Investment Securities (Available-for-sale Securities with Unrealized Loss) (Details)	3 Months Ended
	Mar. 31, 2020 USD ($) securities	Dec. 31, 2019 USD ($) securities
Available-for-sale securities that had an unrealized loss:
Less than 12 months, fair value	$ 59,728,000	$ 291,292,000
Less than 12 months, unrealized loss	$ 524,000	$ 2,622,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | securities	30	65
12 months or more, fair value	$ 25,059,000	$ 95,701,000
12 months or more, unrealized loss	$ 684,000	$ 1,970,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	21	45
Total unrealized loss position, fair value	$ 84,787,000	$ 386,993,000
Total unrealized loss	1,208,000	4,592,000
Other than Temporary Impairment Losses, Investments, Available-for-sale Securities	0
Available-for-sale Securities, Amortized Cost Basis	932,179,000	929,395,000
Gross Unrealized Gains	29,864,000	11,298,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	1,208,000	4,592,000
Available-for-sale investment securities, at fair value (amortized cost of $932,179 at March 31, 2020 and $929,395 at December 31, 2019) (a)	960,835,000	936,101,000
States and political subdivisions
Available-for-sale securities that had an unrealized loss:
Less than 12 months, fair value	2,337,000	6,226,000
Less than 12 months, unrealized loss	$ 75,000	$ 74,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | securities	2	2
12 months or more, fair value	$ 2,251,000	$ 2,441,000
12 months or more, unrealized loss	$ 43,000	$ 57,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	1	1
Total unrealized loss position, fair value	$ 4,588,000	$ 8,667,000
Total unrealized loss	118,000	131,000
Available-for-sale Securities, Amortized Cost Basis	105,904,000	111,217,000
Gross Unrealized Gains	3,026,000	3,018,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	118,000	131,000
Available-for-sale investment securities, at fair value (amortized cost of $932,179 at March 31, 2020 and $929,395 at December 31, 2019) (a)	108,812,000	114,104,000
Residential mortgage-backed securities
Available-for-sale securities that had an unrealized loss:
Less than 12 months, fair value	57,391,000	284,096,000
Less than 12 months, unrealized loss	$ 449,000	$ 2,527,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | securities	28	62
12 months or more, fair value	$ 18,948,000	$ 88,993,000
12 months or more, unrealized loss	$ 334,000	$ 1,657,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	15	39
Total unrealized loss position, fair value	$ 76,339,000	$ 373,089,000
Total unrealized loss	783,000	4,184,000
Available-for-sale Securities, Amortized Cost Basis	798,762,000	787,430,000
Gross Unrealized Gains	25,914,000	7,763,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	783,000	4,184,000
Available-for-sale investment securities, at fair value (amortized cost of $932,179 at March 31, 2020 and $929,395 at December 31, 2019) (a)	823,893,000	791,009,000
Commercial mortgage-backed securities
Available-for-sale securities that had an unrealized loss:
Less than 12 months, fair value	0	970,000
Less than 12 months, unrealized loss	$ 0	$ 21,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | securities	0	1
12 months or more, fair value	$ 2,050,000	$ 2,409,000
12 months or more, unrealized loss	$ 117,000	$ 114,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	3	3
Total unrealized loss position, fair value	$ 2,050,000	$ 3,379,000
Total unrealized loss	117,000	135,000
Available-for-sale Securities, Amortized Cost Basis	16,899,000	18,135,000
Gross Unrealized Gains	279,000	88,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	117,000	135,000
Available-for-sale investment securities, at fair value (amortized cost of $932,179 at March 31, 2020 and $929,395 at December 31, 2019) (a)	17,061,000	18,088,000
Bank-issued trust preferred securities
Available-for-sale securities that had an unrealized loss:
Less than 12 months, fair value	0	0
Less than 12 months, unrealized loss	$ 0	$ 0
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Less than One Year | securities	0	0
12 months or more, fair value	$ 1,810,000	$ 1,858,000
12 months or more, unrealized loss	$ 190,000	$ 142,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	2	2
Total unrealized loss position, fair value	$ 1,810,000	$ 1,858,000
Total unrealized loss	$ 190,000	142,000
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions | securities	2
Available-for-sale Securities, Amortized Cost Basis	$ 4,696,000	4,696,000
Gross Unrealized Gains	202,000	137,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	190,000	142,000
Available-for-sale investment securities, at fair value (amortized cost of $932,179 at March 31, 2020 and $929,395 at December 31, 2019) (a)	$ 4,708,000	4,691,000
Mortgage-backed securities, issued by US government sponsored agencies
Available-for-sale securities that had an unrealized loss:
Percentage of mortgage-backed securities in unrealized loss position for more than 12 months	99.00%
Mortgage-backed securities, privately issued
Available-for-sale securities that had an unrealized loss:
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions, Greater than or Equal to One Year | securities	2
Percentage of mortgage-backed securities in unrealized loss position for more than 12 months	1.00%
Fair value within book value	90.00%
U.S. government sponsored agencies
Available-for-sale securities that had an unrealized loss:
Available-for-sale Securities, Amortized Cost Basis	$ 5,918,000	7,917,000
Gross Unrealized Gains	443,000	292,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0	0
Available-for-sale investment securities, at fair value (amortized cost of $932,179 at March 31, 2020 and $929,395 at December 31, 2019) (a)	6,361,000	$ 8,209,000
Fair Value Less Than 90 Percent of Book Value [Member] | Mortgage-backed securities, privately issued
Available-for-sale securities that had an unrealized loss:
12 months or more, fair value	165,000
Book value of privately issued mortgage-backed securities in unrealized loss position for more than 12 months	$ 202,000