Allowance for Credit Losses (Held-to-Maturity Debt Securities by Credit Quality Indication) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
Amortized Cost	$ 560,330	$ 1,134,400	$ 1,095,802
Less: Allowance for credit losses	(63)	0	0
Held-to-maturity securities, net of allowance for credit losses	560,267	1,134,400	1,095,802
U.S. Government agencies
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2020	124,575
2019	119,995
2018	101,450
2017	0
2016	0
Prior	3,395
Amortized Cost	349,415	902,974	859,011
U.S. Government agencies | 1-4 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2020	124,575
2019	119,995
2018	101,450
2017	0
2016	0
Prior	3,395
Amortized Cost	349,415
U.S. Government agencies | 5-7 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Amortized Cost	0
U.S. Government agencies | 8-10 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Amortized Cost	0
Municipal
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2020	0
2019	162
2018	7,590
2017	43,863
2016	10,172
Prior	149,128
Amortized Cost	210,915	$ 231,426	$ 236,791
Municipal | 1-4 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2020	0
2019	162
2018	7,590
2017	43,863
2016	10,172
Prior	149,128
Amortized Cost	210,915
Municipal | 5-7 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Amortized Cost	0
Municipal | 8-10 internal grade
Debt Securities, Held-to-maturity, Credit Quality Indicator [Line Items]
2020	0
2019	0
2018	0
2017	0
2016	0
Prior	0
Amortized Cost	$ 0