses	(36)	(5,348)
Held-to-Maturity: Fair Value	1,033,047	1,075,315
Residential - U.S. Government-Sponsored Enterprises
Available-for-Sale:
Available-for-Sale: Amortized Cost	635,161	584,272
Available-for-Sale: Gross Unrealized Gains	26,178	4,363
Available-for-Sale: Gross Unrealized Losses	(166)	(1,874)
Available-for-Sale	661,173	586,761
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Held-to-Maturity: Amortized Cost	1,648,144	1,546,479
Held-to-Maturity: Gross Unrealized Gains	57,554	13,871
Held-to-Maturity: Gross Unrealized Losses	0	(2,478)
Held-to-Maturity: Fair Value	1,705,698	1,557,872
Commercial - Government Agencies
Available-for-Sale:
Available-for-Sale: Amortized Cost	287,441	224,372
Available-for-Sale: Gross Unrealized Gains	5,790	2,889
Available-for-Sale: Gross Unrealized Losses	(869)	(2,541)
Available-for-Sale	292,362	224,720
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Held-to-Maturity: Amortized Cost	78,192	84,238
Held-to-Maturity: Gross Unrealized Gains	763	317
Held-to-Maturity: Gross Unrealized Losses	(310)	(1,407)
Held-to-Maturity: Fair Value	78,645	83,148
Mortgage-Backed Securities
Available-for-Sale:
Available-for-Sale: Amortized Cost	2,081,868	1,973,110
Available-for-Sale: Gross Unrealized Gains	73,328	18,879
Available-for-Sale: Gross Unrealized Losses	(2,447)	(7,682)
Available-for-Sale	2,152,749	1,984,307
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Held-to-Maturity: Amortized Cost	2,720,741	2,698,133
Held-to-Maturity: Gross Unrealized Gains	96,995	27,435
Held-to-Maturity: Gross Unrealized Losses	(346)	(9,233)
Held-to-Maturity: Fair Value	2,817,390	2,716,335
Debt Securities Issued by the U.S. Treasury and Government Agencies
Available-for-Sale:
Available-for-Sale: Amortized Cost	202,065	222,365
Available-for-Sale: Gross Unrealized Gains	262	213
Available-for-Sale: Gross Unrealized Losses	(1,238)	(1,447)
Available-for-Sale	201,089	221,131
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Held-to-Maturity: Amortized Cost	214,612	274,375
Held-to-Maturity: Gross Unrealized Gains	2,111	1,319
Held-to-Maturity: Gross Unrealized Losses	0	(31)
Held-to-Maturity: Fair Value	216,723	275,663
Debt Securities Issued by States and Political Subdivisions
Available-for-Sale:
Available-for-Sale: Amortized Cost	53,981	54,480
Available-for-Sale: Gross Unrealized Gains	1,043	631
Available-for-Sale: Gross Unrealized Losses	(1)	(14)
Available-for-Sale	55,023	55,097
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Held-to-Maturity: Amortized Cost	54,543	54,811
Held-to-Maturity: Gross Unrealized Gains	1,017	1,236
Held-to-Maturity: Gross Unrealized Losses	0	0
Held-to-Maturity: Fair Value	55,560	56,047
Debt Securities Issued by Corporations
Available-for-Sale:
Available-for-Sale: Amortized Cost	274,353	335,553
Available-for-Sale: Gross Unrealized Gains	1,969	1,401
Available-for-Sale: Gross Unrealized Losses	(5,146)	(633)
Available-for-Sale	271,176	336,321
Debt Securities, Held-to-maturity, Fair Value to Amortized Cost [Abstract]
Held-to-Maturity: Amortized Cost	14,243	14,975
Held-to-Maturity: Gross Unrealized Gains	104	0
Held-to-Maturity: Gross Unrealized Losses	0	(138)
Held-to-Maturity: Fair Value	$ 14,347	$ 14,837

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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

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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity).

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

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